Valentin Journé

Ecological forecasting is essential for climate change adaptation and mitigation. In reforestation and restoration, seed production forecasting has the potential to support planning and resource allocation, while providing benefits for wildlife management and public health. We hind‐ and forecast seed production using statistical models based on weekly weather and high‐resolution seed data of six tree species recorded in two Austrian old‐growth forest sites. Using a sliding‐window approach and model selection, we model annual reproduction for three coniferous (Silver fir, European larch, Norway spruce) and three broad‐leaved species (Sycamore maple, European beech, European ash). We investigate changes in explained variance with decreasing time before seed rain and evaluate hindcasting proficiency and the potential forecast horizon using quantitative and categorical measures useful to practitioners. Furthermore, we compare the performance of local models with a general forecasting model for seed production in European beech. Most local models show unbiased but partly imprecise predictions with a broad range in explained variance (0.15–0.93) in the year prior to seed rain. Nonetheless, within this timeframe, hindcasting seed rain above 10% of the long‐term maximum, a threshold relevant to practitioners, works well for all species. Previous seed rain explains a considerable proportion of the variation in seed rain of fir, ash, and maple. We forecast seed rain for 2022 to 2025, with mixed results for 2022 and 2023. Overall, categorical year out predictions seem feasible for most studied species, but model refinement is required. Local models for beech outperformed the general model in predicting crop failures and bumper crops. Synthesis and applications: Seed production can be predictable with a promising degree of accuracy for six European tree species in the year prior to seed rain, if combined with on‐site monitoring of seeds, phenology, and weather. This holds value for seed harvesters, nurseries and forest managers, and may inform orchard management and public health risk anticipation. Seed forecasts will help address seed scarcity and support climate change adaptation and mitigation. Future efforts should prioritise species based on rarity and seed storability, as well as forming stronger partnerships with potential forecast users.

Summary: Mast‐seeding trees can vary seed output by orders of magnitude among years, but it remains unclear whether high seed production reduces per‐seed investment, as predicted by fixed‐budget allocation models. We quantified individual seed production with seed mass in European beech across 2792 trees and 123 populations spanning the species' range and quantified seed protein and lipid content in 35 populations. Seed mass increased with seed production, with seeds from high‐seeding years being 14% heavier than those from low‐seeding years, providing no evidence for a seed size–number trade‐off and instead supporting variable reproductive allocation. By contrast, protein content decreased by 31% with increasing seed production, whereas lipid content increased (by 14%), indicating that nitrogen becomes constraining at high reproductive output while carbon‐based provisioning is maintained. Climate further structured provisioning: seed mass and protein content were the lowest at climatic range margins, being 28% and 32% lower, respectively, than at the center of the climatic range. European beech can increase seed output without reducing per‐seed biomass, but that nitrogen limitation and climatic constraints may strengthen regeneration bottlenecks at both trailing and leading margins, especially as climate warming intensifies.

Summary: To survive climate change, forest trees will have to shift seed production poleward. However, warming will not stimulate tree fecundity in the north if it is limited by other habitat variables. We evaluated the responses of tree fecundity to climate change for 292 tree species in North America and Europe, using response velocity, defined as (climate sensitivity) × (climate‐change rate). The sensitivities to climate were estimated for each species and combined with rates of climate change to quantify how temperature, moisture deficits, and late freeze are influencing biogeographic shifts in tree reproduction. The results show that moisture deficit and late freeze, not annual temperature, drive changing seed production. Unlike annual temperature, which is increasing generally, change in these climate variables is not driving poleward shifts in seed production. These findings do not challenge the expectation that forests might eventually shift poleward. Rather, they show why current efforts offer divergent interpretations. The changes happening now are not consistent with annual temperature trends. As warming continues, fecundity changes can best be anticipated from temperature interactions with precipitation and extremes that impact flowering and fruiting in winter and spring.

Les changements globaux affectent la répartition des espèces et le fonctionnement des écosystèmes et sont soumises à des dépérissements massifs. Un des autres processus majeurs de la dynamique forestière est la reproduction sexuée, qui permet aux espèces de persister au fil des générations, notamment grâce à l’adaptation et la migration. Mais il reste de nombreuses incertitudes sur l’effet du climat sur la reproduction des arbres, notamment sur quand et comment sont allouées les ressources au pollen et aux graines. L’objectif principal de cette thèse est d’utiliser une approche basée sur la modélisation des ressources et de la phénologie pour comprendre et prédire l’effet du climat sur la reproduction des arbres forestiers. J’ai d’abord étudié les déterminants impliqués dans les variations spatiales et temporelles de fructification. Puis j’ai quantifié l’allocation des ressources entre les fonctions de croissance et de reproduction à l’aide d’un modèle bayésien hiérarchique. Ce modèle, basé sur des mesures de croissances et de reproduction mâle et femelle sur plusieurs années m’a permis d’identifier des compromis lors de l’initiation florale, et l’importance de la quantité de ressources pour assurer la reproduction. Enfin j’ai développé un modèle mécaniste de reproduction, au sein d’un modèle déjà existant de fonctionnement des forêts (CASTANEA). Le modèle de reproduction, fonctionnant de l’initiation florale jusqu’à la maturation des graines, simule les variations spatiales et temporelles de production de graines. Le modèle a été calibré et validé sur le sapin pectiné au Mont-Ventoux et utilisé pour étudier les effets du changement climatique sur la reproduction

Tree mast seeding i.e., synchronous, highly variable seed production among years, has wide consequences for ecosystem functioning. In a large-seeding year, a pulse of resources is made available to wildlife over the large spatial scale by virtue of the synchronous reproduction by millions of trees. But how trees synchronize masting over the vast geographical areas? A major mechanism governing the annual allocation of resources to seed production is weather variation. If individuals respond to the same “weather cue” across extensive regions, masting synchrony can emerge. This requires, however, the timing of the cue window being well conserved across species range, but mechanisms facilitating such stability remain unknown. Here, we investigated factors driving masting synchrony in European beech, which extends to the thousands of kilometres throughout its geographic range. We used a moving window analysis to determine how correlations between annual seed production and mean temperatures in 61 populations of European beech sampled across the species' range fluctuate at a fine temporal scale. We found that correlation coefficient values between seed production and temperature rapidly increased after the summer solstice, compared to before it. Moreover, using temporally-restricted permutation tests we showed that this abrupt increase in masting-weather correlations at the solstice is not driven purely by chance. Beech achieves high spatial synchrony in seed production by anchoring the weather cue window to the summer solstice - the longest day of the year that occurs simultaneously across the whole Northern Hemisphere. Beech abruptly opens its temperature-sensing window on the solstice, hence widely separated populations all start responding to weather signals in the same week. This enables cohesive timekeeping across distant populations inhabiting diverse climatic regions and creates a high precision timing of the Moran effect, leading to the subcontinental-scale synchrony of beech mast seeding.

Human-wildlife conflicts, particularly the damage to agricultural crops caused by ungulates, pose significant ecological and economic challenges. Understanding the role of natural food availability in driving these conflicts is important for developing effective management strategies. We investigated how the pulsed availability of forest tree seeds, i.e., mast seeding, influences the extent of agricultural crop damage in Poland. Using a 19-year national dataset (2001-2020), we analyzed the relationship between oak Quercus spp. and European beech Fagus sylvatica seed production, the abundance of wild boar Sus scrofa and red deer Cervus elaphus, and the area of damaged agricultural crops. We found a negative relationship between oak seed production and the level of crop damage, with estimated damage decreasing by 30% from years of seed failure to years of abundant seed production, supporting the hypothesis that a diet shift occurs in ungulates during years of seed abundance that averts ungulates from damaging the crop. In contrast, beech seed production showed no significant effect on crop damage. Our findings demonstrate that pulsed resource dynamics in forests are an important driver of human-wildlife conflict in adjacent agricultural landscapes.

Perennial plants reproduce through a high interannual variability in seed production, with irregular years of high reproductive output—a phenomenon known as mast seeding or masting. While mast seeding is linked to fluctuations in weather and resource availability, the molecular mechanisms that trigger or suppress flower production remain poorly understood. Here, we explored molecular basis of tree flowering, and relationships between gene expression and environment. We collected a unique nine-year dataset of seasonal gene expression from leaf samples of Japanese beech (Fagus crenata) at Mt. Naeba in central Japan. From a total of 40,000 genes, we identified 20 genes whose expression levels were associated with heavy flowering, including key floral regulators and additional genes related to sulfur deficiency. Nitrogen has previously been shown to be a key trigger of flowering in F. crenata. We found that increased nitrogen availability elevates sulfur demand, and when sulfur supply fails to keep pace, plants experience sulfur limitation and activate a low-sulfur response that suppresses flowering. These findings provide new insights into how seasonal molecular regulation of tree flowering shapes reproductive phenology and its relationship with resource availability.

In 2023, more than half of olive harvests ( Olea europaea ) across Spain, Greece, and Türkiye were lost to drought. The same year late freeze destroyed 90% of the peach crop ( Prunus persica ) on the Georgia Piedmont and the apple crop ( Malus domestica ) in central New York, Vermont, and southern Quebec. Climate extremes now rank with the costliest threats to agriculture, but their role in forest recovery from diebacks that are happening globally is unknown for lack of tree fecundity estimates in forests. Tolerance of climate extremes could depend on past exposure but constrained by phylogenetic conservatism. We report a continental scale analysis of climate extremes and forest fecundity across North America and Europe showing that responses to late freeze and drought are happening now. Species differences are not explained by the traits typically included in ecological studies and they are weakly associated with phylogeny. Late freeze, that is, freezing temperatures that follow the onset of flower development in spring, is shown to be “normal” in North America, but not Europe, potentially explaining failed seed production due to delayed onset and the resultant shorter growing period by North American transplants dating back at least to the 18th century. Drought has thus far had the greatest impacts in dry forested regions, but here too, species differences are not explained by traditional trait values. If responses have been buffered from drought and late freeze by past exposure, acclimation and local adaptation prove inadequate as extremes intensify.

Seed production in perennial plants often shows strong year-to-year variation, a phenomenon known as masting. Masting is typically adaptive and driven by weather cues that synchronize reproduction by promoting or suppressing flowering and seed set. These cues, involving temperature, precipitation, and drought, differ across species and regions, yet a global synthesis is needed. Understanding such patterns is crucial, as reproductive responses to climate interact with resource constraints to shape plant fitness under climate change. Here, we analyze climate-reproduction relationships in 746 populations of 331 species worldwide using moving-window methods. We find that temperature is the dominant cue for reproductive investment, though its influence is weaker in the wettest and driest climates. In temperate and boreal regions, reproduction is most often linked to warmer conditions, whereas in the tropics, reproduction is disproportionately linked to cooler temperatures. Therefore, we expect rising temperatures to increase reproductive failures in the tropics, but to reduce interannual variability in temperate and boreal zones as favorable cues become more frequent. These results confirm long-standing theoretical predictions and provide a global framework for evaluating climate change risks to mast seeding.