Spring’s Signal: Can Bud Burst Timing Enhance Resistance to Ash Dieback in Europe?

Abstract

Ash dieback (ADB), driven by the invasive fungus Hymenoscyphus fraxineus, poses a significant environmental and financial risk throughout Europe. Fraxinus excelsior (European ash), an essential part of forest ecosystems, has seen death rates as high as 85% in impacted areas, threatening its ecological roles and economic importance. This study examines the relationship between the phenological traits of ash clones, particularly the timing of spring bud burst, and their susceptibility to H. fraxineus infection. The study was conducted in a clonal seed orchard located in Northeastern Poland, encompassing 31 ash clones from different bioclimatic regions. Phenological analyses of bud burst were carried out from early April to late May during the years 2018–2020, and crown damage and defoliation levels were assessed multiple times throughout the growing season. The results confirm that clones with earlier bud burst exhibit significantly higher survival rates and reduced crown damage. Observations revealed that clones with earlier bud burst showed a 30% higher survival rate and up to 40% less crown damage compared to clones with later phenology. The timing of bud burst was strongly correlated with susceptibility to ash dieback (R² = 0.37, p < 0.001). Statistical analyses, including ANOVA and mixed models, revealed significant differences in susceptibility to infection among clones from different bioclimatic regions. These findings underscore the importance of biological timing as a key factor in selecting genotypes resilient to ash dieback. The study highlights the potential of breeding approaches that focus on early bud burst traits to enhance the survival and vitality of ash populations. The results provide essential insights for developing adaptive forest management practices aimed at conserving ash resources and maintaining biodiversity in the face of climate change and the ongoing spread of the pathogen.

1. Introduction

Ash (Fraxinus excelsior L.) is a significant component of forest ecosystems in Europe, particularly in countries such as the United Kingdom and other EU nations, where it fulfills important economic and ecological functions. In Central European regions, including Poland, it is a key species in riparian forests and plays a crucial role in maintaining global biodiversity [1]. Economically, ash is not considered a priority tree species [2]; however, its economic importance may increase due to dynamic climate change [3]. Studies by Dyderski et al. [4] confirm a significant and positive impact of climate change on the potential distribution range of ash. However, modeling the effects of climate change on ash decline is challenging, as current models do not account for the impact of biotic stressors such as the fungus Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz and Hosoya 2014 (anamorph Chalara fraxinea) [5,6,7,8], which causes ash dieback. Additionally, other factors like the invasive emerald ash borer (Agrilus planipennis) and root rot caused by Armillaria species further complicate predictions of the species’ future distribution and health.

One factor limiting the occurrence of ash in forests, and potentially leading to its complete elimination from some forest stands [9], is H. fraxineus, which causes ash dieback (ADB). Hymenoscyphus fraxineus was first detected in Northeastern Poland in 1992 [7,10] and has since rapidly spread across the European continent. Infested stands decline rapidly [11], making sustainable forest management in ecosystems dependent on ash nearly impossible. The mortality rate of ash trees in forest stands across Europe is estimated at 69%, while in ash seed orchards, mortality can exceed 85% [12]. Infection by H. fraxineus primarily occurs in summer when mature apothecia, formed on petioles of leaves from the previous growing season, release wind-dispersed ascospores [10]. Characteristic symptoms of the disease include the wilting of leaves, extensive shoot necrosis, and dieback of entire crowns, ultimately leading to tree death. In Poland, due to the high infection potential of H. fraxineus, the main administrator of the country’s forests (State Forests) has ceased using ash trees in artificial forest regeneration efforts [13].

Scientific forecasts predict no decrease in the occurrence of H. fraxineus in Central Europe, and ash dieback is listed on the warning list of the European and Mediterranean Plant Protection Organization [14]. Due to the ongoing infection pressure and the ecological significance of F. excelsior, attempts are being made to reintroduce the species into forested areas. Initial results of reintroduction efforts under various site conditions have shown that the lowest ash mortality occurs in mesotrophic and dry habitats [15,16]. Grosdidier et al. [17] demonstrated that lower ash mortality is observed in forests with lower ash density per unit area and in park or backyard settings where ash occurs as solitary trees. In Southern and Western Europe [18], the spread of H. fraxineus is strongly declining due to a climate that is unfavorable to the pathogen. The dynamics of ash dieback are favored by heavy summer rainfall, high humidity, and low air temperatures, with optimal growth of H. fraxineus mycelium occurring at 20–25 °C [19]. Additionally, the decreasing density of ash trees and increasing fragmentation of forest stands are factors that limit the spread of the disease [13]. A decrease in mortality and improved health of ash trees in a seed orchard where artificial removal of winter-fallen leaves was performed was also demonstrated [20].

Research focused on identifying factors that contribute to lower susceptibility of ash to dieback has identified spring phenology as one of the key phenotypic markers associated with resistance. The start of the growing season is one of the most important factors determining the potential for growth under local environmental conditions [21]. Smintina [22] and Kleinschmit et al. [23] demonstrated significant differences in phenology between ash provenances and families and emphasized the importance of phenology in the potential selection of species for adaptive traits. There is evidence suggesting that phenological differences, such as early bud burst, may result from genetic adaptations [9]. Studies conducted under natural growing conditions in forest ecosystems have shown that ash trees that develop buds early in spring [24] or show early autumn leaf coloration [8] are less susceptible to crown dieback and exhibit higher resistance. Early bud burst may allow trees to avoid periods of maximum pathogen infection pressure, which occur in later stages of the season, thereby increasing their chances of avoiding infection.

The main objective of the present study was to analyze the relationship between the degree of H. fraxineus infestation in F. excelsior genotypes and phenological observations of spring bud burst. The study was conducted in a clonal seed orchard located in Northeastern Poland. We hypothesized that the following: (a) lower susceptibility to H. fraxineus depends on the bioclimatic region, and (b) ash clones that develop buds early in spring show lower susceptibility to damage by H. fraxineus. Analyses were performed on 31 plus clones growing in a seed orchard infected with H. fraxineus. The pressure of H. fraxineus in the studied seed orchard systematically decreased over the observation years due to the removal of winter-fallen leaves. Therefore, the observations made in the context of forest management aimed to answer the question of whether breeding ash trees for resistant traits can be useful in identifying individuals that are relatively resistant to H. fraxineus. This research is important because it offers information about the genetic and growth characteristics linked to resistance against H. fraxineus. The results aid in creating focused breeding initiatives and forest management plans that aim to protect ash tree groups and improve their ability to withstand biological challenges, helping to maintain the health of forest ecosystems.

2. Materials and Methods

2.1. Study Site

This study was performed in a clonal seed orchard located in the Łomża Forest District (N 22.06, E 53.31) in Northeastern Poland. The orchard (plant materials) was planted on a moist forest site in spring 2002 on an area of 6.22 ha and divided into 6 plots (blocks) where 1544 1-year-old ash trees were planted in a single tree plot design with a spacing of 6 × 6 m. The mean annual temperature in the years of observations ranged from 9.6 °C to 10.1 °C, while the relative humidity ranged from 74.8 to 77.3% (Table S1). The grafts were grown from 31 clones of plus trees originating from 6 populations: Białowieża, Borki, Browsk, Czerwony Dwór, Gołdap, and Hajnówka (

Table 1. Classification of provenances by climatic zones and survival rates of grafts in 2020.

	Group of Climatic Zone	Provenance Name	Individual N° of Plus Trees	Numbers of Grafts in Plot 2 of Seed Orchard	Percentage of Ramet Survival in 2020 [%]	Included in Statistical Analysis [Yes (+)/No (−)]
1	south	Browsk	3362	7	87.5	+
2		Browsk	3367	13	73.3	+
3		Białowieża	3385	3	50.0	−
4		Hajnówka	3418	13	85.71	+
5		Hajnówka	5389	11	57.14	+
6		Hajnówka	5390	13	75.0	+
7		Hajnówka	5517	11	84.62	+
8		Hajnówka	5523	7	43.75	+
9		Hajnówka	5524	2	33.33	−
10		Hajnówka	5525	13	75	+
11		Białowieża	5542	7	58.33	+
12		Hajnówka	5721	9	69.23	+
13		Hajnówka	5722	11	78.57	+
14		Hajnówka	5723	8	72.73	+
15		Hajnówka	5726	10	64.29	+
16		Browsk	6309	12	78.57	+
17		Browsk	6313	7	50	+
18		Hajnówka	6318	2	100	−
19	north	Borki	7246	3	75	−
20		Gołdap	7268	3	50	+
21		Gołdap	7280	8	70	+
22		Borki	7301	10	76.92	+
23		Czerwony Dwór	7312	9	72.73	+
24		Czerwony Dwór	7313	8	60	+
25		Czerwony Dwór	7875	4	75	−
26		Gołdap	7882	3	16.67	+
27		Gołdap	7887	11	76.92	+
28		Gołdap	7888	4	66.67	+
29		Gołdap	7916	6	100	+
30		Gołdap	7918	3	37.5	+
31		Gołdap	7919	7	77.78	+

). For this study, Plot 2 with the highest survival rate (64.0%) was selected. For detailed data on forest habitat and topography of the analyzed seed orchards, see Przybylski et al. [20]. Individual clones were represented by different numbers of grafts (from 2 to 13 individuals). Ramets whose abundance per seed orchard did not exceed 4 were removed from the statistical analyses (Table 1). Clones were excluded from the statistical analysis due to their low sample size, which significantly disrupted statistical outcomes. Including clones with low survival rates could have substantially affected the reliability of the conclusions.

2.2. Climate of Seed Orgin and Study Area

Plus trees whose genotype was analyzed in the present study grew in two Polish climatic zones, i.e., south and north (Table 1). The southern zone is warmer, with an average annual air temperature of approximately 7.5 °C and a growing season approximately 200 days long. The northern zone is colder, with an average annual temperature that does not reach 7.0 °C and a growing season length that does not exceed 180 days. In both, the total annual precipitation is 550–600 mm. The analyzed seed orchard was located at the border between the isotherms and isohyets of the two areas. It was situated in the Łomża Forest District (N 22.06, E 53.31) on a moist forest site with gley soil. The ground cover consisted of turf, and the undergrowth was dominated by grasses (Carex spp.).

2.3. Confirmation of H. fraxineus on Ash Trees

To confirm the presence of H. fraxineus in the seed orchards, shoot samples of the side pedals were collected from trees showing symptoms of infestation (Figure 1a). To minimize sampling bias, shoot samples were collected exclusively from trees showing visible symptoms of infestation to ensure the presence of H. fraxineus, and samples were selected systematically from various locations on the tree to capture potential variability within each ramet. Samples were taken from seed orchards one ramet per clone (clones: 7301, 7888, 7268, 6309, 5542, 5525, 5723, 7919, and 5389). Six to seven samples from each ramet were placed in media. If H. fraxineus was detected in one sample from a clone, it was considered confirmation of infection. Laboratory analyses and macroscopic identification of the obtained cultures were performed according to the methodology described by Kowalski and Bartnik [10] (Figure 1b). The obtained isolates exhibited the characteristics of type B colonies with unevenly discolored white to yellow-orange mycelia (Figure 1c). Morphological identification of selected H. fraxineus isolates was supported by sequencing of the barcode region ITS according to the methodology described by Chandelier et al. [25]. The barcode region sequences were deposited in the GenBank database [ncbi.nlm.nih.gov] under accession numbers MT053856 and MT053857. The medium used was 2% Malt Extract Agar.

2.4. Phenotyping and Defoliation Assessment

The phenology of ash bud burst was assessed for all 238 ramets in block 2 using the methodology developed in the Trees4Future project (Trees4Future—Home). Phenological phase assessment assumes the identification of five spring developmental phases of buds (Figure 2). The phenological phases were as follows:

• Dormant bud.
• Swelling of bud, slight greening of bud scales.
• Buds begin to burst, first green visible.
• Bud burst, petioles of leaves visible, no lengthening of twig.
• Bud burst, petioles of leaves visible, twig has started lengthening, leaves are fully expanded.

Bud development monitoring was conducted every 1–3 days from April to May (in 2018: 111–151 days of the year (DOY); in 2019: 108–146 DOY; and in 2020: 115–154 DOY) by observing the buds from dormancy to full leaf development. These stages provide crucial information about the timing of bud burst, which is associated with the tree’s ability to avoid periods of peak infection pressure from H. fraxineus. Numerical values assigned to individual stages of bud development were averaged (arithmetic mean) for all trees of a given clone (238 ramets).

Plant health was determined in August 2018–2020 by the degree of defoliation of the assimilation apparatus in the central part of the tree crown. Defoliation was assessed from the ground simultaneously by two people who observed the ramets from opposite sides of the crown. The degree of defoliation used in the analysis was the arithmetic mean of two values of crown defoliation measured on a scale from 0 to 100% (dead tree). The methodology described is consistent with international standards adopted by the ICP Forests and ICP-Focus projects [26].

2.5. Bioclimatic Analysis

To determine the position of ash populations in the Polish range of European ash, which is necessary for bioclimatic analysis, we used a dataset of sites compiled from the Polish State Forests IT System (SILP) [27]. After cleaning the dataset for erroneous and doubtful locations, as well as locations outside the country, 1450 European ash stands were available.

Then, 19 bioclimatic parameters were extracted from BIOCLIM 2.0 bioclimatic maps [28] with a resolution of 0.5 arcmin using the R package ‘raster’ version 3.5-2. From this dataset, eight variables were selected (Table 1), removing those that provided too general climatic descriptions (annual temperatures and precipitation) and then excluding variables that were highly correlated with each other (R > 0.7). The selected bioclimatic variables reflect key environmental factors that significantly influence the physiological processes, growth dynamics, and survival of European ash populations. The climatic variables were used to characterize the climatic variability of European ash stands selected from the database.

The analysis was performed to define the climate-related clustering of European ash by means of the partitioning around medoids clustering algorithm, which is an extension of the k-means clustering algorithm [29]. To determine the optimal number of clusters, we applied the within-sum-of-squares method (WSS), which minimizes the distance between points in a cluster. Finally, we defined six clusters as optimal for grouping the European ash distribution dataset. The selection of six clusters was based on maintaining a balance between maximizing climatic differentiation among clusters and ensuring that each cluster retained ecological relevance, enabling a detailed analysis of climatic variability. The optimal number of clusters and PAM clustering were calculated using the ‘cluster’ and ‘factoextra’ packages in R [29]. To determine the climatic position of ash populations within the climatic gradient of European ash distribution in Poland, we performed a principal component analysis (PCA) of bioclimatic variables for all Polish occurrences of European ash based on a correlation matrix. A set of statistically significant axes was determined using a 1000-fold bootstrap analysis. Pearson’s correlation coefficient was used to identify the climatic factors according to which the occurrences of European ash were ranked by principal components (Table 1).

2.6. Statistical Analyses

The mean phenology of spring bud burst in ash clones in 2018, 2019, and 2020 was analyzed using a linear mixed-effects model in R (R Core Team, 2020). The lme4 package [30], which was developed for fitting fixed and random effects models, is therefore particularly suitable for data with hierarchical structures. A linear mixed-effects model was specified to investigate the influence of the year on the timing of bud burst while accounting for the random variability associated with the clone effect. The model included the year as a fixed effect to assess temporal trends, with 2018 serving as the reference year. Clone-specific variability was modeled as a random effect so that each clone has its own intercept value to capture clone-specific deviations from the overall average stage of bud burst. The model can be formally expressed as follows:

$${\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\mathrm{F}\mathrm{e}\mathrm{n}\mathrm{K}\mathrm{l}\mathrm{o}\mathrm{n}}_{ij}={\beta }_0+{\beta }_1\times {\mathrm{Y}\mathrm{e}\mathrm{a}\mathrm{r}}_{2019}+{\beta }_2\times {\mathrm{Y}\mathrm{e}\mathrm{a}\mathrm{r}}_{2020}+u_i+{ϵ}_{ij}$$

(1)

where

${\mathrm{M}\mathrm{e}\mathrm{a}\mathrm{n}\mathrm{F}\mathrm{e}\mathrm{n}\mathrm{K}\mathrm{l}\mathrm{o}\mathrm{n}}_{ij}$ represents the mean bud burst stage for clone i in year j.

${\beta }_0$ is the intercept, reflecting the estimated mean bud burst stage in 2018.

${\beta }_1$ and ${\beta }_2$ are the fixed effect coefficients for the years 2019 and 2020, respectively.

$u_i$ is the random effect associated with clone i, representing the deviation of each clone from the overall mean, assumed to follow a normal distribution with mean 0 and variance ${\tau }^2$.

${ϵ}_{ij}$ is the residual error term, normally distributed with mean 0 and variance ${\sigma }^2$.

The model estimated the fixed effects of the year and the random effects of the individual clones. The fixed effects provided information on the annual variation in the timing of bud burst, while the random effects captured the variability between clones. Confidence intervals were calculated for the fixed effects to assess the precision of the estimates and the statistical significance of the differences from year to year. The random effects for each clone were extracted to understand how each clone differed from the mean of the whole population. This extraction was complemented by tools from the mixedup package, which provided a comprehensive summary of the random effects and facilitated the interpretation of clone-specific patterns.

3. Results

3.1. Bioclimatic Analyses

The analysis of 1450 European ash stands using PCA aimed to examine the influence of bioclimatic variables on tree health and susceptibility to H. fraxineus. Eight components were identified, with two being statistically significant, accounting for 72.60% of the total variance (Figure 3A). These results provided a better understanding of the variability in tree susceptibility across different climatic conditions, which was crucial for assessing phenology and developing breeding strategies for ash trees resistant to the pathogen.

The first principal component (PC1), accounting for 45.06% of the variance (

Table 2. Pearson’s correlation coefficients between eight bioclimatic variables proposed and first two principal components (PC1 and PC2).

Bioclimatic Variables	Abbreviation	Pearson’s Correlation
		PC1	PC2
Temperature Seasonality (STD × 100)	bio4	−0.42	−0.6
Mean Temperature of Wettest Quarter	bio8	0.73	−0.6
Mean Temperature of Driest Quarter	bio9	0.53	−0.33
Mean Temperature of Warmest Quarter	bio10	0.73	−0.62
Mean Temperature of Coldest Quarter	bio11	0.87	0.09
Precipitation Seasonality (CV)	bio15	0.17	0.82
Precipitation of Driest Quarter	bio17	−0.84	−0.28
Precipitation of Coldest Quarter	bio19	−0.77	−0.48

), was strongly correlated (|r| > 0.70) with mean temperatures in the coldest, wettest, and warmest quarters (bio11, bio8, and bio10) and precipitation in the driest and coldest quarters (bio17 and bio19). The second principal component (PC2), explaining 27.54% of the variance, was strongly correlated with precipitation seasonality (bio15) and moderately correlated (|0.50| < |r| < 0.70) with temperature seasonality (bio4) and mean temperatures in the wettest and warmest quarters (bio8 and bio10).

European ash populations were assigned to two different ellipses determined at a 95% confidence level for the variability of climatic conditions in Northeastern and Eastern Poland (Figure 4). The implementation of the partitioning around the medoids clustering algorithm led to the determination of six different bioclimatic regions in Poland for the European ash population (Figure 4). Both the northeastern and eastern clusters are mainly determined by temperature seasonality and precipitation of the driest and coldest quarter conditions (bio4, bio17, and bio19, respectively; Figure 3B). However, the main bioclimatic variables differentiating European ash populations are the mean temperatures of the wettest and warmest quarters (bio8 and bio10, respectively).

3.2. Phenology and Defoliation Analyses

The grafts began bud burst between the end of April and the first week of May, with the entire bud development process lasting approximately 14 days. The most intensive period of growth and differentiation of the grafts occurred between the 5th and 7th days after the initial phenological observations. After this period, 80% of the grafts could be unequivocally assigned to a specific phenological phase. Figure 5 illustrates the distribution of the phenology of ash bud burst in different years. In panel (a), changing patterns of distribution are observed across the different years. The Mfen values from 2018 are characterized by a high concentration around a single peak, indicating lower phenological variability in that year. In contrast, the distributions for 2019 and 2020 show greater dispersion of values with several local peaks, indicating increased complexity and variability in the bud burst process during these years. Panel (b) of the figure presents the density distributions of the differences in phenology of clones relative to the annual mean (MeanFenKlon–MeanFenYear). In 2018, these differences are concentrated near zero, suggesting that the phenological values of the clones were close to the annual mean. In 2019 and 2020, the distributions of differences become more dispersed, and their breadth increases, reflecting greater deviations from the annual mean in the phenology of the clones.

In 2019, the timing of spring bud burst showed no major differences from 2018, demonstrating that the phenological events were consistent across different genotypes during these years. In contrast, 2020 revealed significant changes, with a calculated value of −0.37 (confidence interval: −0.61 to −0.13; p = 0.003), indicating a notable alteration in bud burst timing compared to 2018. The Intraclass Correlation Coefficient (ICC) among the clones was 0.57, suggesting that 57% of the differences in bud burst timing could be linked to genetic variations within the population. The standard deviation for random effects among clones stood at 0.22, showing extra variability stemming from unpredictable factors (

Table 3. Mixed-effects model analysis of spring bud burst phenology in ash clones for 2018, 2019, and 2020.

	MeanFenKlon
Predictors	Estimates	CI	p
Year (2018) (Intercept)	3.27	3.01–3.54	<0.001
Year (2019)	−0.16	−0.40–0.08	0.196
Year (2020)	−0.37	−0.61–−0.13	0.003
Random Effects
σ2	0.16
τ00 Klon	0.22
ICC	0.57
N Klon	22
Observations	66
Marginal R2/Conditional R2	0.058/0.596

).

The study analyzed 66 data points from 22 clones, offering a robust basis for statistical analysis. The marginal R² value was 0.058, while the conditional R² value reached 0.596, implying that the model successfully accounted for a significant amount of the variation in spring bud burst timing by integrating the influences of both random and fixed factors (Table 3).

Figure 6 illustrates the phenological variability of ash clones in two bioclimatic regions from 2018 to 2020. The distinct differences in phenological timing, particularly in clones 6313 and 7887, indicate substantial phenological diversity within the regions. The number of grafts allows for the assessment of clone health, which can be useful in analyzing their viability. An overall decline in the number of grafts was observed in most clones across both bioclimatic regions, which may suggest ongoing issues with infection by the fungus H. fraxineus. While the trends are similar, some clones exhibit greater resistance or susceptibility to conditions affecting their viability, potentially due to their specific genetic traits.

Based on the analysis of available data, the Bioclim 1 region (Figure 6) demonstrates greater adaptive capacity. This is due to the greater stability in the number of grafts and favorable phenological deviations, which suggest better adaptation to environmental conditions. Clone 6313, with a constant number of grafts over three years, deserves special attention. Its positive phenological deviations suggest that it is better adapted to earlier phenological onset, which may be advantageous under certain stress conditions. Clone 5723 shows smaller declines in the number of grafts compared to other clones in the Bioclim 1 region. On the other hand, it should be noted that in Bioclim 3, clone 7301 maintains a relatively stable number of grafts compared to other clones, despite some declines (Figure 6).

The phenological distribution of the clones shows that Bioclim 1 has more clones with early phenological onset, while Bioclim 3 is characterized by a greater number of late clones. This differentiation may indicate the adaptation of clones to specific bioclimatic conditions in these regions. It should be noted that clones 6313 from Bioclim 1 and 7301 from Bioclim 3 show early phenological onset, which is consistent with their high survival rate.

Figure 7 illustrates the degree of defoliation in a group of ash clones over three years (2018–2020) based on their phenological stages. Each year is presented in a separate panel, highlighting the relationship between phenological stages and the degree of defoliation. Figure 7 reveals a clear association between phenological stages and the severity of defoliation in ash clones. Similarly to 2018, in 2019, the correlation between phenological stages and defoliation is generally weak, except for the highly defoliated clones (purple). The correlation for this group is somewhat more pronounced, with p < 0.005. In 2020, the correlation is more evident for the highly defoliated clones (purple), showing R² = 0.37 and p < 0.001. This moderate correlation indicates that clones in the latest phenological stages are more susceptible to severe defoliation. The phenological stage appears to be a key factor influencing the severity of defoliation caused by H. fraxineus infection, with late-developing clones being more vulnerable to damage. This late development may be influenced by genetic or physiological factors, further increasing the susceptibility of these clones to the pathogen.

Figure 8 illustrates the changes in defoliation levels from 2018 to 2020 in two bioclimatic regions, Bioclim 1 and Bioclim 3, highlighting differences in the rate of defoliation reduction between these regions. In Bioclim 1, there is a notable decrease in the mean defoliation levels over the observed years, with mean defoliation values (Dmean) of 32.57 in 2018, 28.43 in 2019, and 26.11 in 2020. Similarly, Bioclim 3 also shows a decrease in defoliation levels over the years. The better vitality of trees is more pronounced in Bioclim 3, suggesting a more effective response to defoliation factors. Fisher’s statistic (F = 22.84) indicates highly significant differences in defoliation levels across the years in Bioclim 3, and Bayesian analysis confirms a significant reduction trend during the observed years. The variance components (σG² = 0.05 and σE² =0.05) underscore the role of genetic factors in shaping the dynamics of defoliation.

4. Discussion

Ash dieback caused by the fungal pathogen Hymenoscyphus fraxineus is a serious ecological problem in Europe. It is estimated that in some regions of Europe, even 70%–90% of ash stands were severely affected, with many trees dying as a result of the infection. Depending on the region, the intensity of ash dieback can vary; however, in the most severely affected areas, losses can reach up to 95% of ash trees [12]. The estimated economic losses due to ash dieback are projected to reach GBP 15 billion in the UK alone over the next 100 years [31]. Currently, there are no effective methods to protect ash trees from infection with H. fraxineus [15]. Ongoing scientific research focused on mitigating the ecological consequences of the epidemic is investigating evolutionary mechanisms of resistance in ash trees by selecting genotypes that are less susceptible to dieback.

In the present study, we evaluated the natural evolutionary mechanism of spring bud burst timing and its impact on the dynamics of infection. We confirmed the well-known relationship between phenology timing and provenance, and also demonstrated higher survival and vitality in clones with earlier bud burst times. These findings may have practical implications, providing guidelines for selecting genotypes to be used as parent generations for generative and vegetative reproduction.

The obtained results are a logical consequence of the nature of the epidemic, whose gradual expansion, combined with the high intensity of the disease, suggests that H. fraxineus is an invasive alien organism [32]. In its native range (Japanese populations), the fungus acts as a leaf endophyte, and it is possible that its invasiveness in Europe would be negligible with limited inoculum. However, the currently observed high inoculum density allows the fungus to challenge host defense responses [33], leading to an epidemic state. The dynamic nature of ash dieback was confirmed in the present study, where 30% of plants died within the first few years of growth; now, the dynamics of dieback are decreasing (Figure 7). The significant reduction in defoliation levels was a result of removing fallen ash leaves during winter, which are a source of reinfection by the pathogen in the spring—a treatment and its effectiveness are described by [23]. Observations of the relationship between leaf shedding and the level of secondary infections by H. fraxineus—along with the spring bud burst phenology—are natural defense mechanisms of ash trees against pathogen infection. The present study demonstrated a significant difference in clones regarding the timing of spring bud burst (Figure 8), which had a significant impact (p < 0.001) on the health of ash trees most affected by H. fraxineus (Figure 7). The level of ash tree infection is influenced by the evolutionary adaptations discussed in this study, genetic factors [34], and environmental interactions [15].

Conversely, the absence of distinct population structure in H. fraxineus throughout Europe [6,34], along with a noticeable decline in allelic richness compared to native groups [35], underscores the necessity for effective natural strategies to mitigate epidemics. Field observations indicate that only a minimal number of foliar infections progress to shoot infections [6], likely because infected leaves are often shed before the pathogen can invade the host’s phloem or xylem. Consequently, extensive research is being conducted to identify genotypes with greater disease tolerance [36].

In addition to phenotypic investigations, there is a significant demand for molecular research, including gene-focused studies to better identify genetic resistance traits, potentially improving the selection process for resistant genotypes. One of the most significant phenotypic traits influencing the severity of H. fraxineus infestation is the timing of spring bud burst, as explored in this study [37,38]. These results underscore the critical role of phenology as a resistance trait against H. fraxineus. By highlighting the timing of spring bud burst as a vital phenotypic indicator, this research advances the scientific understanding of the mechanisms enabling ash tree resistance. Recognizing phenology as a resistance trait opens new avenues to integrate this knowledge into breeding and conservation measures, enhancing ash population resilience against increasing pathogen pressure.

Significant genetic correlations between phenological traits and crown damage caused by H. fraxineus infection were found in Danish and Swedish studies [9,39]. The genetic correlation between spring phenology (bud burst) and clonal health (percentage of crown defoliation) was significant. Additionally, the same studies found a strong correlation between ash infestation levels and leaf senescence (measured as autumn leaf color), with fewer infested ash clones exhibiting earlier yellowing. Recent studies suggest that phenological traits, such as the timing of spring bud burst, may have a genetic basis influencing resistance to H. fraxineus. Early bud burst was associated with lower susceptibility to crown damage, potentially due to the avoidance of peak infection periods. These findings indicate that genetic elements underlying phenological timing may play a key role in the evolutionary mechanisms of resistance [9,39].

The results confirm that leaves, as one of the primary symptoms of infection, play a key role in the dynamics of disease recognition in ash trees. Similar results were obtained by Baliuckas and Pliura [38] in experiments with Lithuanian and Western European populations, in which populations with earlier spring phenology had significantly (r = 0.81) better health. The aforementioned analysis by Baliuckas and Pliura [39] clearly demonstrates the relationship between health status and spring frost occurrence, as well as differences among populations. Baliuckas and Pliura [39] showed that populations from higher latitudes and greater longitudes were less damaged. These findings demonstrate that the spring development of ash trees is an epigenetic trait inherited by offspring. Our study confirms the results obtained in previous studies [9,38,39]. The present study also confirms the genetic dependence of phenological timing on the provenance of the clones. Although the northern clones grew under different climatic conditions, they retained the genetic pattern of later spring phenology. This phenomenon likely has ecological consequences related to the evolutionary escape of northern ash trees from late frosts [40].

The analysis of climatic data related to the occurrence of ash in Poland confirmed the bioclimatic regionalization of ash (Figure 3 and Figure 4). Eight bioclimatic variables responsible for the occurrence of the species were identified (Table 2). The studied ash trees, distinguished by their regions of origin, were grouped into two basic provenances (bioclimatic region 1 and bioclimatic region 3), which differed from each other in terms of the timing of phenological events and health status. Trees from Northern Poland (bioclimatic region 3) were significantly characterized by later timing of phenological appearance. The differences noted among bioclimatic zones highlight how climate influences the schedule of biological events and the condition of ash trees. In Northern Poland (bioclimatic zone 3), ash populations, which exhibit delayed phenological phases, are potentially more vulnerable to H. fraxineus because of extended periods that are conducive to pathogen growth in early spring. These variations indicate that the timing of phenological events, shaped by local climate, plays a key role in assessing both vulnerability and the ability to adapt. Grasping these relationships is vital for creating conservation and breeding plans tailored to specific regions.

It is necessary for the selection and breeding of ash to determine whether the reduced damage is due to adaptation to growing conditions or to reduced susceptibility to H. fraxineus. In this context, data showing the degree of defoliation of ramets as a function of their origin and phenology are important. It should be noted that the spore pressure of H. fraxineus in the present study decreases in years when fallen leaves, which serve as a source of infection the following spring, are removed (Figure 8). Our results revealed that the ramets from different bioclimatic regions had the greatest differences in damage levels in 2018, while the difference between ramets in 2020 was not significant (Figure 8). Therefore, it can be hypothesized that the cause of damage to ash crowns is H. fraxineus, which in 2018 significantly damaged ash crowns at a later spring bud break due to high spore pressure. On the other hand, with low spore pressure from H. fraxineus, variations in the extent of damage to ash trees were negligible. The stronger correlation between phenology and damage observed in 2020 can be attributed to the reduction in pathogen pressure achieved through the removal of fallen leaves, which are the primary source of inoculum. This practice allowed the resistance traits of clones to become more evident, highlighting the importance of environmental management practices in mitigating pathogen impact and differentiating genotypes. This conclusion confirms the general knowledge about the development of H. fraxineus. The pathogen forms its apothecia mainly in petioles and rachitic leaves lying on the ground, releasing ascospores that infect ash trees [32]. The number of airborne ascospores is highest in the early morning and depends on the humidity protecting the ascospores from drying out [41]. Thus, despite the release of spores over a long period [42], ecological conditions for the infection of ash trees by the pathogen are most favorable for a few days in early spring; later, high temperatures reduce the effectiveness of infection.

The ideas and results derived from this research may be relevant to areas beyond Europe, where other ash tree species are impacted by invasive illnesses. For example, analogous methods might be utilized in North America, where Fraxinus varieties are significantly endangered by pathogens like H. fraxineus and invasive insects, including the emerald ash borer (Agrilus planipennis) [43,44]. Incorporating phenological and genetic indicators in breeding and management plans could offer essential knowledge for reducing the effects of invasive species worldwide. These approaches might also aid in preserving genetic variation and enhancing the resilience of ash populations across various ecological environments. Future research should integrate phenological, genetic, and molecular analyses to better understand resistance mechanisms and develop global breeding and conservation strategies.

5. Conclusions

Ash dieback (Fraxinus excelsior), caused by the invasive pathogen H. fraxineus, poses a significant threat to European forests. Our study demonstrated that phenological variations, particularly the timing of early spring bud burst, play a crucial role in reducing infection risks. Clones with earlier bud burst exhibited higher survival rates and less crown damage, underscoring the importance of phenological timing in selecting resistant genotypes.

These findings emphasize the need for continued research into the genetic and ecological factors underpinning resistance. Phenology, shaped by environmental factors such as temperature, not only dictates the timing of bud burst but also influences the trees’ resilience to stressors.

Understanding these dynamics could enable the development of more adaptive forest management practices that account for the interplay between phenology, disease resistance, and environmental stressors. Providing forestry practitioners with region-specific guidelines for selecting genotypes based on phenological traits, such as early spring bud burst, could enhance the efficiency of breeding programs and natural regeneration efforts. These practical measures could significantly contribute to sustaining ash populations and ensuring the ecological stability of forest systems.