Next Article in Journal
Indigenous Contestations of Carbon Markets, Carbon Colonialism, and Power Dynamics in International Climate Negotiations
Previous Article in Journal
Recent Advances in Long-Term Wind-Speed and -Power Forecasting: A Review
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Climate
Volume 13
Issue 8
10.3390/cli13080156
climate-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Analysis of Olive Tree Flowering Behavior Based on Thermal Requirements: A Case Study from the Northern Mediterranean Region

by
Maja Podgornik
1,*,
Jakob Fantinič
1,
Tjaša Pogačar
2 and
Vesna Zupanc
2
1
Science and Research Centre of Koper, Institute for Oliveculture, 6000 Koper, Slovenia
2
Biotechnical Faculty, University of Ljubljana, 1000 Ljubljana, Slovenia
*
Author to whom correspondence should be addressed.
Climate 2025, 13(8), 156; https://doi.org/10.3390/cli13080156
Submission received: 28 May 2025 / Revised: 17 July 2025 / Accepted: 19 July 2025 / Published: 23 July 2025
(This article belongs to the Section Climate Adaptation and Mitigation)
Browse Figures
Versions Notes

Abstract

In recent years, early olive fruit drop has been observed in the northern Mediterranean regions, causing significant economic losses, although the exact cause remains unknown. Recent studies have identified several possible causes; however, our understanding of how olive trees respond to these environmental stresses remains limited. This study includes an analysis of selected meteorological and flowering data for Olea europaea L. “Istrska belica” to evaluate the use of a chilling and forcing model for a better understanding of flowering time dynamics under a changing climate. The flowering process is influenced by high diurnal temperature ranges (DTRs) during the pre-flowering period, resulting in earlier flowering. Despite annual fluctuations due to various climatic factors, an increase in DTRs has been observed in recent decades, although the mechanisms by which olive trees respond to high DTRs remain unclear. The chilling requirements are still well met in the region (1500 ± 250 chilling units), although their total has declined over the years. According to the Chilling Hours Model, chilling units—referred to as chilling hours—represent the number of hours with temperatures between 0 and 7.2 °C, accumulated throughout the winter season. Growing degree hours (GDHs) are strongly correlated with the onset of flowering. These results suggest that global warming is already affecting the synchrony between olive tree phenology and environmental conditions in the northern Mediterranean and may be one of the reason for the green drop.

1. Introduction

Climatic conditions generally play a critical role in tree phenology and adaptation. In the Northern Hemisphere, the seasonal climate of temperate regions has led deciduous trees to adapt by going dormant in winter, during which there is no active growth, low metabolic activity, and high frost tolerance [1]. This dormancy allows fruit trees to survive the cold winter and undergo synchronized bud break when cold accumulation removes dormancy and spring temperatures favor bud growth and development [2]. As an evergreen species, olive trees do not have a true dormancy, i.e., they do not completely stop physiological activity during the winter. A major difference between olive trees and deciduous trees, where reproductive differentiation takes place within the bud before the onset of dormancy, is that the morphology of olive tree buds remains undifferentiated, i.e., neither vegetative nor reproductive, until the release of dormancy the following year, when bud reproductive induction and initiation are evident [3,4,5].
Defining dormancy is difficult due to its complexity; however, a nomenclature has been established: endo-dormancy (internal physiological signals such as chilling and photoperiod), para-dormancy (physiological signals from other parts of the plant such as apical dominance), and eco-dormancy (environmental factors such as temperature and water stress) [6]. Temperature is the main factor determining the phenology of the olive tree. It regulates the release from endo-dormancy once sufficient cold units have accumulated during winter (measured as the number of hours with an air temperature below 7.2 °C according to the Chilling Hours Model) and the release from the eco-dormancy period, the duration of which is based on the accumulated forcing units between the end of endo-dormancy and flowering [7].
Current and future global warming could disrupt this complex synchronization between endo-dormancy, eco-dormancy, and environmental conditions, leading to changes in phenological timing, especially flowering [7,8,9]. An increase in temperature can deteriorate the chilling conditions [10]. According to some authors, the period (from October to February) is when the process is reversible and the buds induced to flower can return to the vegetative state if they are not cold enough or if they lack sugars or substances produced by the leaves [11,12,13]. Depending on the variety, the cold requirement of flower buds varies, in contrast to vegetative buds, which do not require chilling to sprout [12]. Accordingly, olive trees planted under tropical conditions usually do not produce fruit, which is usually due to the lack of sufficient chilling accumulation [10,14]. Inadequate chilling reduces the number of inflorescences, increases flower abortion, deforms flower buds, and leads to asynchronous bud burst, which ultimately results in a low fruit set and a lower final yield [15,16,17].
Unsuitable environmental conditions (moisture stress, low or high temperatures, dry winds, fog, or precipitation) during flowering can severely affect pollen quality (pollen viability, embryo sac viability, rapid growth of pollen tubes, and pollen germination), and consequently, fruit set and yield [10]. Previous studies have shown that drought stress during inflorescence development can lead to incomplete flower development [18]. Water deficit during inflorescence development reduced many different flowering parameters, including inflorescence number, flower number, imperfect flower number and percentage, and ovule development. When a water deficit occurs during flowering and initial fruit set, the petals of many flowers dry and abscise as a unit, exposing a senescent stigma that is no longer receptive to pollination. Fruit formation closely follows the parameters of flowering, with a low fruit number when the flower number is reduced or fertilization is inhibited [18]. In addition, flowering and fertilization in non-irrigated olives are highly sensitive to water deficit during the winter months [19]. However, adding water during inflorescence and flower formation, as well as during fertilization and the initial stages of fruit development, can increase the number of fruits per inflorescence and fruit density per shoot [20]. In this context, it should also be noted that olive fruits do not only develop from the normally fertilized ovaries of olive flowers. Olive fruits often appear normal, but the endocarp lacks a seed because the embryo was aborted in the early stages after fertilization [21]. If olive trees are exposed to environmental stress during the early stages of fruit development, fruit drop can occur. Fruit drop is a self-defense mechanism that allows trees to adapt their fruit load to the environmental conditions of a given year [22].
Olives are one of the most important fruit crops in Slovenia. Olive growing is of great socio-economic importance in the southwestern part of Slovenia (Slovenian Istria), which is the main olive-growing area in the country. According to the Slovenian Ministry of Agriculture, Forestry, and Food, this crop is grown in an area of about 2600 ha.
In recent years, a new phenomenon has emerged in the northern Mediterranean growing areas, including Slovenia, northern Croatia, and northern Italy: premature fruit drop in olives, which leads to significant production losses [23,24]. The real cause of the premature drop of green olives in the first stage (BBCH 71–75—fruit size is about 10–50% of the final size) of fruit development is still unknown. Recent studies have identified several possible causes, including infestation by the invasive brown stink beetle (Halyomorpha halys) [24], fungal diseases, and climate change [23,25]. However, knowledge of the reactions of olive trees to these environmental stresses remains limited.
Therefore, our research aims to determine whether recent climatic changes in the northern Mediterranean region, particularly reduced winter chilling, influence important phenological processes by analyzing long-term meteorological and flowering data.

2. Materials and Methods

2.1. Experimental Site

This study was conducted in Slovenian Istria, which, in a geographical sense, is the only indisputably Mediterranean part of Slovenia [26] and covers just over 300 km2 [27] (Figure 1). The Mediterranean is a highly heterogeneous region with a highly diverse climate, leading to diverse climate types throughout the region due to its unique geographical location [28]. Therefore, the northern Mediterranean regions tend to be temperate and humid, while the southern Mediterranean regions are relatively warm and dry [29,30]. This study was conducted in the coastal area of Slovenian Istria, focusing on the general agri-environmental conditions that characterize olive-growing areas in this Mediterranean region. No specific experimental plantations were selected, but an area analysis of olive groves in the vicinity of the Portorož meteorological station was conducted. In this area, the olive groves share similar cultivation characteristics, including a planting density of approximately 240 trees ha−1 (tree spacing 7 m × 6 m). Similar agronomic practices have been applied throughout the region, especially in terms of pruning and plant protection, which correspond to the standard practices in Slovenian Istria.
Slovenian Istria belongs to the northern Mediterranean region, and its relief and climate are its most pronounced natural features. The relief results from the close alternation of flysch and limestone bedrocks and the low altitude (up to 400 m a.s.l.) near the sea [31]. According to climate data, the area has a sub-Mediterranean climate. It is characterized by high but irregular rainfall, with high precipitation in autumn, at the end of spring, and at the beginning of summer, and low precipitation in winter and summer [32]. The region has a semi-arid climate, characterized by low rainfall and high summer temperatures, which have contributed to increasingly frequent droughts in recent years [33].
During the 1991–2020 period (ARSO—Slovenian Environment Agency, 2023a [34]: data for Portorož station), it was characterized by sunny days (up to 2700 h year−1) with monthly mean air temperatures ranging from 4.9 °C in January to 23.4 °C in July. The mean annual precipitation for the period was 958 mm, and the mean annual reference evapotranspiration was 1086 mm, representing the loss of water to the atmosphere through evaporation and the transpiration of a reference plant, usually a well-watered grass.

2.2. Olive Variety

The leading olive variety in Slovenia is “Istrska belica”, which currently accounts for 70 % of all olive trees in productive orchards, according to the Register of Agricultural Holdings. “Istrska belica” is also the most widespread olive variety in the northern part of the Adriatic region (Slovenian Istria and Italian Venezia Giulia) [35,36]. Since the 1956 frost, this variety has been known for many positive characteristics (excellent adaptability to pedoclimatic conditions, resistance to low temperatures, very good and regular fertility, and high oil content), which have contributed to its rapid spread [35]. “Istrska belica” is the main variety for two protected designations of origin: “Extra virgin olive oil from Slovenian Istria” (Slovenia) and “Extra virgin olive oil Tergeste P.D.O.” (Italy). The most notable characteristic of this olive oil is its high biophenol content, which gives it a specific taste characterized by bitterness and pungency [33].
Owing to its wide distribution and importance in the northern Adriatic region, the analyses in this study were performed using this variety.

2.3. Meteorological Data

The meteorological data for the last thirty-four years (1989–2022), covering the period from January to December of each year, are available for the Portorož meteorological station and were obtained from the Slovenian Environment Agency [37]. The daily and hourly average, minimum (Tmin), and maximum (Tmax) air temperatures at 2 m above the surface were used. The diurnal temperature range (DTR) was calculated as the difference between Tmax and Tmin. Daily precipitation, average temperature, maximum temperature, minimum temperature, average wind speed, relative air humidity, and sun hours were used to evaluate the degree of drought using the standardized precipitation evapotranspiration index calculated with the help of the R package SPEI [38], which uses precipitation and potential evapotranspiration. In this study, we used the SPEI at a 12-month timescale (SPEI-12), which reflects long-term drought conditions by accumulating monthly climatic data over the preceding 12 months. Values were classified into different categories, namely, extremely wet, severely wet, moderately wet, normal climate, moderately dry, severely dry, and extremely dry, according to the literature [39]. It is important to use SPEI-12, as bud differentiation starts in summer and affects the occurrence of inflorescences in the following year. The correlation between the diurnal temperature range (DTR) and the onset of flowering was assessed using the Pearson correlation method.
To assess the conditions that cause frost damage to plants, the average daily maximum (Tmax) and minimum (Tmin) temperatures from 1 November to 1 April were analyzed for all years between 1989 and 2022. For each year, the temperatures in the month before the onset of frost were analyzed because early spring thaws can influence the initiation of physiological processes in plants. In addition, the duration of consecutive days with Tmax below 0 °C was quantified.

2.4. Phenological Data

Phenological data (BBCH 60—First flowers open) [40] covering the last 34 years (1989–2022) were used in this work and were obtained from the publicly available database of the Pan European Phenology Project PEP725, which contains data on the main phenological phases of olive trees for the Portorož meteorological station since 1960. PEP725 is a project funded by the Zentralanstalt für Meteorologie und Geodynamik, the Austrian Federal Ministry for Education, Science, and Research, and EUMETNET (the network of European meteorological services), with the aim of creating a publicly accessible database of phenological datasets for science, research, and education.

2.5. Calculations of Chill Units and Growing Degree Hours

Different olive cultivars show variations in their chilling requirements, which must be satisfied to promote bud break, and exhibit different phenological patterns. Geographical features, management practices, and biotic factors also impact physiological development and determine changes in thermal requirements [41]. Given this phenotypic plasticity, the numerical equations used in thermal models must be calibrated for each specific location, and thermal accumulation periods must be optimized for different olive-growing areas [41,42]. To determine the accumulation periods for the chilling and forcing units, we applied a methodology based on the available thermal models and meteorological data.
The first step was to determine the appropriate chilling accumulation periods, taking into account the temperature thresholds and phenological stages of the olive trees, using historical temperature data to determine when the temperatures fell within the required ranges for the chilling and forcing units. Following the statistical approach proposed by Luedeling et al. [42] and refined by Rojo et al. [41], we estimated the chilling and heat accumulation periods for Portorož (Slovenia) using partial least squares (PLS) regression.
Meteorological and phenological data were used as inputs in four different models from the R library. The models were selected based on their suitability. The first was the Chilling Hours Model [43], which accumulates all hours with temperatures between 0 and 7.2 °C [44,45]; the model interval chosen was the most suitable for olives. The second is the Utah Model, developed by Richardson et al. (1974) [43], which calculates chilling accumulation based on hourly temperature thresholds and assigns different weights to the temperature ranges. It assigns an accumulation of 0.5 for temperatures between 1.4 and 2.4 °C and between 9.1 and 12.4 °C, and an accumulation of 1 for temperatures between 2.4 and 9.1 °C. This model assigns a reduction in chilling effectiveness based on a negative accumulation of −0.5 for temperatures between 15.9 and 18 °C and −1 for temperatures above 18 °C [43]. The Dynamic/Chill Portions Model [46] assumes that winter chill accumulation results from a two-step process. First, an intermediate chilling product is generated at lower temperatures, which can be converted into permanent chill accumulation at moderate temperatures or removed at higher temperatures. The model sums the “chill portions” over the course of winter [47]. The last is the Heat Accumulation or growing degree hours (GDHs) model [48], which is based on three temperature thresholds for physiological effects. The critical temperature was set at 36 °C, and the lower and upper thresholds were optimized for olive trees at the study site. The lower threshold was evaluated in the range of 0 to 10 °C, and the upper threshold in the range of 0 to 35 °C. The chilling and heat accumulation periods and models used were described by Rojo et al. [41].
The equations for these models were described in [49] and implemented for R (v1.4.1717; R Core Team), a free software environment for statistical computing and graphics, using the chillR package [41,42]. Reference evapotranspiration was determined using the FAO Penman–Monteith method, which is the standard method recommended by the Food and Agriculture Organization (FAO) for such calculations. The computations were performed using the FAO56 R package version 0.1.0 [50].

3. Results and Discussion

3.1. Late Spring Frost

Extraordinary cold air intrusions are an increasingly frequent phenomenon in the northern Mediterranean regions, causing frosts during olive cultivation (1929, 1956, 1985, 1996, 2012, 2018, and 2021) (Table 1). The frequency of winter frost is apparently increasing with global warming, as shown by Žnidaršič et al. [51] for various grapevine, apple, and sweet cherry varieties at locations across Slovenia. In addition to the occasional cold waves, frost in olives is also influenced by warm weather or, better, high temperatures before the frost, which have a favorable effect on the onset of physiological processes in the plant. Analysis of meteorological data (Tmin and Tmax) between November and March in years with cold winters in the period from 1989 to 2022 for Slovenian Istria showed that olives were not affected by winter frost in cold winters when Tmin was below 0 °C and Tmax was above 0 °C. Catastrophic effects on olive leaves, branches, and trunks were mainly observed in years (1985, 1996, 2012, and 2018) when Tmax fell below 0 °C for at least two consecutive days after being above 6 °C on average for the previous 14 days.
As highlighted by Petruccelli et al. [52] in a review article, temperatures below 4 °C or around 0 °C markedly compromise plant growth and productivity and cause a delay in flowering. Furthermore, the reproductive organs, flower buds, flowers, and fruit are seriously damaged.
In most regions where olives are cultivated, trees cease vegetative growth in autumn and enter a period of winter dormancy that lasts until the return of favorable temperature conditions in early spring. This dormancy period appears to be essential for increasing frost tolerance, as is the case for many perennial species [53,54], and it coincides with the period when the chilling requirement for flower differentiation in axillary buds is fulfilled [55,56]. Thus, the onset of spring growth often coincides with the mean long-term probability of frost risk in a given region, such that perennial plants are rarely damaged by frost [57]. This synchronization is the result of the complex response of plants to environmental cues, such as chilling duration in winter, which is responsible for triggering dormancy [58] as well as the increasing photoperiod and warmer spring temperatures, which promote bud break (reviewed in [1,59]). Current and future global warming could disrupt this complex synchrony in natural vegetation [45]. The disrupted synchrony between olive phenology and environmental conditions was clearly demonstrated in early April 2021, when the lowest minimum air temperature fell below −2 °C and Central Europe experienced four days of severe frost.
Table 1. Frost damage on olives in Slovenian Istria over the last 100 years, with information on Tmin and Tmax.
Table 1. Frost damage on olives in Slovenian Istria over the last 100 years, with information on Tmin and Tmax.
YearTimeframe of FrostLowest Tmax [°C]Number of Days with the Tmax Below 0 °CLowest Tmin [°C]14-Day Average Tmax [°C] Before the Occurrence of Tmax Below 0DamageReference
192911 February–13 February 1929
Winter frost		//−14.3/90% of trees affected by frost[60]
19562 February–20 February 1956
Winter frost		//−12.8/30% of trees affected by frost[60]
19851 January–18 January 1985
Winter frost		−5.66−9.37.6460% of trees affected by frost[60]
199627 December–29 December 1996 Winter frost−4.63−8.510.52Young trees completely damaged[61]
20121 February–12 February 2012
Winter frost		−2.44−5.96.7910% of trees affected by frost[62]
201825 February–1 March 2018
Winter frost		−3.22−5.79.230% of trees affected by frost[62]
20214 April–9 April 2021
Spring frost		10.80−2.3/More than 50% reduced yield[30]
A sudden cold snap in April 2021 damaged numerous crops at a crucial time in their development, including olive trees, especially in the northern Mediterranean, where an anomalously warm March shortened winter dormancy and hastened the vegetative growing season. Our research showed that the damaging frost in April 2021 was unprecedented after 1929, when 90% of olive trees in the Gulf of Trieste, Slovenian Istria, and eastern Friuli were destroyed by the winter frost. The damaging April 2021 frost was unprecedented in the northern part of the Mediterranean in terms of the calendar date (late spring frosts) and the type of damage to olive trees over the last 93 years. Since 1929, destructive frosts affecting olive trees have been recorded in December (1996), January (1985), February (1929, 1956, and 2012), and March (2018); however, 2021 was the first time in which a frost was recorded in April. Therefore, the typical symptoms used to assess the extent of frost damage, such as burns on shoot tips, complete defoliation, or cracks in the bark of branches or the trunk, as reported by Sanzani et al. [63], were not observed. In 2021, an abnormally premature fall was observed in green olives. This is one of the main reasons for the drastic decline in olive production in the northern Mediterranean region. Similar consequences occurred in California after the cold spring of 1967, which delayed flowering by several weeks and resulted in blossom abnormalities and a crop yield of only 14,000 tons, the smallest in California’s modern history. The prolonged, unusually cold weather in April and May, when olive tree buds develop rapidly, negatively affected the later stages of flowering, pollination, and fruit set [64].

3.2. Spring Drought

Studies from the 1980s indicated that drought was not a significant threat to olive production in the northern parts of the Mediterranean [65], but SPEI-12 of Vicente-Serrano et al. [39] showed that severe droughts occur in the region approximately once per decade (Figure 2).
Notably, the damage caused by drought stress cannot be attributed solely to a lack of rainfall, as several factors must be considered, such as the growth stage of the plants, soil properties, resistance of the variety, and other variables. Owing to the interactions of various factors, olive growers in the Slovenian Mediterranean region have faced nine highly severe droughts over the last decade (2011, 2012, 2013, 2015, 2016, 2017, 2019, 2021, and 2022). According to the Joint Research Center of the European Commission (EC-JRC), the extreme drought of 2022 was the most severe in the last 30 years and unprecedented in the last 500 years in this region [66]. This was most likely the result of a precipitation deficit between winter and spring, which was exacerbated by early heat waves. For example, the experimental site received only 138 mm of precipitation between January and May 2022, which is a significant deviation from the long-term average of 310 mm (1991–2020). The decrease in soil water availability caused by this precipitation deficit and concurrent high temperatures resulted in widespread vegetation stress during olive flowering, which persisted until the stone fruits were fully hardened.
Olive-growing areas in the northern Mediterranean are expanding due to climate change, especially in the flat and mountainous areas of northern Italy and western Slovenia [67,68]. Further research on premature fruit drop will provide vital information for adapting to sustainable and economically viable olive production under changing climatic conditions.

3.3. Changes in the Flowering of Olive Trees and Diurnal Temperature Range

Analysis of the phenological data shows that the onset of flowering in Slovenian Istria occurs on average on May 29 (DOY 150 ± 8, mean ± standard deviation), which is later than in warmer climates such as Tunisia (Chaal) (April 18—DOY 108 ± 7), southern Italy (Lecce) (May 14—DOY 134 ± 5), and central Spain (Toledo) (May 15—DOY 135 ± 8), as reported by [31]. However, the Slovenian Environment Agency has reported that warmer-than-average years have become highly frequent since the mid-1980s, and most of the warmest years since the middle of the last century have occurred recently. In Slovenia, the mean annual temperature increased significantly by about 0.36 °C per decade from 1961 to 2019 [69]. As temperature is one of the most important factors in the flowering phenology of olive trees, we investigated how the timing of flowering initiation has changed over the last three decades (Figure 3). The results emphasize a trend that has persisted throughout the study period, according to which the onset of flowering (i.e., the timing of blooming) has occured earlier over time.
To detect changes in trends during the study period, we applied three complementary statistical methods: breakpoint analysis using linear regression with structural change detection (strucchange package) [70,71], Pettitt’s test for identifying a single change-point in the time series (trend package) [72], and the Standard Normal Homogeneity Test (SNHT) for detecting shifts in the mean (trend package) [72]. All three methods consistently identified 2006 as the primary breakpoint, indicating a significant change in the trend that began in that year (Figure 4). In the first period (1989–2005), flowering began on average on DOY 153.2, 7.7 days later than in the second period (2006–2022), in which flowering began on DOY 145.5. In addition, the second period showed much greater variability, as demonstrated by the increase in the standard deviation from 4.89 in the first period to 8.21 in the second. This increased variability contributes to greater uncertainty and instability in olive cultivation. This is supported by other studies that have found that the timing of flowering onset has shifted in recent decades because of rising air temperatures [73,74,75].
However, the effects of climate change on shifts in flowering times are complex. DTRs have detrimental effects on many morphological and physiological characteristics of plants. They exponentially increase forward and reverse biochemical reactions, resulting in enzyme denaturation. Plants evolve resilient mechanisms to survive under temperature stress by limiting the adverse effects on metabolic and physiological processes [76]. Considering that DTRs play a key role in aspects of plant development and that little information is available on their effects on olive trees, although a 28-year study has revealed remarkable climatic trends affecting phenological phases [77], we analyzed the meteorological data to determine whether olive groves in the northern Mediterranean region are exposed to high DTRs.
The analysis of meteorological data for all months showed that the highest DTRs occurred in April between 1989 and 2022. Despite year-to-year variations due to various climatic factors, a steady increase in the DTR values with higher variability has been observed since 2006. Mann-Whitney U test revealed a statistically significant DTR divergence (p < 0.001) in April when we compared the data from 1989 to 2005 with the data from 2006 to 2022. The April DTR averages for the first and second periods were 10.1 °C and 11.1 °C, respectively.
The influence of the average monthly DTRs on the onset was also observed. A moderate to strong statistically significant negative correlation was found between the average monthly DTR values during the pre-flowering period and the onset of flowering (p < 0.001) (Figure 5). Lower DTRs in the month before flowering may result in a delayed onset of flowering. However, the higher April DTRs observed after 2006 may have resulted in earlier flowering.
These findings align with those of a similar study in central Italy [78], which showed that temperature changes significantly affect the spring phenology of olive trees, which respond to rising temperatures with earlier flowering. Olive trees adjusted the Growing Degree Days (GDDs) required for flowering once a threshold of maximum advancement was reached. Before 2004, the trees accumulated similar GDD values, but as temperatures increased, earlier flowering occurred at fixed GDD values of 650 and 750. After this point, higher GDD values accumulated, which stabilized the flowering date and prevented further advancement. This phenological plasticity enables olive trees to adjust their life cycle and reproductive stages in response to environmental changes. This helps them survive under changing conditionsand prevents excessively early flowering [78].
However, other factors may influence the onset of flowering, such as the amount of light, soil water availability [33,79], altitude, exposition [79], nutrients, and the genetic makeup of the plant [80]. DTR plays a decisive role in the onset of flowering. In most parts of Slovenia, the strongest warming can be observed in the spring and summer, at about 0.4 and 0.5 °C per decade, respectively, with warm climate regions warming more than cold ones [81]. The increasingly frequent phenomenon of shorter winter dormancy caused by anomalously warm conditions could lead to an earlier start of the vegetative growth phase. In addition, changes in atmospheric circulation patterns also contribute to extreme temperature trends. Cold air masses with temperatures below freezing can move from northern Europe to the northern Mediterranean region in spring [81]. The combination of warmer winters and earlier springs, interrupted by cold air intrusions and sudden cold snaps in late spring in the northern Mediterranean, leads to high DTR values in late spring, which can delay the start of flowering. These temperature fluctuations can influence the onset of flowering. Therefore, it is important to consider the DTR along with other factors, such as water availability, to gain a comprehensive understanding of its effects on the onset of olive flowering.

3.4. Cold (Chilling) and Heat (Forcing) Requirements

3.4.1. Estimation of the Thermal Accumulation Period

The PLS regression analysis revealed the effect of daily temperature on the onset of the flowering period, and the most influential days were highlighted using a variable importance in projection (VIP) value of 0.7. VIP quantifies the importance of independent variables in explaining the variation in dependent variables. In this study, we examined the importance of temperature on flowering date.
According to the data presented in Appendix A (Figure A1), the critical period for chilling accumulation was between October 4 and November 15, about, one month earlier than in the southern (Lecce, Italy, and Chaal, Tunisia) and central Mediterranean (Toledo, Spain) (between late autumn and early winter) [41]. The heat accumulation period was between November 15 and May 10, which is longer than that in the southern Mediterranean (Lecce, Italy, and Chaal, Tunisia) and similar to that in the central Mediterranean (Toledo, Spain).
This might be explained by the impact of the continental climate, which, according to Rojo et al. [41], establishes the future conditions necessary for inducing ecodormancy.
These findings are also supported by data from El Yaacoubi et al. [82] and Gabaldón-Leal et al. [45], suggesting that heat accumulation is more important than chilling requirements in olive flowering, and that the timing of flowering is closely related to heat accumulation. Further analysis of the data obtained using the chilling and forcing models revealed a strong statistically significant negative correlation between the growing degree hours and chilling (Appendix B, Figure A2). Although it is not known whether the negative correlation between chilling and heat accumulation is correctly explained by these ecosystem models, Wang et al. [83] showed that more than half (seven out of 12) of the current chilling models are invalid, as they have a positive relationship between chilling and heat accumulation because they do not account for the effects of freezing temperatures on the release of dormancy. Freezing temperatures (<0 °C) can slow down or interrupt the release of dormancy. If this is not taken into account, the models may incorrectly record the chilling during ineffective periods, leading to falsified results.

3.4.2. Models for Calculating Chilling Requirements Using Different Chilling Models

The difference between the northern and southern and central Mediterranean regions was also noted in terms of chilling requirements. Figure 6 and Appendices C (Figure A3) and D (Figure A4) illustrate the data from the three models used to calculate chilling requirements. Although Omran et al. [84] reported that the Chilling Hours Model for conditions in Egypt does not reflect the number of actual chilling units and requires more complicated mathematical methods, the results of our study showed that this model was the most appropriate for the conditions in this study compared with the others tested (the Utah Model and Chill Portions Model). A medium statistically significant positive correlation was found between chilling hours and the onset of flowering (Pearson’s r = 0.680, p < 0.001).
The average chilling accumulation at the Portorož station was 1588 ± 235 chilling units, calculated according to the Chilling Hours Model. As most olive cultivars have a chilling requirement of 200–600 chilling units [85,86,87], global warming will not lead to losses in olive production in the northern Mediterranean region. However, given this trend, the replacement of cultivars with high chilling requirements must be considered. These results are partially consistent with those of De Melo-Abreu et al. [85], who reported that global warming could lead to significant losses in olive production in Córdoba and similar areas and/or force the replacement of some varieties with high chilling requirements. Strong warming in the winter slows the fulfillment of chilling requirements, which may delay spring phenology [88]. In this context, it is necessary to conduct studies to predict the spread of olive cultivation in new regions using models that consider topographic and climatic factors such as slope, soil type, solar radiation, temperature, and precipitation. By better understanding how these factors interact with olives, we can develop effective strategies to mitigate the negative effects of climate change on olive production and ensure the long-term sustainability of the olive industry.

3.4.3. Model for Calculating Growing Degree Hours (GDHs)

The Heat Accumulation Model results showed a strong statistically significant negative correlation (p < 0.001) between the growing degree hours (GDHs) and day of onset of flowering (Figure 7). This indicates that higher heat accumulation (higher GDHs) is associated with an earlier onset of flowering. These data are consistent with those of a study in which chilling and heat accumulation times were estimated based on the timing of olive pollination [41]. The average heat accumulation in Portorož was 17.132 ± 2.760 growing degree hours.
Although the general trend indicates an increase in the growing degree hours, there is considerable variability in the data. GDH values [41] were lower in recent years, as there was also a late onset of flowering, which could be explained by early high temperatures and cold-air intrusions from the north. Temperatures in previous months cause significant changes in phenological events, and plants are more sensitive to spring temperatures when their heat requirements are fulfilled [89]. These data suggest that the northern Mediterranean regions have problems with occasional cold air intrusions arriving from the north in the spring months before flowering, which affect the course of flowering and further fruit development.

4. Conclusions

The results of this study suggest that global warming is already affecting the synchrony between the phenology of the olive, Olea europaea L., and environmental conditions in the northern Mediterranean.
A trend toward earlier flowering and intermittent cold air intrusions during the spring months disrupts the normal flowering process, which is furthermore affected by high diurnal temperature ranges (DTRs) during the period before flowering. An increase in DTRs has been observed in recent decades, and further research is needed to understand the stress response mechanisms of olives to high-temperature fluctuations.
The chilling requirements are still being met in the region (1500 ± 250 chilling units), although the total number of chilling units has decreased over the years. There is also a strong correlation between the growing degree hours (GDHs) and the onset of flowering. with the results showing that an increase in growing degree hours (GDH) is associated with the earlier onset of olive flowering.
We also determined that winter frost occurred mainly when the Tmax fell below 0 °C for at least two consecutive days after it had been, on average, above 6 °C for 14 days. In addition, the occurrence of spring frosts when Tmax does not fall below 0 °C can lead to incomplete fertilization, resulting in seed damage and subsequent fruit drop.
The climate change effects described here are interrelated, and several factors may amplify their effects on olive grove ecosystems in the northern Mediterranean. These results highlight the urgent need for further research on other factors that influence flowering dynamics, such as heat waves, warm winds, air mass circulations, and others in combination with climate change scenarios to prepare projections for the future. Sustainable management practices, conservation measures, and adaptation strategies are essential for mitigating the effects of climate change and ensuring the long-term resilience of olive grove ecosystems.

Author Contributions

Conceptualization, M.P. and J.F.; Methodology, M.P.; Software, J.F.; Validation, T.P., V.Z., M.P. and J.F.; Formal Analysis, J.F. and M.P.; Investigation, T.P., V.Z., M.P. and J.F.; Resources, T.P., V.Z., M.P. and J.F.; Data Curation, T.P., V.Z., M.P. and J.F.; Writing—Original Draft Preparation, M.P. and J.F.; Writing—Review and Editing, T.P. and V.Z.; Visualization, T.P., V.Z., M.P. and J.F.; Supervision, M.P.; Project Administration, M.P.; Funding Acquisition, M.P. All authors have read and agreed to the published version of the manuscript.

Funding

The contributions of M.P. and J.F. were partially funded by the Slovenian Research and Innovation Agency through grants V4-2216, V4-2381, and P5-0453. The contributions of T.P. and V.Z. were also supported by ARIS through grant P4-0085.

Data Availability Statement

Phenological data were obtained from the publicly available database of the Pan European Phenology Project PEP725. The meteorological data for the last thirty-four years (1989–2022), covering the period from January to December of each year, are available for the Portorož meteorological station and were obtained from the Slovenian Environment Agency.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of this study; in the collection, analysis, or interpretation of the data; in writing the manuscript; or in the decision to publish the results.

Appendix A

Climate 13 00156 g0a1
Figure A1. Estimation of the chilling and heat accumulation periods according to thermal requirements in Portorož from 1989 to 2022. Green indicates that daily temperatures were positively correlated with flowering dates. Red indicates when the temperatures were negatively correlated with flowering dates. The gray area represents the range (min–max) of the onset of flowering during the study periods. VIP quantifies the importance of temperature in explaining the variation in flowering date.
Figure A1. Estimation of the chilling and heat accumulation periods according to thermal requirements in Portorož from 1989 to 2022. Green indicates that daily temperatures were positively correlated with flowering dates. Red indicates when the temperatures were negatively correlated with flowering dates. The gray area represents the range (min–max) of the onset of flowering during the study periods. VIP quantifies the importance of temperature in explaining the variation in flowering date.
Climate 13 00156 g0a1

Appendix B

Climate 13 00156 g0a2
Figure A2. Pearson’s correlation between chilling (chilling hours) and heat accumulation (growing degree hours). The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95% confidence interval.
Figure A2. Pearson’s correlation between chilling (chilling hours) and heat accumulation (growing degree hours). The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95% confidence interval.
Climate 13 00156 g0a2

Appendix C

Climate 13 00156 g0a3
Figure A3. The onset of olive flowering vs. chill units calculated using the Utah Model for the 1989–2022 period in Portorož. The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95% confidence interval.
Figure A3. The onset of olive flowering vs. chill units calculated using the Utah Model for the 1989–2022 period in Portorož. The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95% confidence interval.
Climate 13 00156 g0a3

Appendix D

Climate 13 00156 g0a4
Figure A4. The onset of olive flowering vs. chill units calculated using the Chilling Portions Model for the 1989–2022 period in Portorož. The black dots represent each data point.
Figure A4. The onset of olive flowering vs. chill units calculated using the Chilling Portions Model for the 1989–2022 period in Portorož. The black dots represent each data point.
Climate 13 00156 g0a4

References

  1. Vitasse, Y.; Rebetez, M. Unprecedented risk of spring frost damage in Switzerland and Germany in 2017. Clim. Change 2018, 149, 233–246. [Google Scholar] [CrossRef]
  2. Campoy, J.A.; Ruiz, D.; Egea, J. Dormancy in temperate fruit trees in a global warming context: A review. Sci. Hortic. 2011, 130, 357–372. [Google Scholar] [CrossRef]
  3. Rubio-Valdés, G.; Cabello, D.; Rallo, L.; Rapoport, H.F. Bud dormancy release dynamics in the olive tree and validation of the use of explants to determine chilling requirement fulfillment. SSRN Electron. J. 2022, 11, 3461. [Google Scholar] [CrossRef]
  4. Haberman, A.; Bakhshian, O.; Cerezo-Medina, S.; Paltiel, J.; Adler, C.; Ben-Ari, G.; Mercado, J.A.; Pliego-Alfaro, F.; Lavee, S.; Samach, A. A possible role for FT-encoding genes in interpreting environmental and internal cues affecting olive (Olea europaea L.) flower induction. Plant Cell Environ. 2017, 40, 1263–1280. [Google Scholar] [CrossRef] [PubMed]
  5. Ramos, A.; Rapoport, H.F.; Cabello, D.; Rallo, L. Chilling accumulation, dormancy release temperature, and the role of leaves in olive reproductive budburst: Evaluation using shoot explants. Sci. Hortic. 2018, 231, 241–252. [Google Scholar] [CrossRef]
  6. Lang, G.A.; Early, J.D.; Martin, G.C.; Darnell, R.L. Endo-, Para-, and Ecodormancy: Physiological Terminology and Classification for Dormancy Research. HortScience 1987, 22, 371–377. [Google Scholar] [CrossRef]
  7. Farga, H.; Moriondo, M.; Leolini, L.; Santos, J.A. Mediterranean Olive Orchards under Climate Change: A Review of Future Impacts and Adaptation Strategies. Agronomy 2020, 11, 56. [Google Scholar] [CrossRef]
  8. Osborne, C.P.; Chuine, I.; Viner, D.; Woodward, F.I. Olive phenology as a sensitive indicator of future climatic warming in the Mediterranean. Plant Cell Environ. 2000, 23, 701–710. [Google Scholar] [CrossRef]
  9. Galán, C.; García-Mozo, H.; Vázquez, L.; Ruiz, L.; de la Guardia, C.D.; Trigo, M.M. Heat requirement for the onset of the Olea europaea L. pollen season in several sites in Andalusia and the effect of the expected future climate change. Int. J. Biometeorol. 2005, 49, 184–188. [Google Scholar] [CrossRef] [PubMed]
  10. Gómez-del-Campo, M.; Rapoport, H. Descripción de la iniciación floral, floración, cuajado, caída de frutos y endurecimiento del hueso. Agric. Rev. Agropecuaria 2008, 907, 400–406. [Google Scholar]
  11. Tombesi, A. Biologia fiorale e di fruttificazione. In Olea. Trattato di Olivicoltura; Fiorino, P., Ed.; Edagricole: Bologna, Italy, 2003. [Google Scholar]
  12. Rallo, L.; Cuevas, J. Fructificación y producción. In El Cultivo del Olivo; Barranco, D., Fernández-Escobar, R., Rallo, L., Eds.; Mundi-Prensa y Junta de Andalucía: Madrid, Spain, 2004. [Google Scholar]
  13. Lavee, S. Biología y fisiología del olivo. In Enciclopedia Mundial del Olivo; Consejo Oleícola Internacional: Madrid, Spain, 1996. [Google Scholar]
  14. Ayerza, R.; Sibbett, G.S. Thermal adaptability of olive (Olea europaea L.) to the Arid Chaco of Argentina. Agric. Ecosyst. Environ. 2001, 84, 277–285. [Google Scholar] [CrossRef]
  15. Torres, M.; Pierantozzi, P.; Searles, P.; Rousseaux, M.C.; García-Inza, G.; Miserere, A.; Bodoira, R.; Contreras, C.; Maestri, D. Olive Cultivation in the Southern Hemisphere: Flowering, Water Requirements and Oil Quality Responses to New Crop Environments. Front. Plant Sci. 2017, 8, 1830. [Google Scholar] [CrossRef] [PubMed]
  16. Castillo-Llanque, F.J.; Rapoport, H.F.; Baumann Samanez, H. Irrigation withholding effects on olive reproductive bud development for conditions with insufficient winter chilling. Acta Hortic. 2014, 1057, 113–119. [Google Scholar] [CrossRef]
  17. Medina-Alonso, M.G.; Cabezas Luque, J.M.; Ríos Mesa, D.; Lorite, I.J. Flowering Phenology of Olive Cultivars in Two Climate Zones with Contrasting Temperatures (Subtropical and Mediterranean). Agriculture 2023, 13, 1312. [Google Scholar] [CrossRef]
  18. Rapoport, H.F.; Hammami, S.; Martins, P.; Pérez-Priego, O.; Orgaz, F. Influence of water deficits at different times during olive tree inflorescence and flower development. Environ. Exp. Bot. 2011, 77, 227–233. [Google Scholar] [CrossRef]
  19. Gucci, R.; Fereras, E.; Goldhamer, D.A. Olive. In Crop Yield Response to Water, FAO Irrigation and Drainage Paper 66; Steduto, P., Hsiao, T.C., Fereres, E., Raes, D., Eds.; FAO: Rome, Italy, 2012; pp. 300–313. [Google Scholar]
  20. Grattan, S.R.; Berenguer, M.J.; Connell, J.H.; Polito, V.S.; Vossen, P.M. Olive oil production as influenced by different quantities of applied water. Agric. Water Manag. 2006, 85, 133–140. [Google Scholar] [CrossRef]
  21. Rapoport, H.F.; Moreno-Alías, I. Botánica y morfología. In El Cultivo del Olivo, 7th ed.; Barranco, D.R., Fernández-Escobar, R., Rallo, L., Eds.; Editorial Mundi-Prensa: Madrid, Spain, 2017; pp. 35–65. [Google Scholar]
  22. Stephenson, A.G. Flower and fruit abortion: Proximate causes and ultimate functions. Annu. Rev. Ecol. Syst. 1981, 12, 253–279. [Google Scholar] [CrossRef]
  23. Linaldeddu, B.T.; Rossetto, G.; Maddau, L.; Vatrano, T.; Bregant, C. Diversity and pathogenicity of Botryosphaeriaceae and Phytophthora species associated with emerging olive diseases in Italy. Agriculture 2023, 13, 1575. [Google Scholar] [CrossRef]
  24. Sanna, F.; Mori, N.; Santoiemma, G.; Pozzebon, A.; Scaccini, D.; Marangoni, F.; Sella, L. Halyomorpha halys (Hemiptera: Pentatomidae) as the major contributor to early olive drop in northern Italy. J. Econ. Entomol. 2024, 117, 1336–1346. [Google Scholar] [CrossRef] [PubMed]
  25. Kolainis, S.; Koletti, A.; Lykogianni, M.; Karamanou, D.; Gkizi, D.; Tjamos, S.E.; Paraskeuopoulos, A.; Aliferis, K.A. An integrated approach to improve plant protection against olive anthracnose caused by the Colletotrichum acutatum species complex. PLoS ONE 2020, 15, e0233916. [Google Scholar] [CrossRef] [PubMed]
  26. Staut, M.; Kovačič, G.; Ogrin, D. The spatial cognition of Mediterranean in Slovenia: (in)consistency between perception and physical definitions. Acta Geogr. Slov. 2007, 47, 105–131. [Google Scholar] [CrossRef]
  27. Urbanc, M. Perception of Land among Slovenians in the Context of Landscape Changes in Slovenian Istria [Istra]. Mitteilungen Osterr. Geogr. Ges. 2011, 153, 199–220. [Google Scholar] [CrossRef]
  28. Segnalini, M.; Nardone, A.; Bernabucci, U.; Vitali, A.; Ronchi, B.; Lacctera, N. Dynamics of the temperature-humidity index in the Mediterranean basin. Int. J. Biometeorol. 2011, 55, 253–263. [Google Scholar] [CrossRef] [PubMed]
  29. Rosenzweig, C.; Tubiello, F.N. Impacts of global climate change on Mediterranean agriculture: Current methodologies and future directions. An introductory essay. Mitig. Adapt. Strateg. Glob. Change 1997, 1, 219–232. [Google Scholar] [CrossRef]
  30. Podgornik, M.; Valenčič, V.; Bešter, E.; Butinar, B.; Fantinič, J.; Hladnik, T.; Kozlovič, G.; Volk, S.; Fičur, K.; Bučar-Miklavčič, M. Climate change is already taking its toll on olive growing. Olive Olive Grow. Assoc. Newsl. 2021, 22, 16–17. [Google Scholar]
  31. Repolusk, P. The Koper Hills. In Slovenia: Landscapes and People; Perko, D., Oražen Adamič, M., Eds.; Mladinska Knjiga: Ljubljana, Slovenia, 2001; pp. 268–280. [Google Scholar]
  32. Ogrin, D. Modern climate change in Slovenia. In Slovenia: A Geographical Overview; Orožen Adamič, M., Ed.; Association of the Geographical Societies of Slovenia: Ljubljana, Slovenia, 2004; p. 159. [Google Scholar]
  33. Podgornik, M.; Pintar, M.; Bučar Miklavčič, M.; Bandelj, D. Different quantities of applied water on Olea europaea L. cultivated under humid conditions. J. Irrig. Drain. Eng. 2017, 143, 9. [Google Scholar] [CrossRef]
  34. ARSO, 2023a. Climate Data. Available online: https://meteo.arso.gov.si/met/sl/climate/tables/statistike_1950_2020/letalisce_portoroz// (accessed on 20 May 2023).
  35. Bandelj, D.; Jakše, J.; Javornik, B. DNA fingerprinting of olive varieties by microsatellite markers. Food Technol. Biotechnol. 2002, 40, 185–190. [Google Scholar]
  36. Parmegiani, P.; Scarbolo, E. Olive trees and oil in the Friuli Venezia Giulia Region: Historical notes, production, characterization, specificity. In Proceedings of the Perspectives for Horticultural and Viticulture in the Alpine Region in the Third Millennium, Udine, Italy, 8–10 November 2000; pp. 604–606. [Google Scholar]
  37. ARSO, 2023b. Archive Data. Available online: https://meteo.arso.gov.si/met/sl/archive/ (accessed on 20 May 2023).
  38. Beguería, S.; Vicente-Serrano, S.M. SPEI: Calculation of the Standardized Precipitation-Evapotranspiration Index. R package version 1.8.1, 2023. Available online: https://CRAN.R-project.org/package=SPEI (accessed on 20 April 2023).
  39. Vicente-Serrano, S.M.; Beguería, S.; López-Moreno, J.I. A Multiscalar Drought Index Sensitive to Global Warming: The Standardized Precipitation Evapotranspiration Index. J. Climate 2010, 23, 1696–1718. [Google Scholar] [CrossRef]
  40. Sanz-Cortés, F.; Martinez-Calvo, J.; Badenes, M.L.; Bleiholder, H.; Hack, H.; Llácer, G.; Meier, U. Phenological growth stages of olive trees (Olea europaea). Ann. Appl. Biol. 2005, 146, 73–80. [Google Scholar] [CrossRef]
  41. Rojo, J.; Orlandi, F.; Ben Dhiab, A.; Lara, B.; Picornell, A.; Oteros, J.; Msallem, M.; Fornaciari, M.; Pérez-Badia, R. Estimation of Chilling and Heat Accumulation Periods Based on the Timing of Olive Pollination. Forests 2020, 11, 835. [Google Scholar] [CrossRef]
  42. Luedeling, E.; Kunz, A.; Blanke, M.M. Identification of Chilling and Heat Requirements of Cherry Trees—A Statistical Approach. Int. J. Biometeorol. 2013, 57, 679–689. [Google Scholar] [CrossRef] [PubMed]
  43. Richardson, E.A.; Seeley, S.D.; Walker, D.R. A model for estimating the completion of rest for Redhaven and Elberta peach trees. HortScience 1974, 9, 331–332. [Google Scholar] [CrossRef]
  44. Weinberger, J.H. Chilling Requirements of Peach Varieties. In Proceedings of the American Society for Horticultural Science; American Society for Horticultural Science: Alexandria, VA, USA, 1950; Volume 56, pp. 122–128. [Google Scholar]
  45. Gabaldón-Leal, C.; Ruiz-Ramos, M.; de la Rosa, R.; León, L.; Belaj, A.; Rodríguez, A.; Santos, C.; Lorite, I.J. Impact of Changes in Mean and Extreme Temperatures Caused by Climate Change on Olive Flowering in Southern Spain. Int. J. Climatol. 2017, 37, 940–957. [Google Scholar] [CrossRef]
  46. Erez, A.; Fishman, S.; Linsley-Noakes, G.C.; Allan, P. The dynamic model for rest completion in peach buds. Acta Hortic. 1990, 276, 165–174. [Google Scholar] [CrossRef]
  47. Fishman, S.; Erez, A.; Couvillon, G.A. The Temperature Dependence of Dormancy Breaking in Plants: Mathematical Analysis of a Two-Step Model Involving a Cooperative Transition. J. Theor. Biol. 1987, 124, 473–483. [Google Scholar] [CrossRef]
  48. Anderson, J.L.; Richardson, E.A.; Kesner, C.D. Validation of Chill Unit and Flower Bud Phenology Models for “Montmorency” Sour Cherry. Acta Hortic. 1986, 84, 71–78. [Google Scholar] [CrossRef]
  49. Darbyshire, R.; Webb, L.; Goodwin, I.; Barlow, S. Winter Chilling Trends for Deciduous Fruit Trees. Agric. For. Meteorol. 2011, 151, 1074–1085. [Google Scholar] [CrossRef]
  50. Ahani, A.; Mousavi, S. FAO56 1.0; November 2023. Available online: https://cran.r-project.org/package=FAO56 (accessed on 20 November 2023).
  51. Žnidaršič, Z.; Gregorič, G.; Sušnik, A.; Pogačar, T. Frost risk assessment in Slovenia in the period of 1981–2020. Atmosphere 2023, 14, 683. [Google Scholar] [CrossRef]
  52. Petruccelli, R.; Bartolini, G.; Ganino, T.; Zelasco, S.; Lombardo, L.; Perri, E.; Durante, M.; Bernardi, R. Cold stress, freezing adaptation, varietal susceptibility of Olea europaea L.: A review. Plants 2022, 11, 1367. [Google Scholar] [CrossRef] [PubMed]
  53. Arora, R.; Rowland, L.J.; Tanino, K. Induction and release of bud dormancy in woody perennials: A science comes of age. HortScience 2003, 38, 911. [Google Scholar] [CrossRef]
  54. van der Schoot, C.; Rinne, P.L.H. Dormancy cycling at the shoot apical meristem: Transitioning between self-organization and self-arrest. Plant Sci. 2011, 180, 120–131. [Google Scholar] [CrossRef] [PubMed]
  55. López-Bernal, A.; García-Tejera, O.; Testi, L.; Orgaz, F.; Villalobos, F.J. Studying and modelling winter dormancy in olive trees. Agric. For. Meteorol. 2020, 280, 107776. [Google Scholar] [CrossRef]
  56. Lavee, S.; Rallo, L.; Rapoport, H.F.; Troncoso, A. The floral biology of the olive: Effect of flower number, type and distribution on fruit set. Sci. Hortic. 1996, 66, 149–158. [Google Scholar] [CrossRef]
  57. Lenz, A.; Hoch, G.; Körner, C.; Vitasse, Y. Convergence of leaf-out timing towards minimum risk of freezing damage in temperate trees. Funct. Ecol. 2016, 30, 1480–1490. [Google Scholar] [CrossRef]
  58. Coville, F.V. The influence of cold in stimulating the growth of plants. Proc. Natl. Acad. Sci. USA 1920, 6, 434–435. [Google Scholar] [CrossRef] [PubMed]
  59. Delpierre, N.; Vitasse, Y.; Chuine, I.; Guillemot, J.; Bazot, S.; Rutishauser, T.; Rathgeber, C.B.K. Temperate and boreal forest tree phenology: From organ-scale processes to terrestrial ecosystem models. Ann. For. Sci. 2016, 73, 5–25. [Google Scholar] [CrossRef]
  60. Ogrin, D. Olive growing in Slovenian Istria and climatic limitations to its development. Morav. Geogr. Rep. 2007, 15, 34–40. [Google Scholar]
  61. Ogrin, D. On the Olive Tree Frost in Slovenian Istria in December 1996. Disaster 1997, 11, 34–38. [Google Scholar]
  62. Vrhovnik, I.; Vesel, V.; Podgornik, M. Characteristics of the 2018 Olive Tree Frost Compared to Previous Frost Events. Olive Newsl. Olive Grow. Assoc. 2018, 21, 23–25. [Google Scholar]
  63. Sanzani, S.M.; Schena, L.; Nigro, F.; Sergeeva, V.; Ippolito, A.; Salerno, M.G. Abiotic diseases of olive. J. Plant Pathol. 2012, 294, 469–491. [Google Scholar]
  64. Sibbett, G.S.; Osgood, J. Site selection and preparation, tree spacing and design, planting, and initial training. In Olive Production Manual; Sibbett, G.S., Ferguson, L., Coviello, J.L., Lindstrand, M., Eds.; University of California, Agriculture and Natural Resources: Oakland, CA, USA, 2005; pp. 49–54. [Google Scholar]
  65. Sedmak, D. Slovenian Littoral Municipalities—Koper, Izola, Piran: Investment Plan for Olive Orchards 1986–1990; SOZD Timav, DO Droga Portorož; TOK Agraria: Koper, Slovenia, 1985; p. 37. [Google Scholar]
  66. Toreti, A.; Masante, D.; Acosta Navarro, J.; Bavera, D.; Cammalleri, C.; De Jager, A.; Di Ciollo, C.; Hrast Essenfelder, A.; Maetens, W.; Magni, D.; et al. Drought in Europe July 2022; EUR 31147 EN; Publications Office of the European Union: Luxembourg, 2022; ISBN 978-92-76-54953-6. [Google Scholar] [CrossRef]
  67. Gambella, F.; Bianchini, L.; Cecchini, M.; Egidi, G.; Ferrara, A.; Salvati, L.; Colantoni, A.; Morea, D. Moving toward the north? The spatial shift of olive groves in Italy. Agric. Econ. 2021, 67, 129–135. [Google Scholar] [CrossRef]
  68. Zupanc, V.; Pintar, M.; Podgornik, M. Olive production on cultivated terraces in northern Istria. Ann. Ser. Hist. Sociol. 2018, 28, 795–810. [Google Scholar] [CrossRef]
  69. Bertalanič, R.; Dolinar, M.; Honzak, L.; Lokošek, N.; Medved, A.; Vertačnik, G.; Vlahović, Ž. Climate Change Projections for Slovenia over the 21st Century; Ministry of the Environment and Spatial Planning, Slovenian Environment Agency: Ljubljana, Slovenia, 2019.
  70. Zeileis, A.; Leisch, F.; Hornik, K.; Kleiber, C. strucchange: An R Package for Testing for Structural Change in Linear Regression Models. J. Stat. Softw. 2002, 7, 1–38. [Google Scholar] [CrossRef]
  71. Zeileis, A.; Kleiber, C.; Krämer, W.; Hornik, K. Testing and Dating of Structural Changes in Practice. Comput. Stat. Data Anal. 2003, 44, 109–123. [Google Scholar] [CrossRef]
  72. Pohlert, T. trend: Non-Parametric Trend Tests and Change-Point Detection. R Package Version 1.1.6, 2023. Available online: https://CRAN.R-project.org/package=trend (accessed on 25 May 2025).
  73. Pareja-Bonilla, D.; Arista, M.; Morellato, L.P.C.; Ortiz, P.L. Better Soon than Never: Climate Change Induces Strong Phenological Reassembly in the Flowering of a Mediterranean Shrub Community. Ann. Bot. 2025, 135, 239–254. [Google Scholar] [CrossRef] [PubMed]
  74. Fitter, A.H.; Fitter, R.S. Rapid Changes in Flowering Time in British Plants. Science 2002, 296, 1689–1691. [Google Scholar] [CrossRef] [PubMed]
  75. Büntgen, U.; Piermattei, A.; Krusic, P.J.; Esper, J.; Sparks, T.; Crivellaro, A. Plants in the UK flower a month earlier under recent warming. Proc. R. Soc. B 2022, 289, 20212456. [Google Scholar] [CrossRef] [PubMed]
  76. Aslam, M.A.; Ahmed, M.; Hassan, F.; Afzal, O.; Mehmood, M.Z.; Qadir, G.; Asif, M.; Komal, S.; Hussain, T. Impact of temperature fluctuations on plant morphological and physiological traits. In Building Climate Resilience in Agriculture: Theory, Practice, and Future Perspective; Jatoi, W.N., Mubeen, M., Ahmad, A., Cheema, M.A., Lin, Z., Hashmi, M.Z., Eds.; Springer: Cham, Switzerland, 2022; pp. 25–52. [Google Scholar] [CrossRef]
  77. Bartolini, S.; Viti, R. Olive Floral Biology and Climatic Elements: Twenty-Eight Years of Observations. Acta Hortic. 2018, 1229, 45. [Google Scholar] [CrossRef]
  78. Fornaciari, M.; Orlandi, F.; Tedeschini, E. Long term analysis on Olive flowering and climatic relationships in central Italy. Eur. J. Agron. 2025, 162, 127435. [Google Scholar] [CrossRef]
  79. Oteros, J.; García-Mozo, H.; Vázquez, L.; Mestre, A.; Domínguez-Vilches, E.; Galán, C. Modelling olive phenological response to weather and topography. Agric. Ecosyst. Environ. 2013, 179, 62–68. [Google Scholar] [CrossRef]
  80. Maniriho, F. Flower differentiation and fruiting dynamics in olive trees (Olea europaea): Eco-physiological analysis in the Mediterranean basin. Adv. Hortic. Sci. 2022, 36, 53–62. [Google Scholar] [CrossRef]
  81. Horton, D.E.; Johnson, N.C.; Singh, D.; Swain, D.L.; Rajaratnam, B.; Diffenbaugh, N.S. Contribution of Changes in Atmospheric Circulation Patterns to Extreme Temperature Trends. Nature 2015, 522, 465–469. [Google Scholar] [CrossRef] [PubMed]
  82. El Yaacoubi, A.; Oukabli, A.; Legave, J.-M.; Ainane, T.; Mouhajir, A.; Zouhair, R.; Hafidi, M. Response of almond flowering and dormancy to Mediterranean temperature conditions in the context of adaptation to climate variations. Sci. Hortic. 2019, 257, 108687. [Google Scholar] [CrossRef]
  83. Wang, H.; Wang, H.; Ge, Q.; Dai, J. The interactive effects of chilling, photoperiod, and forcing temperature on flowering phenology of temperate woody plants. Front. Plant Sci. 2020, 11, 443. [Google Scholar] [CrossRef] [PubMed]
  84. Omran, M.A.A.; Mofeed, A.S.; Mohamed, H.M. Effect of chilling requirements on flowering and productivity of some native and foreign olive cultivars under the Egyptian conditions. Middle East J. Appl. Sci. 2019, 9, 421–433. [Google Scholar]
  85. De Melo-Abreu, J.P.; Barranco, D.; Cordeiro, A.M.; Tous, J.; Rogadoe, B.M.; Villalobos, F.J. Modelling olive flowering date using chilling for dormancy release and thermal time. Agric. For. Meteorol. 2004, 125, 117–127. [Google Scholar] [CrossRef]
  86. Belaj, A.; de la Rosa, R.; León, L.; Gabaldón-Leal, C.; Santos, C.; Porras, R.; de la Cruz-Blanco, M.; Lorite, I.J. Phenological diversity in a world olive germplasm bank: Potential use for breeding programs and climate change studies. Span. J. Agric. Res. 2020, 18, e0701. [Google Scholar] [CrossRef]
  87. Aybar, V.E.; De Melo-Abreu, J.P.; Searles, P.S.; Matias, A.C.; Del Rio, C.; Caballero, J.M.; Rousseaux, M.C. Evaluation of olive flowering at low latitude sites in Argentina using a chilling requirement model. Span. J. Agric. Res. 2015, 13, e0901. [Google Scholar] [CrossRef]
  88. Yu, H.; Luedeling, E.; Xu, J. Winter and spring warming result in delayed spring phenology on the Tibetan Plateau. Proc. Natl. Acad. Sci. USA 2010, 107, 22151–22156. [Google Scholar] [CrossRef] [PubMed]
  89. Menzel, A.; Sparks, T.H.; Estrella, N.; Koch, E.; Aasa, A.; Ahas, R.; Alm-Kübler, K.; Bissolli, P.; Braslavská, O.; Briede, A.; et al. European phenological response to climate change matches the warming pattern. Glob. Change Biol. 2006, 12, 1969–1976. [Google Scholar] [CrossRef]
Climate 13 00156 g001
Figure 1. Sampling site in Slovenian Istria (dark green) next to Friuli Venezia Giulia (Italy) in the Adriatic region, the northernmost part of the Mediterranean.
Figure 1. Sampling site in Slovenian Istria (dark green) next to Friuli Venezia Giulia (Italy) in the Adriatic region, the northernmost part of the Mediterranean.
Climate 13 00156 g001
Climate 13 00156 g002
Figure 2. The standardized precipitation evapotranspiration index at a 12-month scale (SPEI-12) was evaluated for Portorož between 1990 and 2022.
Figure 2. The standardized precipitation evapotranspiration index at a 12-month scale (SPEI-12) was evaluated for Portorož between 1990 and 2022.
Climate 13 00156 g002
Climate 13 00156 g003
Figure 3. The occurrence of the onset of olive flowering and its trend in Portorož in the period 1989 to 2022.
Figure 3. The occurrence of the onset of olive flowering and its trend in Portorož in the period 1989 to 2022.
Climate 13 00156 g003
Climate 13 00156 g004
Figure 4. Olive flowering onset trend in Portorož in the most recent decade of measurements (2006–2022) compared with the previous decade (1989–2005). The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95 % confidence interval.
Figure 4. Olive flowering onset trend in Portorož in the most recent decade of measurements (2006–2022) compared with the previous decade (1989–2005). The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95 % confidence interval.
Climate 13 00156 g004
Climate 13 00156 g005
Figure 5. The onset of olive flowering and average monthly diurnal temperature range in April in Portorož from 1989 to 2022.
Figure 5. The onset of olive flowering and average monthly diurnal temperature range in April in Portorož from 1989 to 2022.
Climate 13 00156 g005
Climate 13 00156 g006
Figure 6. The onset of olive flowering vs. chilling hours calculated using the Chilling Hours Model for the 1989–2022 period in Portorož. The black dots represent the each data point, the trend is represented with a regression line and the grey area represent the 95 % confidence interval.
Figure 6. The onset of olive flowering vs. chilling hours calculated using the Chilling Hours Model for the 1989–2022 period in Portorož. The black dots represent the each data point, the trend is represented with a regression line and the grey area represent the 95 % confidence interval.
Climate 13 00156 g006
Climate 13 00156 g007
Figure 7. The onset of olive flowering vs. heat accumulation (growing degree hours), calculated using the optimum threshold (15 Nov–10 May) for the 1989–2022 period in Portorož. The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95 % confidence interval.
Figure 7. The onset of olive flowering vs. heat accumulation (growing degree hours), calculated using the optimum threshold (15 Nov–10 May) for the 1989–2022 period in Portorož. The black dots represent each data point, the trend is represented with a regression line and the grey area represent the 95 % confidence interval.
Climate 13 00156 g007
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Podgornik, M.; Fantinič, J.; Pogačar, T.; Zupanc, V. Analysis of Olive Tree Flowering Behavior Based on Thermal Requirements: A Case Study from the Northern Mediterranean Region. Climate 2025, 13, 156. https://doi.org/10.3390/cli13080156

AMA Style

Podgornik M, Fantinič J, Pogačar T, Zupanc V. Analysis of Olive Tree Flowering Behavior Based on Thermal Requirements: A Case Study from the Northern Mediterranean Region. Climate. 2025; 13(8):156. https://doi.org/10.3390/cli13080156

Chicago/Turabian Style

Podgornik, Maja, Jakob Fantinič, Tjaša Pogačar, and Vesna Zupanc. 2025. "Analysis of Olive Tree Flowering Behavior Based on Thermal Requirements: A Case Study from the Northern Mediterranean Region" Climate 13, no. 8: 156. https://doi.org/10.3390/cli13080156

APA Style

Podgornik, M., Fantinič, J., Pogačar, T., & Zupanc, V. (2025). Analysis of Olive Tree Flowering Behavior Based on Thermal Requirements: A Case Study from the Northern Mediterranean Region. Climate, 13(8), 156. https://doi.org/10.3390/cli13080156

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop