Sustainable Intensification of Olive Agroecosystems via Barley, Triticale, and Pea Intercropping

Abstract

In the Mediterranean basin, olive cultivation occupies the largest share of agricultural land, due to the region’s favorable soil and climatic conditions. However, the intensification of farming systems has had negative environmental impacts, for which diversified approaches such as agroforestry offer a potential solution. The objective of the present study was to determine the growth of barley, triticale, and pea as cover crops, as well as the respective intercrops in olive orchards and their productivity. The results showed that the intercropping of pea with barley and triticale had the highest yields in dry biomass compared to the other treatments, while barley monoculture recorded the highest yield in terms of grain. The findings demonstrated that intercropping enhances resource-use efficiency, particularly in terms of land productivity, Radiation-Use Efficiency, and Water-Use Efficiency. However, competitive dynamics varied significantly between species and across years, with pea often exhibiting dominance in biomass production, while cereals showed trade-offs in seed yield components due to shading and interspecific competition. These findings can be used for sustainable intensification strategies, ensuring higher productivity while minimizing external inputs in climate-vulnerable regions.

1. Introduction

The use of synthetic inputs and more specifically fertilizers and pesticides, together with the simplification of agri-food systems, have caused strong adverse effects on the environment, leading to soil degradation, water and air pollution, greenhouse gas emissions that contribute to climate change, and biodiversity loss [1]. On the other hand, the significant increase in the world population and the increasing demand for meat and other agricultural products in developing countries are challenging modern agriculture to continuously increase food production. One of the ways that agriculture can continue to produce and preserve environmental resources is the use of high species diversified (HSD) cropping systems which rely more on ecosystem services, also called agroecological intensification [2,3]. One of the diversified cropping systems that can be used in the Mediterranean region is multi-services cover crops in orchards and other perennial crop species [4].

In most countries in the Mediterranean basin, olive cultivation occupies the largest proportion of the cultivated area, due to the soil and climate conditions in these regions [5]. However, in light of climate change, agricultural production in recent years has been turning to alternative ways of cultivation so it can increase crop yield with the minimum use of inputs. Intercropping of olive trees with cereals and legumes can increase the sustainability of the agroecosystem, yield higher profits with biomass and grain production than arable crops, and have a positive impact on the productivity of olive trees [5,6].

On the other hand, olive orchards are increasingly managed with permanent and annual or seasonal intercrops to increase soil organic matter and water retention, and to prevent erosion [7,8]. There are several studies that have explored the use of intercrops in olive orchards and they focused on the impact on soil health, water management, and tree productivity in both irrigated and rain-fed systems [5,9,10]; however, there are less studies that have focused on the performance of the use of intercrops with winter cereals and legumes compared with sole crops under these systems [11]. In addition, there is a need to determine how intercropping systems are responding to agroforestry systems and whether they are more productive compared with the sole crops [10].

Several annual crop plants like barley, triticale, and pea are well adapted to Mediterranean conditions; however, they were not tested extensively as intercrops in an agroforestry system, and they were not compared with the sole crops [5,10]. In Mediterranean trials, barley intercropped with olive trees showed 58% higher seed yields and improved chlorophyll content under drought conditions compared to monocultures [10]. The olive canopy provided a microclimate that reduced evapotranspiration and mitigated heat stress [7,8]. Additionally, intercropping can increase olive tree growth and fruit/oil yields by increasing soil nitrogen and organic matter [9]. Agroforestry systems also promoted stable microbial networks, enhancing nutrient cycling and drought resilience [7,8,9]. Barley–pea intercropping in olive agroforestry systems enhances land productivity, economic returns, and drought resilience compared to when they are grown as sole crops. While direct studies are limited, evidence from Mediterranean and South Asian trials highlight its potential for sustainable farming in water-scarce regions [5,9,10].

Intercropping barley with peas in olive agroforestry systems affects biomass production, seed yield, and yield components in several ways, primarily influenced by resource competition, microclimate, and legume–cereal interactions [5,10]. Biomass production for barley and peas tend to be lower under olive agroforestry systems compared to conventional cropping systems due to shading and competition for water and nutrients from olive trees. For example, legumes like peas generally exhibit better resilience to environmental stress than cereals near olive trees, but overall biomass production is reduced under low-rain-fed conditions [5,10]. Peas contribute positively to soil fertility through nitrogen fixation, which indirectly supports barley growth and biomass accumulation [10]. However, the proximity to olive trees limits the full potential of biomass production for both crops.

In addition, intercropping generally demonstrates superior efficiency compared to sole cropping, as reflected in its higher land equivalent ratios (LER) [12,13,14]. This indicates a more effective use of key environmental resources such as light, water, and nutrients. Studies comparing legumes and cereals in intercrops versus monocultures benefit from the ability of legumes to fix atmospheric nitrogen, thereby improving soil fertility, supporting the growth of companion crops, and reducing the need for synthetic fertilizers [12,14]. Moreover, the greater ground cover provided by intercrops can effectively suppress weed growth, which in turn lowers reliance on herbicides. Intercropping systems are also generally more resilient than monocultures, offering greater yield stability under variable climatic conditions, including the irregular rainfall patterns characteristic of Mediterranean environments. Collectively, these benefits suggest that intercrops not only perform as well as, but frequently surpass, sole crops in terms of productivity and ecological sustainability [12,13,14]. Although evidence from olive-based systems remains limited, the consistent advantages documented in other Mediterranean agroecosystems provide strong support for their integration into sustainable olive agroforestry practices.

The intercropping systems are quite complex, since there are several interactions between the different species that are involved [12,13]. These interactions are termed as the “four Cs” principles [12,13]: Competition, Complementarity, Cooperation, and Compensation [12,13]. The interaction that is most pronounced is competition, since it occurs for water, light, nutrients, and other environmental resources [12,13]. Therefore, competition between the different species should be at a minimum and the plants species that are involved in the cropping system should utilize the environmental resources more effectively, which can lead to higher yields [12,13,14]. Moreover, competition is the predominant interaction in the intercropping systems, but there is limited information about the interactions between the different crops in an agroforestry system and the evaluation of different indices to determine the different interactions [13,15,16,17,18,19]. Several indices can be used to quantify the interactions between the different crop species that are used in intercropping systems. In particular, the indices that have been used extensively are the partial land equivalent ratio and total Land Equivalent Ratio (LER), Relative Crowding Coefficient (Κ), Competitive Ratio (CR), Aggressivity (A), Actual Yield Loss (AYL), Intercropping Advantage (IA), and Monetary Advantage Index (MAI), which have been developed to describe competition and economic advantage in intercropping [16,20,21,22,23,24,25]. More indices have been proposed such as the Land Equivalent Coefficient (LEC), System Productivity Index (SPI) and Percentage Yield Difference (PYD) [26,27,28,29]. However, these indices have not been used in agroforestry systems where mixtures of different annual crops are used [26,27,28,29].

The objectives of the present study were to: (1) determine the growth performance of different cover crops sown in olive orchards, (2) evaluate the productivity of the different intercropping systems, and (3) examine the interactions among the intercropped species using different indices.

2. Materials and Methods

2.1. Site Characteristics

Soil analysis was performed to assess its properties and nutrient availability (

Table 1. Soil properties and nutrient availability of the olive orchard at Thermi, Greece.

Characteristics	Olive Orchard (0–30 cm)	Olive Orchard (30–60 cm)
Sand (%)	42.8	43.2
Silt (%)	36.4	36.4
Clay (%)	20.8	20.4
Soil type	Loam	Loam
pH (H2O 1:1) (25 °C)	7.4	7.31
Electrical conductivity (25 °C) (mS/cm)	0.531	0.607
CaCO3 (%)	1.5	1.46
Organic matter (%)	2.4	1.5
P (POlsen) (mg/kg of soil)	34.4	27.8
Total N (Kjeldahl) (mg/kg of soil)	6.85	9.70
K (mg/kg of soil)	920	580
Mg (mg/kg of soil)	227	400
Cu (CuDTPA) (mg/kg of soil)	15.4	26.3
Fe (FeDTPA-) (mg/kg of soil)	8.49	11.66
Mn (MnDTPA) (mg/kg of soil)	11.8	13.7
Zn (ZnDTPA) (mg/kg of soil)	2.8	6.2
B (HwsB) (mg/kg of soil)	1.38	1.68

). The soil type was loam, with organic matter 24 g kg⁻¹, pH (1:1 H₂O) 7.4, EC (dS m⁻¹) 0.531, and CaCO₃ 1.5%. Furthermore, the average annual temperature and rainfall were recorded (

Table 2. The main weather parameters (mean temperature and rainfall) for the three growing seasons of experimentation at Thermi, Greece, and their comparison to the 30-year average. The weather data were recorded by an automatic weather station close to the experimental site.

	Mean Temperature (°C)				Rainfall (mm)
Month	2021	2022	2023	30-Year Average	2021	2022	2023	30-Year Average
January	8.5	6.0	9.9	5.2	104.6	34	60.4	29.0
February	9.0	8.7	8.5	6.4	14.4	39.6	15.4	31.0
March	10.2	7.7	11.8	9.6	7.4	50.4	36.2	31.0
April	13.2	14.6	14.0	13.9	31.0	24.2	75.6	38.0
May	20.5	20.7	18.0	19.3	12.2	20.6	70	44.0
June	24.7	25.5	23.2	24.5	12.2	48.6	85.8	32.0
July	28.9	27.4	28.7	26.7	10.2	54.4	9	31.0
August	28.7	27.6	27.7	26	11.2	40.2	40	24.0
September	22.6	22.4	23.8	21.7	21.4	49	11.4	29.0
October	15.4	17.5	16.5	16.3	136.2	7.6	40	42.0
November	13.5	14.1	13.7	10.3	23.6	10	25	51
December	8.0	10.9	10.1	6.5	54.0	10.4	20	61
Mean	16.9	16.9	17.2	15,5
Total					438.4	389.0	488.8	443.0

).

2.2. Experimental Design

The experiments were conducted in an olive grove of the Aristotle University of Thessaloniki Farm for three consecutive growing seasons (2020–2021, 2021–2022 and 2022–2023), in order to study the interactions of different cereal and forage pea cropping systems with the agroforestry systems of olive trees. The olive trees were 30 years old (cv. Chalkidiki) with a canopy height of 3 m. The distance between the trees within rows was 6 m and the distance of each row was 6 m, resulting in a 6 m × 6 m olive orchard (Figure 1). The crop species evaluated were barley (cv. Fatima), triticale (cv. Alkmini), forage pea (cv. Dodoni, a leafy cultivar), and cereal–pea mixtures. Sowing was performed in late November each year and the harvesting was completed in early June. The intercrop was a replacement design with a 25:75 cereal-to-pea ratio. The crops did not receive any irrigation, and all the needs for water of the crops were covered by rainfall. Moreover, no fertilizers, herbicides, or insecticides were applied either to the olive trees or to the annual species throughout the experimental period, as no significant pest or disease incidence was detected. The experimental design was a complete randomized block design with four replications. Each plot consisted of cereal and pea monocultures and their respective cereal–pea intercrops. Plot size was 2.5 m × 12 m. The measurements that were conducted were plant height, chlorophyll content (SPAD), and Leaf Area Index (LAI), at three stages of plant development: during stem elongation (BBCH 30), the beginning of flowering (BBCH 61), and grain filling (BBCH 73) for the cereals. Additionally, the number of tillers/m², the number of spikes and pods/plant, dry biomass, and seed yield were recorded. Finally, intercrop indices based on biomass and seed yield were calculated.

2.3. Plant Height

Plant height was measured at three growth stages. The first measurement was taken during April, at the stem elongation stage of cereal (BBCH 30), as an indicator of early plant vigor. Pea plants were at the stage of two visible nodes (BBCH 32). The second measurement was conducted in early May, at the beginning of cereal flowering. Finally, the third measurement was carried out near the end of May, at the grain filling stage of cereal (BBCH 73), with 50% of pod expansion for peas (BBCH 75). Plant height was measured by placing a measuring tape at the base of the plant and recording the highest point of the plant. Five randomly selected plants were measured for each species in a random area of 1 m² of each experimental plot, and an average value was calculated.

2.4. Leaf Area Index (LAI)

The Leaf Area Index (LAI) was determined using the ACCUPAR LP80 device (Meter, München, Germany) simultaneously with height measurements [30]. One measurement was taken above the experimental plot to record incident radiation, and three measurements were taken within the canopy at the base of plants. Measurements were conducted between 11:00 AM and 1:00 PM under clear skies.

2.5. Leaf Greenness Index (SPAD)

The measurement was performed using the SPAD 502DL (Minolta Camera Co. Ltd., Osaka, Japan). The device makes an estimate, in a non-destructive manner, of the intensity of the green color and, therefore, indirectly of the chlorophyll concentration.

2.6. Dry Matter Yield, Seed Yield and Yield Components

For dry weight calculation, a total area of 1 m² of each plot was harvested in mid-to-late June for the three years. Samples were then dried in a greenhouse for 10 days, weighed, and dry weight per m² was calculated. Seed yield was determined by harvesting an area of 1 m² of each plot in June 2021, June 2022, and June 2023, respectively. After drying for 10 days, the different species were separated. Also, the following yield components were determined: the number of spikes and pods per plant and the number of seeds per spike and per pod. The number of spikes per plant and the number of pods per plant were measured on three randomly selected plants per plot. The number of seeds per spike and the number of seeds per pod were measured on three randomly selected mature spikes and pods per plot. The means were calculated for each experimental plot and used in the statistical analysis.

2.7. Water-Use Efficiency

The experiments were grown under rain-fed conditions and Water-Use Efficiency (WUE) was calculated as yield (biomass or seed in kg/ha) divided by the total rainfall (mm) [31,32].

2.8. Radiation-Use Efficiency

Radiation-Use Efficiency (RUE; g MJ⁻¹) of the different treatments was calculated by dividing the total biomass or seed yield produced by the cumulative radiation intercepted during the growing period. This method follows the approach described by Elhakeem et al. [33].

2.9. Intercrop Indices

The following intercrop indices were calculated both for biomass and seed yield:

Land Equivalent Ratio (LER) was calculated for both crops and it was used to determine the advantage or disadvantage of intercropping [34,35] as follows:

LER = (Y_ci/Y_c) + (Y_p/Y_pi)

where Y_c is the cereal yield in monoculture, Y_ci is the cereal yield in intercrop, Y_p is the pea yield in monoculture, and Y_pi is the pea yield in intercrop. (Y_ci/Y_c) and (Y_pi/Y_p) are the partial LER (pLER) of cereal and pea, respectively. When LER > 1, the intercrop outperforms the respective monoculture, while LER < 1 indicates that intercropping has a disadvantage compared with the monoculture. LER is one of the most widely used indices to evaluate intercrop performance and pLERs are used to describe species complementarity or competitive interaction in a mixture [36].

Actual Yield Loss (AYL) is a measure of proportionate yield loss or gain of intercrop compared to monocrop that takes into account the sown density of each species [24]. It is calculated according to the following formulae [21,37]:

$$\begin{array}{c}{\mathrm{AYL}}_{\mathrm{c}} = {\mathrm{pLER}}_{\mathrm{c}} \times (100/{\mathrm{Z}}_{\mathrm{C}\mathrm{i}})-1\hfill \\ {\mathrm{AYL}}_{\mathrm{p}} = {\mathrm{pLER}}_{\mathrm{p}} \times (100/{\mathrm{Z}}_{\mathrm{p}\mathrm{i}})-1\hfill \\ \mathrm{AYL} = {\mathrm{AYL}}_{\mathrm{c}}+{\mathrm{AYL}}_{\mathrm{p}}\hfill \end{array}$$

where AYLc and AYL_p are the Actual Yield Loss of the cereal and pea, respectively, and Z_ci and Z_pi are the relative sowing proportion of cereal and pea, respectively, in the mixtures. When AYL > 0, it indicates a yield advantage of intercrop compared to monocrop, while when AYL < 0 the intercrop is at a disadvantage [37].

Relative Crowding Coefficient (K) is a measure of relative competitive advantage of one species over the other. It is calculated as follows [29]:

$$\begin{array}{c}\mathrm{K} = {\mathrm{K}}_{\mathrm{c}} \times {\mathrm{K}}_{\mathrm{p}}\hfill \\ {\mathrm{K}}_{\mathrm{c}} = \left[\right({\mathrm{Y}}_{\mathrm{c}\mathrm{i}} \times {\mathrm{Z}}_{\mathrm{p}\mathrm{i}})/({\mathrm{Y}}_{\mathrm{c}} \times {\mathrm{Y}}_{\mathrm{c}\mathrm{i}}) \times {\mathrm{Z}}_{\mathrm{c}\mathrm{i}}]\hfill \\ {\mathrm{K}}_{\mathrm{p}} = \left[\right({\mathrm{Y}}_{\mathrm{p}\mathrm{i}} \times {\mathrm{Z}}_{\mathrm{c}\mathrm{i}})/({\mathrm{Y}}_{\mathrm{p}} - {\mathrm{Y}}_{\mathrm{p}\mathrm{i}}) \times {\mathrm{Z}}_{\mathrm{p}\mathrm{i}}]\hfill \end{array}$$

where K_c and K_p are the Relative Crowding Coefficients of the cereals and pea, respectively. When K_c > K_p, then cereal is more competitive than pea, while when K_p > K_c, pea is more competitive than cereal [29]. Furthermore, if K > 1, then there is a yield advantage, while when K < 1, then the intercrop is at a disadvantage [24].

Competitive Ratio (CR) represents the ratio of pLERs but considers the sowing proportion of each species. It is a ratio that quantifies the competition that occurs between the two crops [38]. It is calculated according to the following formulae [16]:

CR_c = (pLER_c/pLER_p) × (Z_pi/Z_ci)

CR_p = (pLER_p/pLER_c) × (Z_ci/Z_pi)

Aggressivity (A) is another measure of competition, and it indicates to which extent the relative yield of one crop in the mixture was higher than the other. If there is no competition between the two species, values are zero. The aggressor species obtains values above 0, while the suppressed species obtains negative values [38]. It is calculated according to the following equations [29]:

A_c = [Y_ci/(Y_c × Z_ci)] – [Y_pi/(Y_p × Z_pi)]

A_p = [Y_pi/(Y_p × Z_pi)] – [Y_ci/(Y_c × Z_ci)]

Land Equivalent Coefficient (LEC) was calculated according to the following equation [26]:

LEC = pLER_c × pLER_p

From the above equation, it is clear that LEC is a product of pLERs and can better describe complementary and competitive interactions than LER. An LEC threshold value of 0.25 is the minimum before an intercropping advantage. LEC < 0.25 indicates that one of the two species was severely dominated and its yield was compromised [26].

System Productivity Index (SPI) was calculated based on the following equation [29]:

SPI = Y_ci + Y_c/Y_p × Y_pi

SPI has the advantage that it standardizes the yield of the secondary crop (pea) in terms of that of the primary crop [27].

Percentage Yield Difference (PYD) was estimated according to the following equation [29]:

PYD = 100 – [(Y_c – Y_ci)/Y_c + (Y_p – Y_pi) Y_p] × 100

PYD is the percentage yield difference between intercrop and sole crop, assuming a 100% yield in sole crop. The lower the values of PYD, the greater the advantage of intercrop, as it is inversely proportional to yield advantage [28]. On the contrary, Zustovi el al. [38] concluded that PYD is positively correlated with LER and therefore higher values indicate an intercropping advantage.

2.10. Statistical Analysis

Preliminary analyses of all data showed that there was a significant variance between the years, and the effect size of the factor “growth stage” was large so the analysis was performed separately within the year and within the growth stage (Tables S1–S5). The data for plant height, Leaf Greenness Index (SPAD), spikes per plant, and seeds per spike were analyzed for each cereal separately with the analysis of variance (ANOVA) method within the methodological frame of general linear models. For these characteristics, the corresponding model included the effects of the factors “block” (4 blocks) and “treatment” (the monocrops of each cereal and its intercrop). In this model, all factors were entered as fixed-effects factors. In this approach, the analysis was based on an RCB design arrangement [39,40]. Data for the Leaf Area Index (LAI), dry weight, seed yield, Water-Use Efficiency, and Radiation-Use Efficiency ANOVA model included the factors “block” and “treatment” (the monocrops of the three species and the two intercrops). The intercrop indices [Land Equivalent Ratio (LER), Actual Yield Loss, Relative Crowding Coefficient, Competitive Ratio, Aggressivity, Land Equivalent Coefficient, System Productivity Index and Percentage Yield Difference] and corresponding ANOVAs were performed according to an RCB design. The “protected” Least Significant Difference (LSD) criterion was used for testing the differences among mean values. In all hypothesis testing procedures, the significance level was predetermined at a = 0.05 (p ≤ 0.05). All statistical analyses were accomplished with the IBM SPSS Statistics v26.0 statistical software (SPSS Inc., Chicago, IL, USA). The homoscedasticity and normality of residuals and the assumption of additivity were checked for each model, and no significant violations were found.

3. Results

3.1. Leaf Area Index

Leaf Area Index (LAI) was affected by the year, growing stage, and cropping system (Table S1). During the first year, barley monocrop recorded the lowest LAI values in all three growth stages (0.43, 1.28, and 3.26 for jointing, full bloom and grain filling, respectively), while triticale–pea intercrop recorded the highest values of LAI by an average of 70% (1.72, 3.03, and 4.86 for jointing, full bloom, and grain filling, respectively). Also, intercrops of barley and triticale with pea showed higher LAI values in all growth stages, compared to their respective cereal monocultures. The advantage of intercropping was particularly evident during the first season, where both intercropped systems recorded significantly higher LAI values at all stages. A different trend was seen during the following years, 2021–2022 and 2022–2023, as no statistically significant differences were observed among treatments, except for triticale monoculture at the full bloom stage in 2021–2022, which recorded a lower LAI value (1.83) compared to the barley–pea and triticale–pea intercrops (3.93 and 3.82, respectively) (

Table 3. Leaf Area Index for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) across five cropping systems (monoculture of barley, triticale, and pea, as well as intercropping of barley–pea and triticale–pea) at three growth stages (jointing, full bloom, and grain filling).

Year	Treatments	Growth Stage
		Jointing	Full Bloom	Grain Filling
2020–2021	Barley	0.43 c †	1.29 c	3.26 d
	Triticale	0.70 bc	2.21 b	3.96 c
	Pea	1.42 ab	2.39 b	4.63 ab
	Barley–Pea	0.99 bc	2.63 ab	4.21 bc
	Triticale–Pea	1.72 a	3.03 a	4.86 a
2021–2022	Barley	1.28 a	3.13 ab	1.71 a
	Triticale	1.00 a	1.83 b	1.79 a
	Pea	1.58 a	3.33 ab	1.81 a
	Barley–Pea	1.73 a	3.93 a	2.49 a
	Triticale–Pea	1.46 a	3.82 a	2.66 a
2022–2023	Barley	1.12 a	2.08 a	1.21 a
	Triticale	1.01 a	1.95 a	0.97 a
	Pea	1.25 a	2.36 a	1.20 a
	Barley–Pea	1.49 a	3.14 a	1.11 a
	Triticale–Pea	1.46 a	3.41 a	0.88 a
3-year mean	Barley	0.94 b	2.17 b	2.06 c
	Triticale	0.90 b	2.00 b	2.24 bc
	Pea	1.42 a	2.69 ab	2.55 ab
	Barley–Pea	1.40 a	3.23 a	2.60 ab
	Triticale–Pea	1.54 a	3.42 a	2.80 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

). Therefore, intercropping consistently resulted in higher LAI values, particularly for the triticale–pea system.

3.2. Plant Height

Plant height was also affected by the year, cropping systems, and growth stage (Table S1). For the three years, both barley and triticale plant heights were not affected by the intercrop in the jointing stage (

Table 4. Plant height for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) across five cropping systems (monoculture of barley, triticale, and pea, as well as intercropping of barley–pea and triticale–pea) at three growth stages (jointing, full bloom, and grain filling).

Year	Treatments	Growth Stages
		Jointing		Full Bloom		Grain Filling
		Cereal	Pea	Cereal	Pea	Cereal	Pea
2020–2021	Barley	57.3 a †	-	75.0 a	-	88.7 a	-
	Barley–Pea	41.0 a	52.0 b	59.7 a	70.7 b	72.3 a	83.3 c
	Triticale	67.0 a	-	79.7 a	-	85.0 a	-
	Triticale–Pea	67.7 a	87.3 a	78.0 a	103.3 a	93.0 a	102.0 b
	Pea	-	90.3 a	-	108.7 a	-	122.7 a
2021–2022	Barley	39.0 a	-	87.9 a	-	80.0 a	-
	Barley–Pea	37.9 a	46.2 a	73.4 a	104.4 a	78.2 a	116.4 a
	Triticale	53.1 a	-	87.1 b	-	91.8 a	-
	Triticale–Pea	58.2 a	47.9 a	100.2 a	105.8 a	108.7 a	123.6 a
	Pea	-	38.8 b	-	86.4 b	-	135.4 a
2022–2023	Barley	35.8 a	-	79.6 a	-	71.7 a	-
	Barley–Pea	39.9 a	40.9 a	78.7 a	152.6 a	73.7 a	120.5 a
	Triticale	48.2 a	-	99.9 a	-	86.7 b	-
	Triticale–Pea	49.5 a	41.0 a	102.5 a	144.6 a	104.8 a	114.3 a
	Pea	-	42.1 a	-	151.6 a	-	103.7 b
3-year mean	Barley	44.1 a	-	80.8 a	-	80.1 a	-
	Barley–Pea	39.6 a	46.9 b	70.6 b	109.2 a	74.7 a	106.7 b
	Triticale	58.5 a	-	93.5 a	-	102.2 a	-
	Triticale–Pea	56.1 a	58.7 a	88.9 a	117.9 a	87.8 b	113.3 ab
	Pea	-	57.1 a	-	115.4 a	-	120.6 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

). Therefore, barley plant height at jointing ranged from 35.8 cm in 2022–2023 to 57.3 cm in 2020–2021, while Triticale plant height at jointing growth stage ranged from 48.17 cm to 67.7 cm. At jointing growth stage, pea was negatively affected by the barley in the year 2020–2021 with a plant height of 52.0 cm, significantly shorter than the two other treatments, while in the second year, 2021–2022, pea was favored in both intercropping systems compared to the monoculture. At the full bloom stage, only during 2021–2022 was triticale taller in its intercrop with pea (100.2 cm) compared with its monoculture (87.1 cm). Pea plant height was higher in the year 2022–2023 compared with the other two years, probably because of the warmer temperatures and higher rainfall in March and April. At the grain filling stage only triticale height was affected positively by its intercrop with pea in the year 2022–2023. Pea in its intercrops was shorter (83.3 cm in barley–pea and 102.0 cm in triticale–pea) compared to its monoculture (122.7 cm) in the growing season 2020–2021. Lastly, in the growing season 2022–2023, pea monoculture obtained a lower value in plant height (103.7 cm) compared to its intercrops with barley (120.5 cm) and triticale (114.3 cm) (Table 4).

3.3. Chlorophyl Content

Chlorophyl content measured with SPAD was affected by the year, growing season, cropping systems, and crop species (Table S1). In full bloom growth stage during the year 2020–2021, SPAD for barley ranged from 42.7 to 44.9 in its intercrop and monocrop, respectively (

Table 5. Chlorophyl content (SPAD) for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the five cropping systems (monoculture of barley, triticale, and pea, and the intercropping of barley–pea and triticale–pea) and at two growth stages (full bloom and grain filling).

Year	Treatments	Growth Stage
		Full Bloom		Grain Filling
		Cereal	Pea	Cereal	Pea
2020–2021	Barley	44.9 a †	-	49.8 a	-
	Barley–Pea	42.7 a	45.3 b	42.2 a	48.1 b
	Triticale	46.3 a	-	50.8 a	-
	Triticale–Pea	58.6 a	49.1 b	53.3 a	54.2 ab
	Pea	-	59.5 a	-	56.2 a
2021–2022	Barley	31.2 b	-	31.9 a	-
	Barley–Pea	35.8 a	43.8 a	31.3 a	38.5 b
	Triticale	44.8 a	-	53.1 a	-
	Triticale–Pea	48.4 a	47.1 a	50.9 a	45.8 a
	Pea	-	44.8 a	-	39.2 ab
2022–2023	Barley	36.9 a	-	36.4 a	-
	Barley–Pea	35.7 a	36.8 a	40.7 a	39.8 a
	Triticale	34.1 b	-	41.4 b
	Triticale–Pea	51.2 a	34.8 b	50.1 a	40.9 a
	Pea	-	34.2 b	-	42.6 a
3-year mean	Barley	37.7 a	-	39.4 a	-
	Barley–Pea	38.1 a	42.1 a	38.1 a	42.2 b
	Triticale	41.8 b	-	49.3 a	-
	Triticale–Pea	52.7 a	43.35 a	50.6 a	47.0 a
	Pea	-	46.2 a	-	46.0 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

). In the year 2021–2022 it ranged from 31.3 to 35.9 (monocrop and intercrop, respectively) with statistically significant differences, while in the year 2022–2023 there were not any significant differences and values ranged from 35.7 to 36.9 for intercrop and monocrop, respectively. For triticale, only in the year 2022–2023 were there significant differences with higher values in its intercrop with pea compared to its monoculture. Pea SPAD values were lower in its intercrop in the first growing season, while in the third growing season the barley–pea treatment values were higher than the other two treatments. At the grain filling stage, there were no significant differences for both cereals between the two cropping systems. In the third growing season, triticale monoculture recorded a lower value of 41.4 compared to its intercrop’s 50.1. For the pea in the first two years, barley–pea intercrop recorded the lowest SPAD values of 48.1 and 38.5 for 2020–2021 and 2021–2022, respectively. In the third growing season, there were not any significant differences between the treatments (Table 5).

3.4. Yield Components

The yield components were also affected by the year and cropping systems (Table S2). In the year 2020–2021, both cereals recorded a lower value of spikes per plant of 4.7 and 4.3 for barley and triticale intercrops, respectively, compared to the respective monocultures, 8.7 and 9.0, for barley and triticale, respectively (

Table 6. Yield components of the crops for cereals, the number of spikes per plant, and the number of seeds per spike for pea; the number of pods per plant and the number of seeds per pod for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the five cropping systems (monoculture of barley, triticale, and pea, and the intercropping of barley–pea and triticale–pea).

Year	Treatments	Spikes per Plant	Seeds per Spike	Pods per Plant	Seeds per Pod
2020–2021	Barley	8.7 a †	39.0 a	-	-
	Barley–Pea	4.7 b	23.0 b	8.7 b	3.3 b
	Triticale	9.0 a	42.7 a	-	-
	Triticale–Pea	4.3 b	37.3 a	8.0 b	3.7 b
	Pea	-	-	13.7 a	5.3 a
2021–2022	Barley	11.1 a	16.6 a	-	-
	Barley–Pea	8.8 a	18.9 a	9.8 b	4.3 a
	Triticale	8.7 a	38.5 a	-	-
	Triticale–Pea	2.7 b	25.8 a	16.7 a	4.9 a
	Pea	-	-	7.8 b	4.2 a
2022–2023	Barley	8.3 a	20.2 a	-	-
	Barley–Pea	7.1 a	20.3 a	8.4 a	5.5 a
	Triticale	5.4 a	32.1 a	-	-
	Triticale–Pea	4.1 a	34.0 a	6.8 a	4.2 a
	Pea	-	-	7.8 a	4.5 a
3-year mean	Barley	9.4 a	25.3 a	-	-
	Barley–Pea	6.8 b	20.8 a	9.0 b	4.4 a
	Triticale	7.7 a	37.8 a	-	-
	Triticale–Pea	3.7 a	32.4 a	10.5 a	4.3 a
	Pea	-	-	9.8 ab	4.7 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

). In the second year, 2021–2022, only triticale was negatively affected by its intercrop with pea compared with its monoculture, while in the third year, there were not any significant differences. Regarding seed per spike, only in the first year were there significant differences between barley monoculture with 39.0 seeds per spike and its intercrop barley–pea with 23.0 seeds per spike. For the other two years, there were not any differences and the number of seeds per spike ranged from 16.6 to 18.9 for barley and 25.8 to 38.5 for triticale in 2021–2022, and 20.2 to 20.3 for barley and 32.1 to 34.0 for triticale in 2022–2023. In 2020–2021, pea monoculture obtained more pods per plant with 13.7 compared with its intercrops with triticale and barley (8.0 and 8.7, respectively). In the second growing season, pea recorded the highest number of pods per plant in the intercrop with triticale with 16.7 pods per plant, while in the third growing season the differences were minimized and the number of pods per plant ranged from 6.8 in intercrop with triticale to 8.4 pods per plant in intercrop with barley. Similarly to pods per plant, in the first year, pea monoculture obtained the highest number of seeds per pod with 5.3. For the second year, seeds per pod ranged from 4.2 to 4.9 and in the third year, seeds per pod ranged from 4.2 to 5.5, without any significant differences among the treatments (Table 6).

3.5. Dry Biomass and Seed Yield

Dry biomass was also affected by the years and also by the cropping systems (Table S2). For the three years, both cereals’ dry biomasses were affected by the presence of pea in the intercrop. In the first year, barley and triticale had similar dry biomasses (7.27 tn ha⁻¹ and 7.37 tn ha⁻¹, respectively) both in monoculture and in the intercrops (3.69 tn ha⁻¹ and 3.41 tn ha⁻¹, respectively) (

Table 7. Dry biomass and seed yield for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the five cropping systems (monoculture of barley, triticale, and pea, and the intercropping of barley–pea and triticale–pea).

Year	Treatments	Dry Biomass (tn ha−1)			Seed Yield (tn ha−1)
		Cereal	Pea	Total	Cereal	Pea	Total
2020–2021	Barley	7.27 a †	-	7.27 b	5.74 a	-	5.74 a
	Barley–Pea	3.69 b	7.72 a	11.27 a	1.62 c	2.19 a	3.81 b
	Triticale	7.37 a	-	7.37 b	3.54 b	-	3.54 b
	Triticale–Pea	3.41 b	7.65 a	11.07 a	1.31 c	2.59 a	3.90 b
	Pea	-	6.13 a	6.13 b	-	1.95 a	1.95 c
2021–2022	Barley	9.70 ab	-	9.70 ab	1.74 a	-	1.74 ab
	Barley–Pea	0.78 c	2.77 a	3.55 b	0.19 b	0.21 b	0.40 c
	Triticale	12.96 a	-	12.96 a	2.09 a	-	2.09 a
	Triticale–Pea	2.18 bc	2.78 a	4.96 ab	0.26 b	0.54 a	0.79 bc
	Pea	-	2.22 a	2.22 b	-	0.26 b	0.26 c
2022–2023	Barley	8.40 a	-	8.40 a	3.04 a	-	3.04 a
	Barley–Pea	4.70 ab	6.60 a	11.30 a	0.79 b	0.59 a	1.38 b
	Triticale	7.40 a	-	7.40 a	1.56 b	-	1.56 b
	Triticale–Pea	2.48 b	9.25 a	11.72 a	0.44 b	0.70 a	1.14 b
	Pea	-	9.13 a	9.13 a	-	0.68 a	0.68 b
3-year mean	Barley	8.45 a	-	8.45 ab	3.50 a	-	3.50 a
	Barley–Pea	3.05 b	5.69 a	8.74 ab	0.86 c	0.99 a	1.85 b
	Triticale	9.24 a	-	9.24 a	2.39 b	-	2.39 b
	Triticale–Pea	2.68 b	6.56 a	9.24 a	0.66 c	1.27 a	1.93 b
	Pea	-	5.82 a	5.82 b	-	0.96 a	0.96 c

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

). In the second year, triticale monoculture had the highest dry biomass yield with 12.96 tn ha⁻¹, followed by barley monoculture with 9.70 tn ha⁻¹. The lowest cereal dry biomass yield was observed in the barley–pea intercrop with 0.78 tn ha⁻¹. In 2022–2023, cereal dry yield was the lowest in the triticale–pea intercrop with 2.48 tn ha⁻¹. In terms of pea dry biomass yield, for the three growing seasons, there were not any significant differences between the different treatments within each year. The lowest values for pea were observed in the second growing season and ranged from 2.22 tn ha⁻¹ to 2.78 tn ha⁻¹, while the highest were recorded in the triticale–pea intercrop and pea monoculture in the third year. In terms of total dry biomass, in the first growing season both barley–pea and triticale–pea intercrops recorded a higher total dry biomass than their respective cereal and pea monocultures. The highest dry biomass was recorded for barley–pea intercrop with 11.27 tn ha⁻¹, followed by triticale–pea intercrop with 11.07 tn ha⁻¹, while the lowest dry biomass was recorded for pea monoculture with 6.13 tn ha⁻¹. The second-year triticale monoculture obtained the highest dry biomass with 12.96 tn ha⁻¹, followed by the barley monoculture with 9.70 tn ha⁻¹. The lowest dry yield was obtained in 2021–2022 for the pea monoculture, with 2.22 tn ha⁻¹. In the third growing season, there were not any significant differences between the treatments (Table 7).

In the experiment’s first two years, cereal monocultures recorded a higher value of seed yield than their respective intercrops. In 2020–2021, barley monoculture obtained the highest cereal seed yield with 5.74 tn ha⁻¹, followed by triticale monoculture with 3.54 tn ha⁻¹. In the second growing season, triticale monoculture obtained the highest cereal yield with 2.09 tn ha⁻¹, while the lowest was in the barley–pea intercrop with 0.19 tn ha⁻¹. In 2022–2023, the barley monocrop obtained the highest values of cereal seed yield with 3.04 tn ha⁻¹. The second highest seed yield was observed in the triticale sole crop with 1.56 tn ha⁻¹, but it did not differ significantly with the barley–pea and triticale–pea intercrops. In the first and third growing seasons, pea seed yield did not differ significantly between the intercrops and sole crops. In 2021–2022, the highest pea seed yield (0.54 tn ha⁻¹) was recorded in the triticale–pea intercrop, while pea monoculture seed yield was severely impaired and recorded a value of 0.26 tn ha⁻¹. In terms of total seed yield, barley monoculture recorded the highest values compared to the other treatments in 2020–2021 and 2022–2023, while in 2021–2022, triticale sole crop obtained the highest seed yield. In 2020–2021, both intercrops obtained similar seed yields, with 3.81 tn ha⁻¹ and 3.90 tn ha⁻¹ for barley–pea and triticale–pea, respectively. In 2021–2022, the intercrops’ seed yields were lower than 2020–2021 and recorded values of 0.40 tn ha⁻¹ and 0.79 tn ha⁻¹ for barley–pea and triticale–pea, respectively. In 2022–2023, intercrop seed yield ranged from 1.14 tn ha⁻¹ to 1.38 tn ha⁻¹ for triticale–pea and barley–pea, respectively (Table 7).

3.6. Land Equivalent Ratio (LER)

In 2020–2021, cereal obtained similar biomass pLER_c values of 0.52 and 0.49 for barley–pea and triticale–pea, respectively. In 2022–2023, barley–pea obtained a pLER_c value of 0.62, higher than the 0.34 of triticale–pea. In 2021–2022, pLER_c values for both cereals were low and ranged from 0.07 to 0.24 for barley–pea and triticale–pea, respectively. For all the years, pLER_p values were above 1, with the exception of the barley–pea intercrop in 2022–2023. The highest value of pLER_p was recorded in 2021–2022 in the triticale–pea intercrop, while the lowest was found in 2022–2023 in the barley–pea intercrop. For all the treatments in all years, biomass LER was higher than 1, indicating an intercropping advantage. This was due to the higher pLER_p values indicating that pea produced higher biomass yield in intercrop compared to its respective monoculture. Total LER values ranged from 1.75 to 1.80 in 2020–2021, 1.29 to 1.71 in 2021–2022, and 1.32 to 1.35 in 2022–2023, which were higher than 1 (

Table 8. Partial LER and total LER for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the two cropping systems (intercropping of barley–pea and triticale–pea).

LER
Year	Treatments	LER Based on Biomass			LER Based on Seed Yield
		Cereal	Pea	Total	Cereal	Pea	Total
2020–2021	Barley–Pea	0.52 a †	1.28 a	1.80 a	0.28 a	1.17 a	1.45 a
	Triticale–Pea	0.49 a	1.27 a	1.75 a	0.37 a	1.32 a	1.69 a
2021–2022	Barley–Pea	0.07 a	1.21 a	1.29 a	0.12 a	0.81 a	0.93 a
	Triticale–Pea	0.24 a	1.47 a	1.71 a	0.19 a	2.09 a	2.28 a
2022–2023	Barley–Pea	0.62 a	0.71 a	1.32 a	0.28 a	0.86 a	1.14 a
	Triticale–Pea	0.34 a	1.00 a	1.35 a	0.47 a	0.99 a	1.46 a
3-year mean	Barley–Pea	0.40 a	1.06 a	1.46 a	0.23 a	0.94 b	1.17 b
	Triticale–Pea	0.35 a	1.24 a	1.59 a	0.34 a	1.47 a	1.81 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S4).

In all years, seed pLER_c was below 0.5 and lower than that of pLER_p, indicating that pea outcompeted both cereals in the intercrop (Table 8, Figure 2). Pea pLER values in 2020–2021 and 2021–2022 were above 1 with the exception of barley–pea in 2021–2022. Additionally, in 2021–2022 pea monoculture yield was extremely low and therefore triticale–pea obtained a value of 2.09 (Table 8, Figure 2). Seed LER was above 1 in all years, apart from the Barley–Pea intercrop in 2021–2022, indicating that intercrops outperformed the respective monocultures. The high LER values were due to the high pLER_p values. Total LER values were 1.45 and 1.69, 0.93 and 2.28, and 1.14 and 1.46 for barley–Pea and triticale–pea intercrops in 2020–2021, 2021–2022, and 2022–2023, respectively, which were higher than 1 in most intercropping treatments (Table 8 and Table S5).

3.7. Water-Use Efficiency (WUE)

Cereal WUE based on biomass was higher in monocultures compared to the intercrops for all the years of the experiment. Furthermore, intercrop treatments did not differ significantly with each other. The highest WUE was found in the triticale sole crop in 2021–2022 with 47.73 kg ha⁻¹ mm⁻¹, and the lowest in the barley–pea intercrop in 2021–2022 with 2.88 kg ha⁻¹ mm⁻¹. The treatments for all the years did not differ significantly for pea WUE. The highest value was observed in 2020–2021 in the barley–pea intercrop with 42.4 kg ha⁻¹ mm⁻¹ and the lowest in 2021–2022 for the pea monoculture with 8.2 kg ha⁻¹ mm⁻¹. In 2020–2021, total WUE of intercrops did not differ significantly with each other and obtained values almost twice the values of the respective sole crops. In 2021–2022, the triticale–pea intercrop obtained a value of 18.3 kg ha⁻¹ mm⁻¹, which is between those of the cereal sole crops and pea monoculture, while barley–pea crops obtained a lower value of 13.1 kg ha⁻¹ mm⁻¹ and did not differ significantly with the pea sole crop. In 2022–2023, barley–pea and triticale–pea recorded values of 31.6 kg ha⁻¹ mm⁻¹ and 32.8 kg ha⁻¹ mm⁻¹, respectively, which is higher than the respective sole crops, but without statistically significant differences (

Table 9. Water-Use Efficiency (WUE) for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the five cropping systems (monoculture of barley, triticale, and pea, and the intercropping of barley–pea and triticale–pea).

Year	Treatments	Water-Use Efficiency Based on Biomass (kg ha−1 mm−1)			Water-Use Efficiency Based on Seed Yield (kg ha−1 mm−1)
		Cereal	Pea	Total	Cereal	Pea	Total
2020–2021	Barley	27.3 a †	-	27.3 b	21.6 a	-	21.6 a
	Barley–Pea	13.9 b	29.0 a	42.4 a	6.1 c	8.2 a	14.3 b
	Triticale	27.7 a	-	27.7 b	13.3 b	-	13.3 b
	Triticale–Pea	12.8 b	28.8 a	41.6 a	4.9 c	9.7 a	14.7 b
	Pea	-	23.1 a	23.1 b	-	7.3 a	7.3 c
2021–2022	Barley	35.7 ab	-	35.7 ab	6.4 a	-	6.4 ab
	Barley–Pea	2.9 c	10.2 a	13.1 b	0.7 b	0.8 b	1.5 c
	Triticale	47.7 a	-	47.7 a	7.7 a	-	7.7 a
	Triticale–Pea	8.0 bc	10.2 a	18.3 ab	0.9 b	1.9 a	2.9 bc
	Pea	-	8.2 a	8.2 b	-	0.9 b	0.9 c
2022–2023	Barley	23.5 a	-	23.5 a	8.5 a	-	8.5 a
	Barley–Pea	13.2 ab	18.5 a	31.6 a	2.2 b	1.6 a	3.9 b
	Triticale	20.7 a	-	20.7 a	4.4 b	-	4.4 b
	Triticale–Pea	6.9 b	25.9 a	32.8 a	1.2 b	1.9 a	3.2 b
	Pea	-	25.5 a	25.6 a	-	1.9 a	1.9 b
3-year mean	Barley	28.8 a	-	28.8 ab	12.1 a	-	12.1 a
	Barley–Pea	9.9 b	19.2 a	29.1 ab	3.0 c	3.5 a	6.5 b
	Triticale	32.0 a	-	32.0 a	8.4 b	-	8.4 b
	Triticale–Pea	9.2 b	21.6 a	30.8 a	2.3 c	4.5 a	6.8 b
	Pea	-	18.9 a	18.9 b	-	3.3 a	3.3 c

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S3).

In 2020–2021, barley monoculture obtained the highest cereal grain WUE of 21.6 kg ha⁻¹ mm⁻¹, followed by triticale monocrop, and then the respective cereal–pea intercrops. In 2021–2022, cereal species recorded similar values of cereal WUE and were higher than the respective intercrops. In 2022–2023, barley sole crop obtained the highest values of 8.5 kg ha⁻¹ mm⁻¹, while triticale monoculture and the intercrops did not differ with each other. In 2020–2021 and 2022–2023, there were not any significant differences between the treatments for pea WUE. In 2021–2022, triticale–pea intercrop recorded a value of 1.9 kg ha⁻¹ mm⁻¹, which is higher than the pea monoculture and barley–pea intercrop with values of 0.9 kg ha⁻¹ mm⁻¹ and 0.8 kg ha⁻¹ mm⁻¹, respectively. In 2020–2021 and 2022–2023, the barley monoculture obtained higher WUE compared to the other treatments, which did not differ significantly between each other, with the exception of the pea monoculture in 2020–2021 that recorded the lowest WUE at 7.3 kg ha⁻¹ mm⁻¹. In 2021–2022, triticale monoculture obtained the highest WUE, followed by barley monoculture. For all the years, pea monoculture recorded the lowest values of grain WUE, while intercrops were intermediate between that of cereal sole crops and pea monoculture. In 2020–2021, WUE ranged from 7.3 kg ha⁻¹ mm⁻¹ to 21.6 kg ha⁻¹ mm⁻¹, in 2021–2022 WUE ranged from 0.9 kg ha⁻¹ mm⁻¹ to 7.7 kg ha⁻¹ mm⁻¹, and in 2022–2023 WUE ranged from 1.9 kg ha⁻¹ mm⁻¹ to 8.5 kg ha⁻¹ mm⁻¹ (Table 9 and Table S3).

3.8. Radiation-Use Efficiency (RUE)

The Radiation-Use Efficiency (RUE) for biomass and seed yield across three growing seasons (2020–2023) for five cropping systems: monocultures of barley, triticale, and pea, and intercropping systems of barley–pea and triticale–pea were determined (

Table 10. Radiation-Use Efficiency (RUE) for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the five cropping systems (monoculture of barley, triticale, and pea, and the intercropping of barley–pea and triticale–pea).

Year	Treatments	Radiation-Use Efficiency Based on Biomass (g MJ−1)			Radiation-Use Efficiency Based on Seed Yield (g MJ−1)
		Cereal	Pea	Total	Cereal	Pea	Total
2020–2021	Barley	2.00 a †	-	2.00 a	1.58 a	-	1.58 a
	Barley–Pea	1.01 b	2.12 a	3.10 b	0.44 b	0.60 a	1.05 a
	Triticale	2.02 a	-	2.02 a	0.97 b	-	0.97 a
	Triticale–Pea	0.94 b	2.10 a	3.04 b	0.36 b	0.71 a	1.07 a
	Pea	-	1.68 a	1.68 a	-	0.54 a	0.54 b
2021–2022	Barley	2.88 a	-	2.88 a	0.52 a	-	0.52 a
	Barley–Pea	0.23 b	0.82 a	1.05 b	0.02 b	0.05 a	0.08 b
	Triticale	3.84 a	-	3.84 a	0.62 a	-	0.62 a
	Triticale–Pea	0.65 b	0.82 a	1.47 b	0.07 b	0.16 a	0.23 b
	Pea	-	0.66 a	0.66 c	-	0.05 a	0.05 b
2022–2023	Barley	2.64 a	-	2.64 a	0.95 a	-	0.95 a
	Barley–Pea	1.48 b	2.07 a	3.55 a	0.25 b	0.19 a	0.43 b
	Triticale	2.32 a	-	2.32 a	0.49 ab	-	0.49 b
	Triticale–Pea	0.78 c	2.90 a	3.68 a	0.14 b	0.22 a	0.36 b
	Pea	-	2.87 a	2.87 a	-	0.21 a	0.21 b
3-year mean	Barley	7.52 a	-	7.52 ab	3.00 a	-	3.00 a
	Barley–Pea	2.87 b	5.09 a	7.96 ab	0.74 c	0.81 a	1.55 b
	Triticale	8.05 a	-	8.05 ab	2.00 b	-	2.00 b
	Triticale–Pea	2.34 b	6.07 a	8.41 a	0.55 c	1.03 a	1.58 b
	Pea	-	5.50 a	5.50 b	-	0.79 a	0.79 c

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S3). Higher values of RUE reflect more efficient conversion of intercepted radiation into either biomass or seed. From the monocultures, triticale consistently had the highest RUE for biomass among cereals, peaking in 2021–2022 (3.84 g MJ⁻¹). Pea monoculture had lower biomass RUE compared to cereals, ranging from 0.66 to 2.87 g MJ⁻¹, with the highest in 2022–2023. Barley monoculture showed moderate and relatively stable RUE values across seasons. Regarding the intercropping, the highest total RUE values were found in barley–pea (3.55) and triticale–pea (3.68), especially during the third growing season of 2022–2023. In all seasons, intercropping improved total biomass RUE compared to monocultures, though cereal RUE values were lower due to resource sharing with peas.

Regarding the RUE based on seed yield, monocultures of barley and triticale had much higher seed RUE than pea in 2020–2021, but the trend flipped in later years. In 2021–2022, grain RUE dropped sharply across all systems, with values <1 g MJ⁻¹, likely due to unfavorable climatic conditions or stress during grain filling. By 2022–2023, seed RUE recovered for most systems, particularly barley (0.95 g MJ⁻¹) and pea (0.21 g MJ⁻¹). In the case of intercropping, seed RUE for intercropping was generally lower than the sum of those of the monocultures, suggesting competition effects during reproductive stages. However, in 2020–2021, intercropping grain RUE (1.05 g MJ⁻¹ and 1.07 g MJ⁻¹) slightly outperformed triticale monocultures, showing potential benefits under favorable conditions. In 2021–2022, seed RUE in intercropping dropped dramatically, highlighting the potential vulnerability of mixed systems to stress.

3.9. Actual Yield Loss (AYL)

In 2020–2021 and 2022–2023, both intercrops acquired an AYL cereal value of above 0, indicating a yield advantage for cereals in the intercrop. In 2021–2022, the AYL cereal value was below 0 and therefore the cereals were disadvantageous compared to the monoculture. The AYL of pea was above 0 for all the years except for the barley–pea intercrop in 2022–2023. Peas were favored in the intercrop, probably due to the presence of cereals which prevented them from lodging. Total AYL was above 0 in all the treatments, with the exception of the barley–pea intercrop in 2021–2022. According to AYL, there was a significant yield advantage for biomass in the intercrop compared to the sole crops

Table 11. Actual Yield Loss and Relative Crowding Coefficient based on biomass for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the two cropping systems (intercropping of barley–pea and triticale–pea).

Year	Treatments	Actual Yield Loss Based on Biomass			Relative Crowding Coefficient Based on Biomass
		AYL Cereal	AYL Pea	AYL Total	K Cereal	K Pea	K Total
2020–2021	Barley–Pea	1.07 a †	0.71 a	1.78 a	4.41 a	−2.31 a	−10.42 a
	Triticale–Pea	0.94 a	0.69 a	1.63 a	3.32 a	−2.85 a	−13.72 a
2021–2022	Barley–Pea	−0.70 a	0.62 a	−0.09 a	0.25 a	7.00 a	0.12 a
	Triticale–Pea	−0.04 a	0.96 a	0.91 a	1.46 a	−4.30 a	−1.70 a
2022–2023	Barley–Pea	2.84 a	−0.06 a	2.78 a	2.31 a	−3.34 a	0.001 a
	Triticale–Pea	0.37 a	0.34 a	0.71 a	2.26 a	1.32 a	4.48 a
3-year mean	Barley–Pea	1.07 a	0.42 a	1.49 a	2.32 a	0.45 a	−3.43 a
	Triticale–Pea	0.42 a	0.66 a	1.08 a	2.34 a	−1.94 a	−3.65 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S4). K cereal was above 1 and higher than that of K pea in most cases, which indicated that cereals were more competitive in the mixture than the peas. Only in 2022–2023 was the triticale–pea intercrop K above 1, which means that there was a yield advantage in the intercrop compared to the sole crops.

3.10. Land Equivalent Coefficient, System Productivity Index, and Percentage Yield Difference Based on Biomass Yield

For all years and treatments, with the exception of the barley–pea intercrop in 2021–2022, biomass LEC was above the threshold value of 0.25. This indicates that the mixtures were balanced in terms of biomass yield. The highest SPI values both for barley–pea intercropping and triticale–pea intercropping were recorded in year 2021–2022. In 2020–2021 and 2022–2023, triticale–pea obtained similar SPI values as barley–pea, but in 2021–2022, triticale–pea intercrop recorded higher values than barley–pea. PYD for both intercrops were higher in the first year of the experiment. In 2020–2021 and 2022–2023, barley–pea intercrop recorded a higher value of PYD than triticale–pea intercrop, while the opposite occurred in 2021–2022

Table 12. Land Equivalent Coefficient, System Productivity Index, and Percentage Yield Difference based on biomass for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the two cropping systems (intercropping of barley–pea and triticale–pea).

Year	Treatments	LEC Based on Biomass	SPI Based on Biomass	PYD (%) Based on Biomass
2020–2021	Barley–Pea	0.65 a †	1276.4 a	79.8 a
	Triticale–Pea	0.59 a	1293.8 a	75.1 a
2021–2022	Barley–Pea	0.09 a	1447.1 a	28.8 a
	Triticale–Pea	0.50 a	2035.2 a	70.6 a
2022–2023	Barley–Pea	0.43 a	1083.9 a	66.4 a
	Triticale–Pea	0.37 a	1001.6 a	34.9 a
3-year mean	Barley–Pea	0.38 a	1269.1 a	58.3 a
	Triticale–Pea	0.48 a	1443.5 a	60.2 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S4).

3.11. Competitive Ratio and Aggressivity Based on Biomass Yield

In 2020–2021 and 2021–2022, CR of pea was higher than that of the cereals, which indicates that pea was more competitive in the mixture. The exception was found only in 2022–2023, where in triticale–pea intercrop, triticale obtained a higher value of CR than pea. The aggressivity of peas were negative in 2020–2021 and 2022–2023 for both intercrops, which means that peas were outcompeted in the mixture and were less competitive. On the other hand, in 2021–2022, in both intercrops peas outcompeted the cereals and obtained positive values of aggressivity (

Table 13. Competitive Ratio and Aggressivity based on biomass for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the two cropping systems (intercropping of barley–pea and triticale–pea).

Year	Treatments	CR Based on Biomass		A Based on Biomass
		Cereal	Pea	Cereal	Pea
2020–2021	Barley–Pea	1.27 a †	2.23 a	0.36 a	−0.36 a
	Triticale–Pea	1.22 a	2.30 a	0.25 a	−0.25 a
2021–2022	Barley–Pea	0.21 a	15.02 a	−1.32 a	1.32 a
	Triticale–Pea	0.51 a	6.46 a	−1.00 a	1.00 a
2022–2023	Barley–Pea	10.33 a	3.07 a	2.90 a	−2.90 a
	Triticale–Pea	1.01 a	4.35 a	0.02 a	−0.02 a
3-year mean	Barley–Pea	3.94 a	6.77 a	0.64 a	−0.64 a
	Triticale–Pea	0.92 a	4.37 a	−0.24 a	0.24 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S4).

3.12. Actual Yield Loss and Relative Crowding Coefficient Based on Seed Yield

In 2020–2021 and 2022–2023, grain AYL of cereal was above 0 which means that there was a yield advantage for the cereals in the intercrop, while in 2021–2022 both intercrops recorded negative values of AYL. Regarding pea AYL for the three years of the experiments, pea AYL was positive, indicating a yield advantage of intercrop over monoculture. Total AYL was positive for both intercrops in all years, except for the barley–pea intercrop in 2021–2022. Seed K values of cereals were above 1 in both intercrops in 2020–2021, while in 2021–2022 they were below 1. In 2022–2023, barley was more competitive than pea with a value of 1.33. Only in the triticale–pea intercrop in 2022–2023 was the K value of pea above 1 and higher than that of cereal. In terms of total K, triticale–pea intercrop was more advantageous than barley–pea, and obtained a higher yield advantage compared to monoculture, since in 2020–2021 and 2022–2023 it obtained values above 1 (

Table 14. Actual Yield Loss and Relative Crowding Coefficient based on seed yield for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the two cropping systems (intercropping of barley–pea and triticale–pea).

Year	Treatments	Actual Yield Loss Based on Seed Yield			Relative Crowding Coefficient Based on Seed Yield
		AYL Cereal	AYL Pea	AYL Total	K Cereal	K Pea	K Total
2020–2021	Barley–Pea	0.13 a †	0.56 a	0.69 a	1.22 a	0.04 a	0.07 a
	Triticale–Pea	0.49 a	0.76 a	1.25 a	2.15 a	−0.24 a	1.81 a
2021–2022	Barley–Pea	−0.52 a	0.08 b	−0.45 b	0.41 a	−0.15 a	0.09 a
	Triticale–Pea	−0.25 a	1.79 a	1.53 a	0.99 a	−1.64 a	−1.07 a
2022–2023	Barley–Pea	0.11 a	0.14 a	0.25 a	1.33 a	0.41 a	0.80 a
	Triticale–Pea	0.88 a	0.32 a	1.20 a	−3.13 a	3.02 a	6.24 a
3-year mean	Barley–Pea	−0.10 a	0.26 b	0.16 a	0.99 a	0.10 a	0.32 a
	Triticale–Pea	0.37 a	0.95 a	1.33 a	0.01 a	0.38 a	2.32 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S5).

3.13. Land Equivalent Coefficient, System Productivity Index, and Percentage Yield Difference Based on Seed Yield

Seed LEC of triticale–pea intercrop was above the threshold value of 0.25 for the three years, indicating that it was a more balanced mixture than the barley–pea intercrop. Seed SPI of barley–pea was higher than that of the triticale–pea intercrop and therefore was a more productive mixture than the triticale–pea intercrop (

Table 15. Land Equivalent Coefficient, System Productivity Index, and Percentage Yield Difference for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the two cropping systems (intercropping of barley–pea and triticale–pea).

Year	Treatments	LEC Based on Seed Yield	SPI Based on Seed Yield	PYD (%) Based on Seed Yield
2020–2021	Barley–Pea	0.32 a †	804.7 a	44.9 a
	Triticale–Pea	0.45 a	593.9 b	68.9 a
2021–2022	Barley–Pea	0.09 a	164.0 a	−7.4 b
	Triticale–Pea	0.37 a	512.2 a	127.6 a
2022–2023	Barley–Pea	0.22 a	349.6 a	13.5 a
	Triticale–Pea	0.64 a	172.6 b	45.9 a
3-year mean	Barley–Pea	0.21 a	439.5 a	16.9 a
	Triticale–Pea	0.49 a	426.3 a	80.8 a

^† When each year’s mean values are followed by the same letter(s), thy do not differ statistically significantly according to the protected LSD criterion. LSD_0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S5).

During the 2020–2021 growing season, both systems showed similar cereal CR values (barley: 0.83, triticale: 0.98), indicating comparable competitiveness. In the following year of 2021–2022, CR was 0.14 for barley and 0.31 for triticale in their intercrops, suggesting severe suppression by peas. However, during the third year, 2022–2023, CR rebounded sharply (barley: 1.10, triticale: 1.22), with cereals becoming more competitive than peas. Peas showed a different trend as CR values were consistently higher than cereals, peaking in 2021–2022 (barley–pea: 7.76, triticale–pea: 9.11), indicating pea dominance under that season’s conditions. Barley was consistently non-aggressive (negative A values) except in 2022–2023 (−0.04), while triticale shifted from mildly aggressive (0.27 in 2020–2021) to highly suppressed (−2.04 in 2021–2022) before rebounding (0.56 in 2022–2023). Pea dominated in most seasons (positive A values), especially in 2021–2022 (A = 2.04 in triticale–pea). Notably, peas in triticale–pea systems turned non-aggressive in 2022–2023 (A = −0.56), reflecting triticale’s resurgence (

Table 16. Competitive Ratio and Aggressivity for the three growing seasons (2020–2021, 2021–2022, and 2022–2023) of the two cropping systems (intercropping of barley–pea and triticale–pea).

Year	Treatments	CR Based on Seed Yield		A based on Seed Yield
		Cereal	Pea	Cereal	Pea
2020–2021	Barley–Pea	0.83 a †	3.54 a	−0.43 a	0.43 a
	Triticale–Pea	0.98 a	2.85 a	0.27 a	0.27 a
2021–2022	Barley–Pea	0.14 a	7.76 a	−0.60 a	0.60 a
	Triticale–Pea	0.31 a	9.11 a	−2.04 a	2.04 a
2022–2023	Barley–Pea	1.10 a	4.68 a	−0.04 a	0.04 a
	Triticale–Pea	1.22 a	4.37 a	0.56 a	−0.56 a
3-year mean	Barley–Pea	0.80 a	5.33 a	−0.35 a	0.35 a
	Triticale–Pea	0.84 a	5.44 a	−0.58 a	0.58 a

^† When each year’s mean values are followed by the same letter(s), they do not differ statistically significantly according to the protected LSD criterion. LSD0.05: common Least Significant Difference, at significance level a = 0.05 (p ≤ 0.05).

and Table S5).

4. Discussion

4.1. Leaf Area Index

The Leaf Area Index (LAI) is an important index which is critical for resource capture and biomass accumulation. Triticale–pea intercropping showed significantly higher LAI values across all years of experimentation, from 1.46 at jointing up to 4.86 at grain filling. These findings imply a denser and more developed canopy structure beneath the olive trees due to the complementary effects between cereal and legume species [41,42]. Similarly, pea monocrop exhibited high LAI values, higher than the other monocrop treatments, probably since a leafy cultivar for biomass production was selected, as well as the moderated influence of microclimatic conditions (e.g., lower evapotranspiration, milder temperature) [43,44]. Thus, higher LAIs during anthesis and grain filling may contribute to sustained photosynthetic activity and improved assimilate partitioning toward reproductive organs both in cereals and legumes [45,46]. In contrast, cereal monocultures (triticale and barley) demonstrated consistently lower LAIs, especially during 2020–2021, reflecting their limited ability to compensate for reduced light or spatial constraints in agroforestry. Also, cereals with erectophile leaf architecture or short stature may be structurally disadvantaged in cereal-based intercrops, especially when they are implemented beneath perennial crops in agroforestry [47,48].

4.2. Plant Height

Plant height, measured during three growing stages (jointing, flowering and grain filling), revealed significant differences across treatments, especially in triticale and pea. Intercropped triticale with pea consistently exhibited increased height compared to their sole crops, up to 22%, beneath the olive tree canopy. This elongation response can be interpreted as a shade avoidance strategy, which is typically affected by the reduced red/far-red light ratio due to partial tree shading [49]. This morphological plasticity in cereals can lead to optimized light interception in shaded environments, also mentioned by Tsonkova et al. [50]. Furthermore, the presence of a nitrogen-fixing species, such as field pea, in the intercropping systems may have reduced below-ground competition for nitrogen, allowing cereals to allocate more resources toward vegetative growth [14,51]. However, pea exhibited variations in plant height across the three growing seasons. More specifically, at the jointing stage, pea exhibited reduced plant height when intercropped with barley compared to triticale in 2020–2021, indicating early-stage suppression, whereas in the subsequent years, intercropping appeared to enhance pea height, particularly under favorable climatic conditions for both intercropped species development [52]. Notably, in 2022–2023, pea height in intercrops was higher than sole crops, likely due to increased rainfall and milder temperatures during spring, which favored vegetative development [25,53]. This suggests that pea is more competitive under intercropping, especially when climate conditions are optimum and plants are subjected to reduced light availability, as observed in agroforestry conditions [54,55]. Also, pea is well adapted to Mediterranean conditions and can utilize the environmental resources more efficiently [53,56].

4.3. Chlorophyll Content

Chlorophyll content (SPAD) was measured at both flowering and grain filling stages, providing insights into crop nitrogen status and photosynthetic potential across treatments. Cereals which were intercropped with pea consistently showed higher SPAD values, up to 49%, than the respective cereal sole crops, with the most pronounced differences observed in triticale species under agroforestry during the third year at full bloom (34.11 in monocrop compared to 51.23 in intercrop). This pattern suggests a delay in leaf senescence, possibly induced by improved nitrogen-use efficiency in intercropping or by the attenuated radiation and moderate thermal stress under the trees’ canopy [57,58]. Trees in agroforestry systems have been shown to enhance nutrient cycling and reduce abiotic stress in annual crops, potentially prolonging the functional lifespan of their leaves. Therefore, higher chlorophyll concentrations with a better overall crop nutritional status is achieved [59]. Also, the presence of pea may also have contributed indirectly to increased nitrogen availability through niche complementarity, due to biological nitrogen fixation effects [13]. However, SPAD values in pea remained relatively stable, with no notable variability, except in the first year when SPAD values in the intercropping treatments were lower than in the monocrop, supporting its limited physiological response to light or interspecific interactions [43,60].

4.4. Seed Yield Components

Cereal seed yield components (spikes per plant and seeds per spike) did not differ significantly except in the first year. Although tillering in cereal plants is favored under lower plant population, the presence of pea probably competed with cereals for space, while shading by olive groves lowered Photosynthetically Active Radiation (PAR) that reached the cereal canopy. Therefore, there were no differences in spikes per plant between intercrop and monoculture [25,53]. Similar results were found in other studies [36]. Similar results were observed for pea seed yield components (pods per plant, seeds per pod) [25,53]. Pea plants were significantly taller than cereal plants in both intercrops and therefore adequate PAR reached the pea canopy, enabling plants in intercrop to reach similar pods per plant as monocultures. Pankou et al. [25] also did not observe differences in pea seed yield components between the two cropping systems using wheat and pea intercrops.

4.5. Dry Biomass and Seed Yield

Dry biomass was significantly lowered for cereals in the intercrops while pea dry biomass was consistent in all cropping systems. Pea probably outcompeted both cereals for resources such as light and soil nitrogen, particularly due to its ability to fixate nitrogen, and obtained consistent biomass yield even at lower plant population density (75%). Total biomass was increased in the intercrops compared to the monocultures. Niche complementarity between cereals and pea and N-fixation allowed the intercrops to acquire more resources. Intercrops produce higher yields under low input conditions, due to complementary and cooperative interactions that occur between the two species [40]. Similar trends were observed for seed yield, where cereals were unfavored in the intercrops, while pea was not affected. Total seed yield was in between cereal and pea plants. Intercropping produces more than the lower yielding species and reaches that of the higher yielding species. Similar results for seed yield were observed in other studies [25,36,53].

4.6. Land Equivalent Ratio (LER)

Several intercropping indices were used to determine the effect of intercropping on the productivity of the systems. LER for biomass was higher than 1 for all the treatments and years, indicating that intercrops produced higher yields per unit area than monocultures. Furthermore, pea was more competitive than cereals in the intercrop and achieved higher pLER. Intercropping can produce significantly higher yield under low input conditions, probably due to higher resource acquisition. Higher resource acquisition is attributed to niche complementarity and the better exploitation of available soil resources [14]. Meta-analyses have shown that cereal–legume mixtures obtain an LER above 1, indicating a significant advantage of the mixtures [18,49,61,62].

4.7. Water-Use Efficiency (WUE)

In all three years, monoculture cereals (barley and triticale) generally showed higher WUE based on biomass compared to their intercropped counterparts. In particular, in 2020–2021, barley monoculture had a biomass WUE of 27.34 kg ha⁻¹ mm⁻¹, while barley–pea intercropping had 13.89 kg ha⁻¹ mm⁻¹ for barley and 29.03 kg ha⁻¹ mm⁻¹ for pea, totaling 42.39 kg ha⁻¹ mm⁻¹, which is higher than barley alone. This pattern suggests that while individual cereal WUE decreases in intercropping, the total system WUE (cereal + pea) often increases, indicating a complementary effect and better overall water use in intercropping systems. Moreover, pea consistently showed high WUE values, especially in biomass, across all years and treatments. In intercropping systems, pea’s WUE compensates for the reduced cereal WUE, resulting in higher total WUE. The WUE values showed a significant variation between years, likely due to environmental factors such as rainfall distribution, temperature, and soil moisture. Grain WUE also fluctuates, with generally lower values in intercropping systems compared to monocultures, possibly due to competition affecting grain filling. WUE based on grain is consistently lower than biomass WUE across all treatments, reflecting the fact that seed yield is a subset of total biomass and often more sensitive to water stress. This is particularly evident in intercropping systems where grain WUE is markedly lower, possibly due to resource partitioning and competition between species. Intercropping cereals with peas enhances total system WUE by utilizing water more efficiently through complementary resource use (e.g., nitrogen fixation by peas benefiting cereals) [14]. This system can improve sustainability by reducing water wastage and increasing total biomass production per unit of water. The lower grain WUE in intercropping systems suggests a trade-off between total biomass production and seed-yield efficiency. Management practices such as optimizing planting density, timing, species and cultivars, and nutrient management may help mitigate these effects. The year-to-year variation highlights the importance of adapting cropping systems to local climatic conditions to maximize WUE [49,63,64].

4.8. Radiation-Use Efficiency (RUE)

Radiation-Use Efficiency (RUE) quantifies how effectively plants convert absorbed photosynthetically active radiation (PAR) into biomass or seed yield. It is a key parameter for optimizing cropping systems, particularly under variable environmental conditions and in intercropping systems where species interactions influence resource use. Cereals such as barley and triticale generally show moderate to high RUE, with triticale often outperforming barley due to differences in photosynthetic efficiency and growth habits [65]. Pea monocultures initially exhibited lower biomass RUE than cereals but reached comparable levels by 2022–2023 (2.87 g MJ⁻¹), possibly due to improved growing conditions or phenological adaptation. Intercropping systems consistently showed the highest total biomass RUE, indicating effective complementarity between cereals and peas in capturing and converting radiation [33]. However, cereals within intercropping had lower RUE than in monocultures, likely due to interspecific competition, though this was offset by higher RUE in peas.

Seed RUE was lower than biomass RUE across all treatments, reflecting the greater energy cost of seed production. Monocultures achieved higher seed RUE, e.g., the barley monoculture reached 1.58 g MJ⁻¹ in 2020–2021, while barley–pea intercropping was 1.05 g MJ⁻¹. Pea seed RUE remained lower but still contributed to total grain RUE in mixed systems. Reductions in seed RUE in intercropping likely result from competition during grain filling or shifts in resource allocation [66]. RUE values varied notably across years due to environmental factors such as radiation intensity, temperature, and water availability. In 2021–2022, intercropping RUE declined, likely due to less favorable conditions. This highlights the need for adaptive management and crop selection based on climatic variability [67,68].

Overall, cereal–pea intercropping enhances total RUE by leveraging complementary resource use and improving biomass production per unit of radiation. While monocultures favor seed RUE, intercropping increases system resilience and sustainability—especially important in water-limited Mediterranean environments [68].

4.9. Actual Yield Loss (AYL) Based on Biomass Yield

Actual Yield Loss (AYL) measures the yield loss (negative value) or gain (positive value) of the cereal or pea in intercropping relative to their monoculture yields. AYL total is the sum of cereal and pea AYL. A negative value indicates net yield loss in intercropping compared to monocultures, while a positive value suggests a yield advantage (overyielding) [21]. In the barley–pea and in triticale–pea intercrops, the AYL total was 1.78 and 1.63, respectively, indicating a net yield advantage compared to monocultures in 2021. However, in 2022 the barley–pea AYL total was −0.09, showing slight net yield loss. Triticale–pea showed promising coexistence (facilitation); the AYL total was 0.71 (positive), indicating moderate yield advantage, while barley–pea remained competitive. The shift in triticale–pea dynamics suggested better adaptation to 2022–2023 conditions, since it obtained a positive AYL cereal value.

4.10. Relative Crowding Coefficient Based on Biomass Yield

The relative crowding coefficient (K) was used to evaluate species competitiveness within the intercropping systems. A K value greater than 1 indicates a dominant species, while values less than 1 indicate suppression. The total K, derived from the product of cereal and pea K values, provides a system-level indicator: values above 1 suggest mutual facilitation; values below 1 indicate that interspecific competition prevails. Across all years, cereals (barley and triticale) consistently outcompeted peas. In the barley–pea system, barley was dominant while peas were strongly suppressed, resulting in a K total of −10.42 in 2020–2021, signaling intense interspecific competition and reduced system efficiency. In 2022–2023, triticale further amplified this trend (K cereal = 3.32; K pea = −2.85), with an even lower K total (−13.72), indicating severe competition and limited complementarity.

However, environmental conditions significantly influenced species interactions. For instance, the 2021–2022 season, marked by drought stress, reduced cereal performance and enabled modest overyielding in the barley–pea system. In contrast, the 2022–2023 season showed a shift in the triticale–pea system toward more balanced interactions, with a positive K total suggesting facilitation. This shift may reflect improved ecological complementarity or environmental adaptation.

Overall, these results highlight that interspecific competition remains a major constraint in cereal–legume intercropping systems under Mediterranean agroforestry conditions. Cereals frequently dominate, especially under suboptimal conditions, leading to pea suppression [16]. Management strategies such as adjusting planting density, sowing dates, or selecting more compatible genotypes could improve complementarity and system sustainability [62]. The observed facilitative trend in triticale–pea intercropping under favorable conditions warrants further investigation for its potential in enhancing productivity and resilience in olive-based agroforestry systems.

4.11. Land Equivalent Coefficient, System Productivity Index, and Percentage Yield Difference Based on Biomass Yield

Three other indices were proposed recently that can be used to evaluate the intercropping systems: the Land Equivalent Coefficient, System Productivity Index, and Percentage Yield Difference [29]. The Land Equivalent Coefficient (LEC) is used to evaluate the efficiency and productivity advantage of intercropping systems compared to monocultures. Values above 0.25 generally indicate a yield advantage from intercropping. Barley–pea generally had higher LEC values in two out of the three years, except for 2021–2022, when its performance dropped dramatically. Triticale–pea was more stable and outperformed barley–pea only in 2021–2022. The year-to-year fluctuations suggest that environmental or management factors significantly influenced the relative efficiency of these intercropping systems.

System Productivity Index (SPI) generally increased from 2020–2021 to 2021–2022, especially for triticale–pea, indicating improved system productivity possibly due to better growing conditions or management. Similarly, triticale–pea intercropping [16] showed the highest SPI values. In 2022–2023, productivity decreased for both systems, reflecting possible environmental or agronomic challenges [69]. Triticale–pea showed higher peak productivity (2021–2022), while barley–pea maintained more consistent productivity across the years. These trends highlight the dynamic nature of system productivity influenced by cropping system choice and seasonal factors [70].

The Percentage Yield Difference (PYD) reflects the relative yield advantage of intercropping compared to monocultures, capturing how much the yield changes due to the interaction between the two crops [29]. In the barley–pea and triticale–pea systems, PYD fluctuated across the three growing seasons but remained positive, ranging from about 28.79% to 79.77% for barley–pea and 34.87% to 75.08% for triticale–pea. These positive PYD values suggest that intercropping consistently improved overall yield compared to monocultures, despite year-to-year variability. Intercrops are generally more stable in yield variation over the year [71]. Higher PYD values, such as the 79.77% observed in barley–pea during 2020–2021, correspond to substantial productivity gains, indicating strong complementary interactions and efficient resource use between barley and pea in that season. Conversely, lower PYD values, like 28.79% for barley–pea in 2021–2022, imply reduced but still positive yield benefits, possibly due to environmental stress or stronger competition between crops. Research on similar cereal–legume intercropping systems supports that moderate increases in sowing density and optimal crop spatial arrangements can enhance PYD by reducing interspecific competition and improving resource-use efficiency, thereby boosting overall productivity. In summary, the PYD positively impacts the overall productivity of barley–pea and triticale–pea intercropping systems by quantifying the yield advantage over monocultures and reflecting the effectiveness of crop interactions in utilizing resources efficiently. Higher PYD values correspond to greater productivity gains and economic benefits in these systems [28].

4.12. Competitive Ratio and Aggressivity Based on Biomass Yield

Pea consistently showed higher CR values than cereals in most years and systems, indicating pea’s stronger competitive ability for biomass accumulation, which was especially pronounced in 2021–2022 (CR pea = 15.02 for barley–pea and 6.46 for triticale–pea). This suggests that pea dominated biomass production that year. Also, the aggressivity index confirms this pattern as it had positive values for pea in 2021–2022, indicating that pea was highly aggressive and strongly suppressing cereals (negative A for cereals). In 2020–2021 and 2022–2023, cereals showed positive aggressivity in some cases (e.g., barley–pea in 2022–2023 with A cereal = 2.90), indicating that cereals dominated biomass in those seasons. The large fluctuations in CR and A across the years highlight the dynamic nature of crop competition influenced by environmental conditions and crop growth characteristics. The dominance of pea in biomass production, particularly in 2021–2022, aligns with findings from the study by Agegnehu et al. [72], which showed that legumes like faba bean can be highly competitive in mixtures with barley, especially under certain seeding ratios and environmental conditions. The strong year effect on competition indices is consistent with Arlauskienė et al. [73], who reported that soil and weather conditions significantly influence the competitive balance between peas and cereals in intercropping systems, affecting aggressivity and biomass accumulation. The positive aggressivity of barley in 2022–2023 (A = 2.90) is supported by findings in other studies where cereals can dominate under favorable conditions or higher seeding proportions, highlighting the importance of management (e.g., sowing ratio, fertilization) in modulating competition. Understanding CR and aggressivity helps optimize species proportions and spatial arrangements to balance competition and maximize total biomass and yield, as emphasized by Willey and Rao [74] and supported by recent research on cereal–legume intercropping.

4.13. Actual Yield Loss and Relative Crowding Coefficient Based on Seed Yield

All indices were calculated for biomass and seed yield to determine the species interactions and the advantage of the different systems [38]. In 2020–2021, both systems experienced positive AYL for both cereal and pea, indicating yield benefits in both crops, with triticale–pea showing a higher total gain (1.25) than barley–pea (0.69). In 2021–2022, barley–pea showed a negative AYL for cereal (−0.52), suggesting a yield loss or competition effect for barley, while pea still had a positive gain (0.08), resulting in a slight net loss (−0.45) overall. Triticale–pea had a positive AYL for pea (1.79), indicating severe pea yield gain, and a high total gain (1.53). In 2022–2023, both systems again showed positive AYL values, with barley–pea having relatively low gains (0.25 total) and triticale–pea having higher gains (1.20 total). The variability in AYL across years and treatments suggests fluctuating competitive interactions influenced by environmental conditions and crop growth dynamics. Positive AYL values (yield gain) indicate facilitative effects or compensation in intercropping.

In 2020–2021, barley was dominant in barley–pea (K cereal = 1.22) with pea nearly neutral (K pea = 0.04), indicating that barley outcompeted pea. In triticale–pea, triticale was strongly dominant (K cereal = 2.15) while pea was suppressed (negative K). In 2021–2022, both systems showed negative K values for pea, indicating pea was suppressed or had poor competitive ability, while cereals had low K values, reflecting weak dominance. In 2022–2023, barley–pea showed moderate dominance of barley (K cereal = 1.33) and positive K for pea (0.41), suggesting more balanced competition. Triticale–pea showed a strong negative K for triticale (−3.13) but a very high positive K for pea (3.02), resulting in a very high total K (6.24), indicating pea dominance or a shift in competitive balance.

The K values reveal that competitive dynamics vary significantly by year and cropping system. Barley tends to dominate pea in barley–pea intercropping, while triticale–pea shows more fluctuating dominance, sometimes favoring pea. Negative K values suggest strong suppression or poor adaptation under certain conditions. Barley–pea systems generally show more stable and moderate competition, with barley usually dominant. Also, triticale–pea systems exhibit more variability, with strong dominance shifts between crops and higher yield losses, especially for pea in some years. These dynamics could be influenced by environmental factors, crop growth rates, nutrient competition, and planting densities [75].

4.14. Land Equivalent Coefficient, System Productivity Index, and Percentage Yield Difference Based on Seed Yield

In 2020–2021, both systems of barley–pea and triticale–pea intercropping had LEC values above 0.25, indicating a clear yield advantage from intercropping. In 2021–2022, barley–pea’s LEC dropped drastically to 0.07, suggesting little to no intercropping benefit, while triticale–pea’s LEC was very high (0.92), indicating strong intercropping efficiency. In 2022–2023, LEC values rebounded somewhat but remained below 0.25 for barley–pea (0.22) and moderate for triticale–pea (0.64), showing a better intercropping advantage for triticale–pea.

In 2020–2021, barley–pea had a significantly higher SPI than triticale–pea, indicating greater total grain productivity. In 2021–2022, triticale–pea showed a substantial increase in SPI, outperforming barley–pea, suggesting better adaptation or conditions favoring triticale–pea. In 2022–2023, barley–pea again had higher SPI (349.64) compared to triticale–pea (172.61), indicating better productivity. Triticale–pea consistently showed higher PYD values than barley–pea, especially in 2021–2022, where PYD reached an exceptional 127.65%, indicating a massive yield advantage. Barley–pea showed moderate PYD values across all years, with the highest in 2020–2021. The large PYD in triticale–pea in 2021–2022 suggests either very low monoculture yields or exceptional intercropping synergy that year.

4.15. Competitive Ratio and Aggressivity Based on Seed Yield

With regard to Competitive Ratio (CR) across all years and treatments, pea consistently showed a higher CR than cereal, indicating that pea is generally the more competitive crop in these intercropping systems. In particular, in 2020–2021, pea had CR values of 3.54 (barley–pea) and 2.85 (triticale–pea), highlighting strong competitive ability relative to cereals. In 2021–2022, pea’s CR values surged dramatically (7.76 for barley–pea and 9.11 for triticale–pea), suggesting extreme competitive dominance over cereals, possibly due to environmental stress or cereal growth suppression. In 2022–2023, pea’s CR values decreased but remained above 4, still indicating strong competitiveness. Cereal CR values were generally below or near 1, showing cereals were less competitive, except in 2022–2023 where barley (1.10) and triticale (1.22) slightly outcompeted pea in some cases.

Aggressivity values for pea were positive in most cases, confirming its dominance in the intercropping system. For cereals, aggressivity was mostly negative or near 0, indicating suppression or balanced competition. Notably, in 2021–2022, pea aggressivity was very high (0.60 for barley–pea and 2.04 for triticale–pea), while cereals were strongly suppressed (−0.60 and −2.04, respectively), reinforcing the CR findings. In 2022–2023, triticale showed slight positive aggressivity (0.56) in triticale–pea, indicating some reversal in dominance.

The dominance of pea in these systems aligns with findings by Willey [20] and Li et al. [49], who reported that legumes often exhibit strong competitive ability in cereal–legume intercropping due to nitrogen fixation and efficient resource use. The dramatic increase in pea’s competitiveness in 2021–2022 may be linked to environmental stress (e.g., drought or nutrient limitation) that disproportionately affected cereals, a phenomenon documented by Li et al. [76] in cereal–legume systems.

The near parity or slight cereal dominance in 2022–2023 suggests that under favorable conditions or optimized management, cereals can compete more effectively, consistent with findings from Hauggaard-Nielsen et al. [77], who emphasized the role of planting density and timing in balancing competition.

Understanding CR and aggressivity helps optimize seeding ratios and planting arrangements to minimize suppression and maximize complementarity, as recommended by Willey and Rao [74].

5. Conclusions

This study evaluated the performance of cereal–pea intercropping systems under Mediterranean olive agroforestry conditions, analyzing key agronomic, physiological, and competitive interactions over three growing seasons. The findings demonstrate that intercropping enhances resource-use efficiency, particularly in terms of land productivity (LER > 1), Radiation-Use efficiency (higher RUE in intercrops), and complementary water use (improved total WUE). However, competitive dynamics varied significantly between species and across years, with pea often exhibiting dominance in biomass production (higher CR and aggressivity values), while cereals showed trade-offs in seed yield components due to shading and interspecific competition. Despite yield reductions in sole crop comparisons, intercropping provided consistent advantages in total system productivity, particularly under low-input conditions, aligning with niche complementarity and facilitation in cereal–legume mixtures. The stability of triticale–pea systems in certain years suggested that they can be used for optimized agroforestry designs that balance competition through adjusted planting densities or species selection. It is possible that using the interrow space between olive trees to grow annual species offers opportunities to capture more radiation and water for productive use, and that adding a legume to the system may improve nitrogen availability and increase overall yield. These insights can guide sustainable intensification strategies, ensuring higher productivity while minimizing external inputs in climate-vulnerable regions.