Weed Coexistence in Eucalyptus Hybrid Stands Decreases Biomass and Nutritional Efficiency Mid-Rotation

Abstract

Determining the period of weed coexistence with eucalyptus stands assists in the integrated management of weed competition, which reduces the application of herbicides, economic costs, and environmental damage. Therefore, the objectives of the present study were to evaluate the impact of a hybrid stand of Eucalyptus grandis × Eucalyptus urophylla trees on biomass production, the concentration and stock of macro and micronutrients, and the efficiency of nutrient utilization under different periods of coexistence with weeds. Our study is the first to report the impacts of weed management on the biomass and nutrients present in the leaves, branches, stembark, and stemwood at the time of mid-rotation (4.5 years old). The experiment was carried out in southern Brazil in the Pampa biome and followed a randomized block design, with three replications. The treatments consisted of increasing periods of coexistence with or control of weeds. The total biomass in the coexistence up to 378 days treatment was 53.7 Mg ha⁻¹, and in the control treatment up to 168 days, it was 81.4 Mg ha⁻¹, differing statistically. This reduction was in the order of 34%. The continuous presence of weeds led to a significant reduction in the production of wood and total biomass. The highest nutrient utilization efficiency (NUE) was observed for wood. The control of weeds for up to 168 days promoted greater productivity than controlling weeds for 378 days. This recommendation can be adopted by plantation managers for this region, reducing the amount of herbicide applied on the stand.

1. Introduction

The productivity of the planted forest sector in Brazil is 38.9 m³ ha⁻¹ year⁻¹, which is higher than those of second-place China with 27 m³ ha⁻¹ year⁻¹ and third-place Mozambique with 25 m³ ha⁻¹ year⁻¹. In addition, the period from planting to harvest (5 years) is half as long when compared to Oceania and the USA, and up to three times shorter when compared to Scandinavia and Canada, providing a greater number of rotations [1]. Even though the soil and climate variables contribute to the recognized productivity, it should be noted that technical and scientific advances in genetic improvement, forest management, and silviculture were and still are significant contributors to these results.

Maximum productivity in forests, particularly in fast-growing planted stands, is highly dependent on forest nutrition [2]. Effective fertilization management is key to optimizing productivity, but the presence of weeds can nullify the positive effects of fertilization due to competition for limited resources such as water, nutrients, and light [3,4]. Competition for these resources is a significant factor that can compromise productivity [5]. Therefore, it is essential to consider both weed management and fertilization management strategies to achieve optimal forest productivity.

The high capacity of weeds to absorb essential elements for their development makes them strong competitors; in addition, under a low nutritional supply, they preferentially invest in photoassimilates in their below-ground biomass, which is a strategy for exploiting a larger volume of soil [6]. Competition for nutrients depends mainly on the species involved, and the allocated amount of nutrients in the biomass should be considered instead of just the nutrient contents as a base indicator [6,7].

Weeds are able to achieve this domination because they develop in an inappropriate period or place, such as in areas of production and economic interest [8]. Depending on the degree of infestation, productivity is compromised [2]. In the case of forest plantations, greater spacing is a facilitator of infestation [9]. A high degree of infestation increases the control costs, and their incorrect use has caused the selection of more resistant weed individuals [10]. Some herbicides have a residual effect, which varies according to the formulation [11].

Many studies have analyzed the efficiency of different herbicides in weed control or their drift and the impact on eucalyptus growth [7,12,13]. Other studies have evaluated plant spacing as a form of control [7,14]. The nutrient dynamics in eucalyptus biomass are particularly reported in young plants, still seedlings, at 110 days after transplantation [3,15], and no study has evaluated the impacts of weeds on biomass production, stock, and nutrient utilization efficiency in eucalyptus stands over 48 months old.

An evaluation of the biomass and the allocation of nutrients under different periods of weed coexistence in eucalyptus stands is an essential tool for the adoption of an integrated management strategy. Therefore, the objective of this work was to evaluate the biomass and nutrients allocated to Eucalyptus grandis × Eucalyptus urophylla trees, at 55 months of age, under different periods of coexistence with weeds. We hypothesized that long periods of coexistence between weeds and eucalyptus trees would promote a reduction in tree biomass and the less efficient use of nutrients.

2. Materials and Methods

2.1. Study Area

The present study was carried out in an experimental area at Fazenda Aroeira in the municipality of Candiota—RS, at the geographic coordinates of 31°44′39.96″ S and 53°50′48.0″ W (Appendix A). The region is located in the Pampa biome, in the south of Brazil, which is characterized by flat-to-undulating relief and is covered by grassland vegetation [16].

According to the Köppen classification, the climate is of the Cfa type and is characterized as subtropical, in which the temperature of the coldest month ranges between −3 °C and 18 °C and the average of the hottest month is above 22 °C. The average annual rainfall is 1465 mm [17]. Figure 1 shows the meteorological variables of precipitation and temperature during the period of eucalyptus growth.

The soil of the study area is LEPTOSOL Dystroumbric (RLdh) fragmentary, with the presence of gravel. The original material is fine sandstone, claystone, and siltstone. Drainage in the area is considered moderate and water is removed slowly, leaving the horizon wet for a part of the time. The soil has a layer of slow permeability, and the water table affects the lower part of the B horizon [19].

LEPTOSOLS soils do not have a deep horizon and do not have a diagnosed B horizon, as shown in

Table 1. Chemical and physical attributes of the soil in the experimental area.

Horizon		A	AC	CR
Depth (cm)		0–13	13–28/36	28/36–60+
Attributes
Granulometry (%)	C.S.	9.9	29.3	-
	F.S.	3.4	3.4	-
	M.S.	5.0	5.8	-
	Silt	38.5	20.8	-
	Clay	43.3	40.8	-
pH	Water	5.1	5.4	-
(cmolc/dm3)	Ca2+	0.01	0.01	-
	Mg2+	6.2	8.5	-
	K+	0.6	0.4	-
	Na+	0.3	0.3	-
	S	7.1	9.2	-
	Al3+	3.3	4.6	-
	H+	10.9	10.9	-
	t	10.4	13.9	-
	T	21.3	24.8	-
O.M	%	3.6	2.2	-
Ta	clay	41.6	49.5	-
V	%	39.6	45.8	-
m	%	31.3	33.3	-
P	mg/dm3	2.9	1.5	-

Where—C.S.: coarse sand; F.S.: fine sand; M.S.: medium sand; S: sum of cations (Ca²⁺ + Mg²⁺ + K⁺ + Na⁺); O.M.: organic matter determined by wet combustion (K₂Cr₂O₇ + H₂SO₄); Al: exchangeable aluminum, Ca: exchangeable calcium; Mg: exchangeable magnesium, extraction with KCl solution (1 mol L⁻¹); P available and K exchangeable, extraction with Mehlich solution (HCl + H₂SO₄); t: Effective CTC (Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ + Al³⁺); T: CTC pH7 (Ca²⁺ + Mg²⁺ + K⁺ + Na⁺ + (H⁺ + Al³⁺); V: base saturation; m: aluminum saturation; S: available sulfur, extraction with Ca solution (H₂PO₄)₂ 500 mg P L⁻¹; Ta: clay fraction activity. Source: Elaborated by the author.

. Regarding their 2nd categorical level (suborder), LEPTOSOLS soils present an A horizon directly over a C or Cr horizon. However, they may present a B horizon at the beginning of formation. The 3rd categorical level (Dystroumbric) comprises soils with a low base saturation (V < 50%) and a prominent A horizon. The 4th categorical level (fragmentary) comprises soils with fragmentary lithic contact. Among other attributes, clay activity is considered high (Ta) [19].

2.2. Implantation and Experimental Design

Seedlings of the hybrid Eucalyptus grandis × Eucalyptus urophylla trees were planted on 7 December 2005, with a spacing of 3.0 m × 2.0 m. The experiment received a base fertilization of 100 kg ha⁻¹ of triple superphosphate + 0.5% copper (Cu) + 0.3% boron (B) via a mechanized application in a line and 200 kg ha⁻¹ of NPK 06-30-06 (manual application). At 90 days, the seedlings in the field received a cover fertilization of 133 kg ha⁻¹ of NPK 20-0⁻¹0 + 0.3% boron (80 g/plant). The second topdressing fertilization occurred at 180 days with 100 kg of ammonium sulfate + 0.5% boron, and at one year, the third topdressing fertilization was performed with 100 kg of ammonium sulfate + 0.5% boron. Every 28 days, a glyphosate herbicide was applied at a dosage of 3 L ha⁻¹ to the plots that should be free from weed competition.

The experiment was performed using a randomized block design, with three replicates. The dimensions of the plots were 22 m × 18 m. The experimental treatments consisted of periods of coexistence with or control of weeds. In group 1, the stands remained in a state of coexistence with weeds. This treatment group was used to determine the period before interference (PBI). In group 2, the eucalyptus trees received weed control for longer periods. This treatment group determined the total interference prevention period (TIPP) (

Table 2. Treatments for the quantification of eucalyptus biomass and nutrients.

Group	Period of Time	Treatment
(1) Coexistence	378 days	Ce 378
	140 days	Ce 140
(2) Control	28 days	Ct 28
	168 days	Ct 168
	378 days	Ct 378

). To control weed competition, a glyphosate herbicide was applied at a dosage of 3 L ha⁻¹.

2.3. Eucalyptus Biomass and Weed Species

In the forest inventory, the diameter at breast height (DBH) and the tree height were measured. Biomass measurements were performed at 55 months of age, and fifteen trees were felled for these measurements (three for each treatment). The trees were selected by considering the arithmetic mean of the DBH, which was measured in the forest inventory. The selected trees were sectioned at ground level and the biomass was separated into components: leaves, branches, stembark, and stemwood.

For the sampling of the stemwood and stembark biomass, the felled trees were subdivided into three sections of the same length (lower third, middle third, and upper third), and a disc approximately 3 cm thick was collected from the center of each section previously determined (16.7%, 50.0%, and 83.3% of the total height). For the branches and leaves, the sampling was random to represent the entire component.

All samples were weighed in the field using a precision scale to measure the wet weight, then packed in paper bags and dried in the laboratory in an air circulation oven at 70 °C until they reached a constant weight. After determining the dry weight, the % moisture content was calculated according to the formula below. The biomass of each tree was determined indirectly through the moisture content of the samples of each component.

$$Mo(\%)=\frac{Mw-Dw}{Dw}·100$$

where:

Mo = moisture content (%);

Mw = moist weight (grams);

Dw = dry weight (grams).

The nutrient use efficiency of different tree components, including leaves, branches, wood, and bark, was calculated using [20], based on the provided data on nutrient content and dry biomass.

The sampling of weed species was performed in plots where no herbicides were applied, in the Ce378 treatment. In each plot of this treatment, 5 areas with dimensions of 1 m × 1 m were randomly marked along the diagonal of the trees. All plant biomass was collected for subsequent species identification. The ten identified species were: Eragrostis pilosa, Baccharis coridifolia, Paspalum sp., Senecio sp., Aspilia montevidensis, Lolium multiflorum, Spo-robolus sp., Erianthus angustifolius, Conyza bonariensis, and Cynodon dactylon.

2.4. Chemical Analysis

The determination of nutrient contents in the biomass was performed using the methodology in [21]. For the determination of the nitrogen content, digestion with H₂SO₄ + H₂O₂ was used, and the analysis was performed using the Kjeldahl method. To determine the content of the elements P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn, the samples were submitted to nitric–perchloric digestion (HNO₃ + HClO₄). The Ca and Mg analyses were carried out using atomic absorption spectrometry, the K analysis was performed using flame photometry, the P analysis was conducted using spectrophotometry, and the S analysis was carried out using turbidimetry. The Cu, Fe, Zn, and Mn contents were determined using atomic absorption spectrometry with a Perkin Elmer AAnalyst 200. The boron content was determined using dry digestion and a spectrophotometry reading at a wavelength of 460 mm.

2.5. Statistical Analyses

The tests validated that the data satisfied the normality and homoscedasticity assumptions required for the analysis of variance (ANOVA). Statistical analyses were performed using SPSS software version 20.0, Chicago, IL, USA [22].

The effect of the period of coexistence of weeds with eucalyptus trees on the forest inventory (DBH, height, and volume), biomass production, and nutrient concentration was statistically analyzed using Tukey’s test at p ≤ 0.05 error probability. Nutrient concentration and nutrient use efficiency were also compared between biomass components (leaves, branches, stembark, and stemwood). Tukey’s test was used as the mean test, considering an error probability of p ≤ 0.05.

3. Results

3.1. Forest Inventory

The forest inventory, carried out at 55 months of age, showed a variation in the diameter at breast height of 12.2 cm and 13.8 cm for the treatments in which the eucalyptus stands coexisted with weeds for 378 days (Ce 378) and in which the weeds were controlled for up to 168 days (Ct 168), respectively. Tukey’s test of means, with an error probability level of 5%, showed that these treatments differed from each other. The total height and volume without bark also showed statistical differences between treatments. The total height ranged from 15.7 m for the Ce 378 treatment to 17.8 m for the control treatment up to 168 days (Ct 168), with a statistically significant difference. The volume without bark ranged from 121.0 m³ ha⁻¹ for the coexistence treatment up to 378 days (Ce 378) to 152.9 m³ ha⁻¹ for the control treatment up to 168 days (Ct 168) (Figure 2).

The smallest DBH was found in the Ce 378 treatment, representing a reduction of 11.6% in relation to treatment Ct 168. The reduction in the average height of trees in coexistence up to 378 days (Ce 378) was 11.8% compared to the weed control treatment up to 168 days (Ct 168). However, the coexistence of weeds had a greater impact on the volume of wood without bark. A reduction of 20.9% was observed between the Ce 378 and Ct 168 treatments, respectively.

3.2. Forest Biomass

The leaf biomass of E. urophylla × E. grandis stands after different periods of coexistence with a weed community was 7.0 Mg ha⁻¹ and 6.0 Mg ha⁻¹ in the Ct 378 and Ce 378 treatments, respectively (Figure 3). This represents a reduction of 14.3%. A statistically significant difference was found, considering Tukey’s means test at p ≤ 0.05.

The stembark, stemwood, and total biomass showed statistical differences between treatments. The Ce 378 treatment had the lowest recorded amounts of leaves and wood, at 6.0 and 53.7 Mg ha⁻¹, respectively. The Ct 28 treatment had the highest leaf and branch biomass, with both at 7.0 Mg ha⁻¹. The coexistence-with-weeds treatment up to 140 days (Ce 140) showed a stembark biomass of 12.4 Mg ha⁻¹, while the control treatment up to 168 days (Ct 168) showed a biomass of 4.2 Mg ha⁻¹.

For the biomass of branches, the greatest variation occurred between the Ct 28 and the Ce 140 treatments, with 7.0 Mg ha⁻¹ and 5.1 Mg ha⁻¹, respectively. This variation represents a reduction of 27.1%. As with leaf biomass, there was no statistical difference at p ≤ 0.05 through Tukey’s means test.

There was a statistical difference for the wood biomass (p ≤ 0.05) between the control treatment up to 168 days and the continuous coexistence treatment, with 81.4 and 53.7 Mg ha⁻¹, respectively. This represented a reduction of 34%. The wood biomass constituted 81.4% of the total biomass for the control treatment up to 168 days (Ct 168), while the same component represented only 53.7% of the total biomass for the coexistence up to 378 days (Ce 378) treatment.

3.3. Nutrient Concentrations

Through the test of means, it was observed that there was no statistically significant difference in the concentration of nutrients in the leaf biomass, except for the macronutrient potassium (

Table 3. Nutrient concentrations in the different treatments and biomass partitions of the hybrid E. grandis × E. urophylla at 55 months in southern Brazil.

Trat.	N	P	K	Ca	Mg	S	B	Cu	Fe	Mn	Zn
	g kg−1						mg kg−1
Leaves
Ce 378	20.23 a	1.84 a	* 9.23 a	7.94 a	2.02 a	1.33 a	30.72 a	9.46 a	96.75 a	702.86 a	11.86 a
Ce 140	18.62 a	1.66 a	7.60 b	7.37 a	2.26 a	1.07 a	31.12 a	8.88 a	90.83 a	843.52 a	11.19 a
Ct 28	19.41 a	1.62 a	8.37 ab	6.97 a	2.05 a	1.31 a	26.64 a	8.82 a	93.23 a	576.07 a	11.48 a
Ct 168	18.73 a	1.60 a	8.17 ab	7.04 a	2.09 a	1.23 a	27.68 a	10.12 a	92.63 a	696.45 a	10.90 a
Ct 378	17.70 a	1.70 a	8.13 ab	7.51 a	2.26 a	1.22 a	26.56 a	8.61 a	88.23 a	703.63 a	10.71 a
Average	18.94 A	1.68 B	8.30 B	7.37 B C	2.14 B	1.23 A	28.54 B	9.18 A	92.33 B	704.51 B	11.23 B
CV (%)	4.99	5.66	7.14	5.32	5.43	8.33	7.77	6.69	3.40	13.45	4.08
Branches
Ce 378	4.45 a	1.07 a	5.07 a	10.01 a	1.09 a	0.30 a	5.76 a	7.38 a	30.98 a	383.08 a	10.16 a
Ce 140	3.80 a	0.84 a	4.37 a	9.83 a	1.25 a	0.20 a	6.56 a	7.80 a	28.19 a	522.92 a	10.61 a
Ct 28	4.87 a	1.07 a	5.23 a	9.13 a	1.27 a	0.37 a	8.08 a	7.55 a	30.53 a	369.80 a	10.78 a
Ct 168	3.29 a	1.23 a	5.53 a	8.79 a	0.66 a	0.25 a	5.52 a	7.67 a	28.22 a	354.25 a	8.83 a
Ct 378	3.45 a	0.82 a	4.93 a	10.35 a	1.00 a	0.21 a	7.52 a	6.89 a	25.11 a	429.27 a	9.47 a
Average	3.97 C	1.01 C	5.03 C	9.62 B	1.05 C	0.27 C	6.69 C	7.46 B	28.61 B	411.86 B	9.97 B
CV (%)	16.90	17.25	8.54	6.69	23.46	26.40	16.50	4.74	8.18	16.54	8.17
Stembark
Ce 378	9.89 a	3.36 a	13.30 a	88.34 a	13.00 a	0.92 a	* 53.04 a	10.80 a	402.46 a	2129.06 a	28.29 a
Ce 140	9.08 a	2.76 a	13.10 a	78.28 a	12.87 a	0.90 a	37.92 a b	9.61 a	125.62 a	2827.73 a	25.09 a
Ct 28	9.40 a	3.81 a	13.10 a	70.74 a	14.54 a	0.53 a	46.56 a b	9.81 a	160.72 a	2181.49 a	26.66 a
Ct 168	9.12 a	3.81 a	13.40 a	88.80 a	14.32 a	1.01 a	48.48 a b	9.24 a	141.34 a	2700.74 a	28.01 a
Ct 378	8.43 a	2.71 a	13.00 a	71.43 a	12.09 a	0.68 a	34.08 b	11.61 a	212.81 a	2340.72 a	26.42 a
Average	9.18 B	3.29 A	13.18 A	79.52 A	13.36 A	0.81 B	44.02 A	10.21 A	208.59 A	2435.95 A	26.89 A
CV (%)	5.78	16.39	1.25	11.03	7.76	24.40	17.73	9.50	54.29	12.84	4.82
Stemwood
Ce 378	1.04 a	0.41 ab	1.47 a	0.98 a	0.29 ab	0.04 a	2.80 a	2.21 a	45.97 a	30.30 a	* 4.04 a
Ce 140	0.89 a	0.31 b	1.14 a	1.00 a	0.25 b	0.02 a	3.12 a	1.96 a	12.37 a	35.61 a	2.64 b
Ct 28	1.01 a	* 0.49 a	1.61 a	1.14 a	* 0.34 a	0.02 a	3.44 a	1.89 a	11.36 a	32.20 a	* 4.16 a
Ct 168	0.99 a	0.37 ab	1.43 a	0.92 a	0.28 ab	0.05 a	3.20 a	1.82 a	22.31 a	30.35 a	3.13 ab
Ct 378	1.07 a	0.29 b	1.37 a	1.03 a	0.25 ab	0.04 a	3.36 a	3.02 a	18.81 a	38.01 a	3.30 ab
Average	1.00 D	0.37 D	1.40 D	1.01 C	0.28 C	0.03 D	3.18 C	2.18 C	22.16 B	33.29 C	3.45 C
CV (%)	6.86	21.52	12.25	8.00	13.13	39.46	7.83	22.57	63.43	10.23	18.50

Note: Means followed by the same lowercase letter, in the same biomass component, do not differ from each other based on Tukey’s test, at 5% error probability. Means followed by the same uppercase, between different biomass components, do not differ from each other based on Tukey’s test, at 5% error probability. Asterisk (*) indicate a statistically significant difference by Tukey-test (p < 0.05).

). The Ce 378 treatment had an average concentration of 9.23 g kg⁻¹, which differed from that of the coexistence treatment up to 140 days (Ce 140), at 7.6 g kg⁻¹.

The Ce 378 treatment had the highest concentrations of the nutrients N, P, K, Ca, S, B, Fe, and Zn. The Ct 378 treatment had the lowest concentrations of N, B, Cu, Fe, and Zn. This result is explained by the fact that the stand under competition experienced a reduction in the production of dry biomass; consequently, there was a higher concentration of nutrients.

The Ct 28 treatment showed statistically significant differences for the nutrients P, Mg, and Zn in the wood component. In addition, higher concentrations were observed in this treatment for the nutrients K, Ca, and B, although the mean test did not show a statistical difference (p ≤ 0.05).

Considering the concentrations of nutrients between the biomass components, it was observed that the stembark presented the highest contents of P, K, Ca, Mg, B, Fe, Mn, and Zn, differing from the other components. The N and S concentrations in the leaves showed the highest levels, and these levels statistically differed from those in the branch, stembark, and stemwood components.

In the wood component, there was a statistically significant difference (p ≤ 0.05) for the nutrients P, Mg, and Zn. The Ct 28 treatment showed the highest nutrient contents, with 0.49 and 0.34 g kg⁻¹ for P and Mg, respectively, and 4.16 mg kg⁻¹ for Zn. The lowest contents were found for the Ce 378 (P, 0.29 g kg⁻¹) and the Ce 140 (0.25 g kg⁻¹ of Mg and 2.64 mg kg ⁻¹ of Zn) treatments.

3.4. Nutrient Stock

Regarding the amount of nutrients in the leaves, there was a statistically significant difference (p ≤ 0.05) only for S between the treatments. The control treatment for up to 28 days (Ct 28) had the highest average, at 9.2 kg ha⁻¹, and the coexistence treatment for up to 140 days (Ce 140) had 6.6 kg ha⁻¹ of S (Figure 4).

For the stock of nutrients in wood, K, Mg, S, Mn, and Zn showed statistically significant differences (p ≤ 0.05) between the treatments. In general, it was observed that the control treatments had higher nutrient stocks, as was the case with the Ct 168 treatment. This treatment differed statistically for K, Mg, and S.

3.5. Nutrient Utilization Efficiency

The nutrient utilization efficiency showed greater biomass production per amount of nutrient required for the wood component (

Table 4. Nutrient utilization efficiency in the different treatments and biomass partitions of the hybrid E. grandis × E. urophylla at 55 months.

Trat.	N	P	K	Ca	Mg	S	B	Cu	Fe	Mn	Zn
	kg of biomass ha−1/kg of nutrient
Leaf
Ce 378	50	545	109	128	484	759	32,751	10,3986	10,395	1388	84,986
Ce 140	54	590	131	131	443	939	32,141	111,712	11,062	1167	90,247
Ct 28	52	619	120	143	490	761	37,493	113,452	10,806	1712	87,173
Ct 168	54	627	124	143	481	810	36,551	98,009	10,922	1463	91,822
Ct 378	57	583	123	133	446	833	37,574	116,861	11,489	1424	94,340
Average	53 c	593 b	121 c	135 b	468 c	815 b	35,274 c	108,630 c	10,937 c	1417 b	89,684 b
Branches
Ce 378	231	958	200	99	932	3450	173,804	134,766	32,228	2653	100,877
Ce 140	252	1186	228	106	785	4636	147,399	129,442	35,003	1890	91,727
Ct 28	205	921	191	108	778	2692	122,164	132,075	33,097	2665	92,961
Ct 168	301	821	180	113	1500	4059	176,471	128,492	35,312	2829	111,470
Ct 378	289	1190	201	92	1015	4600	133,981	141,684	38,961	2313	104,387
Average	250 b	985 b	197 bc	103 bc	956 b	3685 b	147,615 b	133,333c	34,753 bc	2458 b	100,214 b
Stembark
Ce 378	317	890	229	34	211	3421	68,638	251,938	8802	1405	105,863
Ce 140	331	1097	230	39	234	3351	78,780	322,078	24,096	1063	118,888
Ct 28	318	797	228	43	206	6000	64,353	310,976	18,326	1387	112,335
Ct 168	304	677	207	35	181	2625	59,238	371,681	24,041	993	104,478
Ct 378	351	1098	230	41	245	4870	87,843	245,077	15,577	1263	112,337
Average	328 b	931 b	227 b	39 c	220 c	3973 b	73,071 c	288,774 b	16,461 c	1211 b	112,260 b
Stemwood
Ce 378	975	2569	706	1055	3604	25,571	396,310	434,115	24,774	33,558	254,502
Ce 140	1127	3204	877	993	3954	43,250	322,611	523,054	80,606	27,974	378,556
Ct 28	986	2076	623	886	2939	44,929	294,476	528,571	86,591	30,976	244,082
Ct 168	1021	2634	715	1070	3555	21,421	298,825	589,855	39,855	32,481	328,358
Ct 378	945	3488	743	977	3936	23,000	292,063	336,073	50,380	26,205	303,630
Average	1009 a	2731 a	727 a	994 a	3572 a	28,165 a	313,235 a	465,574 a	46,969 a	29,844 a	298,476 a
Total biomass
Ce 378	323	1575	377	204	1116	5259	161,333	282,895	19,771	5559	177,686
Ce 140	399	1948	459	191	1020	7146	154,988	349,642	44,678	4193	225,814
Ct 28	328	1405	362	206	924	5846	141,373	326,829	40,657	5410	172,783
Ct 168	420	1776	441	327	1545	6593	177,399	368,617	32,983	7298	235,645
Ct 378	385	2008	413	208	1133	6403	159,728	264,131	33,240	5038	204,395
Average	370	1727	409	221	1121	6241	158,472	314,621	32,455	5331	202,153

Averages followed by the same lowercase letter, vertically, between different biomass components, do not differ from each other based on Tukey’s test, at 5% error probability.

). Regarding the macronutrients in stemwood, for each kg of N, P, K, Ca, Mg, and S, there was an average biomass production value of 1.0, 2.7, 0.7, 0.99, 3.6, and 28.1 Mg ha⁻¹, respectively. These values showed a significant difference (p ≤ 0.05) according to Tukey’s test for the other components.

When analyzing the EUN in the wood, the coexistence treatment for up to 140 days presented an above-average efficiency for all nutrients except Mn. This treatment still had the highest EUN for N, K, Mg, and Zn. A possible justification for this result may be related to the ability of weeds to initially absorb nutrients from the initial soil preparation and topdressing fertilization, allocating them to their biomass. This prevents the erosion, leaching, or even volatilization of the applied fertilizers. After the application of pesticides against the weeds, decomposition of the weed plant biomass occurs and, consequently, the nutrients are gradually released to the eucalyptus stand.

Given this aspect, the weeds could be acting as a possible reserve of nutrients while there is a reduced density of fine roots in the eucalyptus stand. In general, the leaf component showed the lowest efficiency in biomass production for the nutrients N, P, K, S, B, Cu, Fe, and Zn. For the nutrients Ca, Mg, and Mn, the lowest efficiency was observed in the stembark.

The EUN for the total biomass was 0.37, 1.73, 0.41, 0.22, 1.12, and 6.24 Mg ha⁻¹ for each kg of N, P, K, Ca, Mg, and S, respectively. The highest efficiencies for N, Ca, Mg, S, B, Cu, Mn, and Zn were also observed in Ct 168, with weed control up to 168 days. For the nutrients P and Fe, the highest EUN values were reported for the Ct378 treatment.

4. Discussion

4.1. Forest Inventory

In an evaluation carried out in the same experimental area, with the same stand, and at 12 months of age, the authors of [23] found reductions of 68.8, 26.8, and 40% in the volume with bark, total height, and DBH, respectively, between treatment 1 (continuous coexistence) and treatment 21 (control up to 294 days). The results demonstrate that with the maturity of the eucalyptus stands, there was a reduction in the relative differences and recovery of the treatments that were submitted to coexistence with weeds. According to [24], as the age of the stand advances, the differences in the diameter at breast height, the height, and the volume are reduced.

In a weed competition study conducted with E. grandis and Urochloa sp. by the authors of [25] in Três Lagoas, Brazil, it was shown that, at 12 months of age, the eucalyptus trees that coexisted with the weed community during the first 364 days had an average diameter of 2.1 cm and an average height of 1.6 m. This represented reductions of 68.2% and 65.7%, respectively, in relation to the stand that grew under weed control.

In [24], at 23 and 30 months after planting, the Eucalyptus trees that coexisted with the weeds for up to 364 days in Três Lagoas, Brazil had an average DBH of 8.7 cm and an average height of 12.3 m, respectively, while the plants that grew free of the presence of weeds had an average DBH of 10.0 cm and an average height of 13.8 m, representing increases of 13.0% and 10.9%, respectively. The results suggest a tendency for eucalyptus trees to recover from this interference. The same author described that the volume of wood, when comparing the 23- and 30-month-old trees, was 17.6 and 41.6 m³ ha⁻¹ in the presence of weeds and 35.1 and 55.3 m³ ha⁻¹ without the presence of weeds. Thus, there was a reduction of 49.9% in the wood volume at 23 months and 24.8% at 30 months.

A study examining the response of E. pellita to weed control in South Sumatra, Indonesia [26] showed that weed control is fundamental for the growth of eucalyptus stands. According to the researchers, at 72 months of age, the volume of wood was 201.1 and 190.6 m³ ha⁻¹ for the stands with full weed control and up to 1 year of weed control, respectively. The authors also observed that eucalyptus trees have the ability to reduce the impact of the presence of weeds with advancing maturity. Our study is the first to report the impacts of weed management on the biomass and nutrients present in the leaves, branches, stembark, and stemwood of E. grandis × E. urophylla trees over 4 years old.

In the municipality of Cerrito, southern Brazil, a study investigating the influence of the coexistence period of hybrid E. grandis × E. urophylla trees at 360 and 630 days after planting [27] measured the diameter and the amount of dry matter of the stems. The authors also concluded that the eucalyptus trees that initially showed less development with weed competition showed a recovery capacity. In practice, this would decrease the critical periods of interference when the evaluation is carried out after longer periods.

The authors of [28] studied the effects of the coexistence period of the main weed species of initial reforestation in E. grandis, at 2.6 years of age in Guatapará, SP. Eucalyptus trees that coexisted with weeds for 28 days showed a decrease in wood production of 19.7% compared to those that grew under weed control for a period of 364 days. The eucalyptus trees that were under weed control for up to 140 days had a production reduction of 14.4% in relation to those plants that were without competition for a period of 168 days; this effect remained constant in the other periods longer than 168 days.

4.2. Forest Biomass

The highest proportion of leaves and branches in relation to the total biomass in each treatment was observed in the Ce378 treatment, at 8.2 and 9.4%, respectively. Possibly, the competition with weeds impeded the eucalyptus development, especially that of the wood biomass. For stembark, the highest proportion was in the treatment with coexistence up to 140 days, at 13.3% of the total biomass, and for wood, the greatest representation was in the control treatment up to 168 days, at 82.3%. By contrast, the lowest proportions for leaves and stembark were in the control treatment up to 168 days, at 6.4 and 4.3%, respectively. The smallest proportion of branches was found in the treatment with coexistence up to 140 days, at 5.5%. For wood, the lowest proportion was in the control treatment up to 28 days, at 72.2% of the total biomass. In 4-year-old E. saligna stands, the stemwood and stembark fractions were responsible for about 80% of the total biomass [29].

An evaluation of biomass production in a stand of E. grandis × E. urophylla trees at 54 months of age in the municipality of Alegrete, southern Brazil [30] reported 3.4, 11.3, 10.1, and 109.9 Mg ha⁻¹ for leaves, branches, stembark, and stemwood, respectively. The total biomass estimated by the authors was 134.7 Mg ha⁻¹, representing an increase of 84.3% in relation to the continuous coexistence treatment and 36.7% in relation to the Ct378 treatment.

In the present study, the average proportion of the wood component was 75.4% and the average proportion of leaves was 7.3%; by contrast, the authors of [31] evaluated the biomass production in E. grandis × E. urophylla trees at 9 years of age and found a total biomass of 170.6 Mg ha⁻¹, of which 2.8, 5.7, 16.7, and 145.5 Mg ha⁻¹ were accounted for by the leaf, branch, stembark, and stemwood components, representing 1.6, 3.3, 9.8, and 85.2%, respectively. The authors of [32] studied biomass production and found contributions of 89.3, 6.2, 3.2, and 1.2% for the stemwood, stembark, branches, and leaves, respectively. In general, younger stands have a higher proportion of leaves and branches in the total biomass compared to mature stands [33].

In a stand of E. grandis in the north of Rio de Janeiro, the authors of [34] found values of 2.7, 11.2, 12.9, and 88.4 Mg ha⁻¹ for the leaves, branches, stembark, and stemwood, respectively, in the first rotation area of 8 years, and 2.2, 6.5, 9.2, and 43.3 Mg ha⁻¹ for the leaves, branches, stembark, and stemwood, respectively, in a 5-year-old coppice management area.

4.3. Nutrient Concentration

In general, the leaf component had the highest concentration of nutrients, and the wood component had the lowest. This same behavior, with a higher concentration of nutrients in the leaves, was found by the authors of [30] in different genotypes and species of 4.5-year-old stands in Alegrete, southern Brazil; by the authors of [35] in stands of different genotypes in São Gabriel, southern Brazil; and by the authors of [31] when evaluating pure and mixed stands of E. grandis × E. urophylla and A. mearnsii trees of 9 years of age in southern Brazil. According to [36], the concentration gradient is as follows: leaves > stembark > branches > stemwood.

According to [37], considering eucalyptus species, the following premises can be established: (a) The higher the foliar concentration of the nutrient, the higher the absorption efficiency, the exhaustion of nutritional reserves in the soil, the chances of overcoming interspecific competition, and the nutritional status. On the other hand, a high foliar nutrient concentration will reduce the nutrient utilization efficiency. (b) The opposite reasoning will be valid for lower concentrations of nutrients in the leaves.

Studies have indicated high concentrations of Mn for the genus Eucalyptus [38,39], corroborating our results. Therefore, the elements N, S, B, Fe, Zn, and K were apparently the most limiting for the maximum growth of the species. According to [40], trees that coexist in a competitive environment with weeds may develop a deficiency in some nutrients. According to [41], productivity is compromised in infested areas due to the reduction in nutritional content.

In the stembark, there was a significant difference (p ≤ 0.05) only for B, with the coexistence up to 378 days treatment (Ce 378) having 53.04 mg kg⁻¹ and the control up to 378 days treatment (Ct 378) having a concentration of 34.08 mg kg⁻¹. The Ce 378 treatment showed above-average values for all nutrients except B, Cu, and Mn. The authors of [42] analyzed stands of E. camaldulensis and E. grandis at 9 years of age and E. torelliana at 12 years of age and observed that the highest concentrations of Ca and Mg were present in the stembark, which matches the results obtained in this study.

Similar to the present study, the authors of [43] compared the nutrient contents in different biomass components of E. grandis × E. urophylla trees at 10 years of age in southern Brazil. According to the authors, the highest levels of the nutrients Ca and Mg were found in the stembark, differing statistically in relation to the levels in the other components. However, as it was a mature stand, the respective concentrations were significantly lower.

The N content, which is much higher in the leaves than in the other components, is present because it participates in most compound metabolism reactions (amino acids, proteins, amines, amides, vitamins, etc.), which occur mainly in the leaves due to photosynthesis [44].

Nitrogen and phosphorus are highly mobile in the plant and, therefore, are concentrated in new tissues; in the case of this study, they were concentrated in the leaf component. In the case of Ca, its immobility in the phloem could explain its high concentration in the stembark fraction, as well as the fact that the element is a structural component, forming part of the middle lamella of the cell membrane [45].

4.4. Nutrient Stock

The authors of [35], when comparing the stock of nutrients in different genotypes and species of eucalyptus trees at 43 months of age, found a greater proportion of the leaf and branch components in the stock of nutrients when compared to more mature stands. According to the authors, the amounts of nutrients allocated to the leaf biomass of hybrid E. grandis × E. urophylla trees were 143, 8, 59, 22, and 7.8 kg ha⁻¹ for N, P, K, Mg, and S, respectively. Regarding micronutrients, the values observed were 65 and 110 g ha⁻¹ for Cu and Zn, respectively. These results are consistent with the present study, possibly due to the similar maturity, except for Ca, B, Fe, and Mn.

The authors of [34] evaluated the nutrient contents in the leaves of a stand of E. grandis trees at 8 years old in the first rotation in Norte Fluminense, and they found 57, 2.81, 43, 23, and 8.9 kg ha⁻¹ of N, P, K, Ca, and Mg, respectively, and 43, 1.2, 36, 16, and 8.4 kg ha⁻¹ of N, P, K, Ca, and Mg for a 5-year-old coppice, respectively. The authors of [46] evaluated the stock of nutrients in the biomass of hybrid E. grandis × E. urophylla trees at 54 months of age and found nutrient contents in the leaves of 54, 4.2, 22.9, 20.2, 9.4, and 4.1 kg ha⁻¹ for N, P, K, Ca, Mg, and S, respectively. It was observed that the lower amounts in the present study were associated with a decrease in the proportion of leaf biomass by the maturity of the stand.

In the branches, only phosphorus showed a statistically significant difference (p ≤ 0.05) between the control treatment up to 168 days (T18) and the coexistence treatment up to 140 days (T6), with 8.4 kg ha⁻¹ and 4.3 kg ha⁻¹, respectively. Considering the average stock of nutrients in the biomass of branches, the values observed in the present study were 26.2, 6.7, 33.2, 63.6, 6.9, and 1.8 kg ha⁻¹ for N, P, K, Ca, Mg, and S, respectively, and 44.4, 49.2, 188.8, 2669.4, and 65.5 g ha⁻¹ for B, Cu, Fe, Mn, and Zn, respectively. These results are similar to those reported in [47], where the authors investigated a hybrid of E. grandis × E. urophylla of the same age in southern Brazil. In the stembark, there was no statistically significant difference (p < 0.05) between the treatments.

By analyzing the nutrient content in 10 species of eucalyptus trees, the authors of [48] also verified a tendency toward the accumulation of nutrients in the stembark, mainly for calcium and magnesium. The leaves had a greater representation of nitrogen, reinforcing the results found in our study.

4.5. Nutrient Utilization Efficiency

Other studies corroborate the results obtained here, such as [29], the authors of which evaluated the EUN in a stand of hybrid E. grandis × E. urophylla trees at 54 months of age in southern Brazil. According to these authors, the EUN in the wood was 0.93, 17.17, 0.73, 4.19, 5.31, and 4.70 Mg ha⁻¹ for each kg of N, P, K, Ca, Mg, and S. The highest EUN for P reported in [30] can be attributed to the low concentration of the nutrient in the biomass tissues. A low concentration tends to increase the efficiency of biomass production. In this case, the P concentration in wood reported by the authors was only 0.10 g kg⁻¹, whereas for the present study, it was 0.37 g kg⁻¹.

The authors of [46] also evaluated the EUN in a hybrid of E. grandis × E. urophylla at 43 months of age in the west of the state of RS. According to the researchers, the EUN was 0.69, 6.92, 0.48, 1.23, 1.93, and 5.51 Mg ha⁻¹ for each kg of N, P, K, Ca, Mg, and S, respectively. In general, only S presented a distinct EUN in relation to the present study. This can be attributed to the fact that the experiment conducted by the authors of [46] was carried out in an initial-stage stand, and consequently, there was a lower biomass stock. Another factor that may explain this is the higher concentration of S in the soil and wood, being 16 mg dm⁻³ and 0.18 g kg⁻¹, respectively.

The NUE variation in total biomass in ten Eucalyptus species [48] was: N (406–583), P (2320–4753), K (270–537), Ca (122–446), and Mg (1144–2048). Only P presented a lower NUE in our study when compared to the results presented by the researchers.

5. Conclusions

According to the two treatment groups, the total interference prevention period was 168 days (group 2). The maintenance of weeds over 378 days promoted a significant reduction in the production of total biomass and wood in hybrid E. grandis × E. urophylla trees, and this reduction in productivity was 34%. The stemwood represented 81.4% of the total biomass for the control treatment up to 168 days, while it represented only 53.7% of the total biomass for the coexistence up to 378 days.

Wood had the lowest average concentration for all nutrients; on the other hand, it was the component that obtained the highest stocks of P, K, B, Cu, Fe, and Zn. Regarding the different coexistence periods, the control treatment up to 168 days promoted the highest stocks of macronutrients and for the micronutrient boron in the wood component. The highest EUN was observed in wood. The leaves had the lowest EUN for N, P, K, S, B, Cu, Fe, and Zn. The control treatment up to 168 days was more efficient in the production of wood biomass for N, Ca, and Mg.

Through the results of the study, it was observed that the treatment involving the control of weeds for up to 168 days promoted greater productivity than controlling weeds for 378 days. This recommendation can be adopted by plantation managers for this region, reducing the amount of herbicide applied on the stand.

More studies are needed to evaluate the impact of weeds on productivity at the end of the rotation. In addition, it is recommended that treatments should be evaluated bimonthly until the closure of the canopies.