PD Dr. Yann Vitasse

Purpose of review: To synthesize new information regarding the environmental sensitivity and impact of climate change on leaf-, wood-, phloem- and root phenology of deciduous forests of the temperate (and boreal) zone, comprising overstory and understory, and both woody and herbaceous species.
Recent findings: The environmental sensitivity and impact of climate change on spring leaf phenology are relatively well understood, with ongoing efforts focusing on the spatial and temporal variability in both overstory and understory. Autumn leaf phenology and cambial phenology have received increasing attention in recent years. The drivers of senescence progression are well understood (current temperature), while the drivers of the onset of senescence are still uncertain but likely relate to spring temperature, water availability and light conditions. Studies on cambial phenology of angiosperm trees have focused on the variability across populations and years, while studies on phloem remain scarce and synthesis studies are unavailable. For fine root phenology, asynchronicity with leaf phenology and high variability among species have been demonstrated, but large uncertainty remains regarding the drivers of the onset and cessation of their growth. Studies on woody and herbaceous understory highlight the importance of microclimate differences within the stand.
Summary: Future phenology research should focus on (i) onset of leaf senescence, (ii) fine roots, (iii) the relationships between overstory and understory species not only regarding leaves, but also wood and fine roots, (iv) variability across multiples scales (e.g. individuals, stands), and (v) interannual legacy effects and connections among phenophases of different organs and forest compartments.

• Tree net carbon (C) uptake may decrease under global warming, as higher temperatures constrain photosynthesis while simultaneously increasing respiration. Thermal acclimation might mitigate this negative effect, but its capacity to do so under concurrent soil drought remains uncertain.
• Using a 5-yr open-top chamber experiment, we determined acclimation of leaf-level photosynthesis (thermal optimum T_opt and rate A_opt) and respiration (rate at 25°C R₂₅ and thermal sensitivity Q₁₀) to chronic +5°C warming, soil drought, and their combination in beech (Fagus sylvatica L.) and oak (Quercus pubescens Willd.) saplings. Process-based modeling was used to evaluate acclimation impacts on canopy-level net C uptake (A_tot).
• Prolonged warming increased T_opt by 3.03–2.66°C, but only by 1.58–0.31°C when combined with soil drought, and slightly reduced R₂₅ and Q₁₀. By contrast, drought reduced T_opt (−1.93°C in oak), A_opt (c. 50%), and slightly reduced R₂₅ and Q₁₀ (in beech). Mainly because of reduced leaf area, A_tot decreased by 47–84% with warming (in beech) and drought, but without additive effects when combined.
• Our results suggest that, despite photosynthetic and respiratory acclimation to warming and soil drought, canopy-level net C uptake will decline in a persistently hotter and drier climate, primarily due to the prevalent impact of leaf area reduction.

1. Predicting phenology, that is the timing of seasonal events of plant life such as leaf emergence and colouration in relation to climate fluctuations, is essential for anticipating future changes in carbon sequestration and tree vitality in temperate forest ecosystems. Existing approaches typically rely on either hypothesis-driven process models or data-driven statistical methods. Several studies have shown that process models outperform statistical methods when predicting under climatic conditions that differ from those of the training data, such as for climate change scenarios. However, in terms of statistical methods, deep learning approaches remain underexplored for species-level phenology modelling.
2. We present a deep neural architecture, PhenoFormer, for species-level phenology prediction using meteorological time series. Our experiments utilise a country-scale data set comprising 70 years of climate data and approximately 70,000 phenological observations of nine woody plant species, focussing on leaf emergence and colouration in Switzerland. We extensively compare PhenoFormer to 18 different process-based models and traditional machine learning methods, including Random Forest (RF) and Gradient Boosted Machine (GBM).
3. Our results demonstrate that PhenoFormer outperforms traditional statistical methods in phenology prediction while achieving significant improvements or comparable performance to the best process-based models. When predicting under climatic conditions similar to the training data, our model improved over the best process-based models by 6% normalised root-mean-squared error (nRMSE) for spring phenology and 7% nRMSE for autumn phenology. Under conditions involving substantial climatic shifts between training and testing (+1.21°C), PhenoFormer reduced the nRMSE by an average of 8% across species compared to RF and GBM, and performed on par with the best process models.
4. These findings highlight the potential of deep learning for phenology modelling and call for further research in this direction, particularly for future climate projections. Meanwhile, the advancements achieved by PhenoFormer can provide valuable insights for anticipating species-specific phenological responses to climate change.

Aim: Plant senescence largely influences the global carbon cycle by regulating the growing season length. However, the driving mechanisms of plant senescence remain unclear, particularly the role of developmental factors. This study aims to investigate how environmental and developmental factors drive autumn senescence and evaluate whether woody and herbaceous plants exhibit divergent responses to these drivers.
Location: Eurasia.
Time Period: 1982–2014.
Major Taxa Studied: Woody and herbaceous species.
Methods: Using 120,833 long-term ground phenological observations, we employed partial correlation analysis to investigate the influence of environmental and developmental factors on senescence termination. Experimental records from literature and pasture survey observations from China were separately utilised to further validate the influence of developmental factors on senescence termination. Structural equation modelling was applied to analyse the pathways of growth onset affecting senescence termination. Additionally, multiple linear regression was used to examine the tendency of the sensitivity of senescence termination to plant development rate.
Results: We find that earlier growth onset primarily leads to earlier senescence termination directly in herbaceous plants, but indirectly in woody plants by accelerating early-season development. The sensitivity of senescence termination to plant development rate shows a declining trend, particularly in early-season negative effects on woody plants and late-season positive effects on herbaceous plants, suggesting diminished impacts of future warming on senescence timing. The impact of growing season photosynthetic activity on senescence termination is not pronounced for both woody and herbaceous plants.
Main Conclusions: The results demonstrate that growth onset may affect woody and herbaceous senescence termination through different pathways, whereas the carry-over effects of growing season photosynthetic activity are not widely discovered. This emphasises that the introduction of developmental factors into phenological models needs to be considered carefully according to plant type.

Changes in vegetation carbon uptake are largely influenced by the timing and magnitude of the peak of the growing season (POS), when vegetation photosynthesis reaches its maximum. However, the factors controlling the timing of POS remain poorly understood, leaving us uncertain about its future trajectory. Using satellite observations and carbon flux measurements, we show that, in recent decades, increased early-season carbon uptake has been driven by both an earlier onset of the growing season and higher temperatures. In 93% of northern (>30°N) vegetation, these increases in early-season carbon uptake were associated with an advancement of POS. This ongoing shift suggests a developmental constraint on seasonal productivity, potentially limiting carbon uptake later in the season. Our findings provide a mechanistic explanation that reconciles previous observations linking earlier growing season onset, rising temperatures, and shifts in POS timing, and suggest a decrease in late-season carbon uptake with climate warming.

Global warming increases the likelihood that temperate tree species will face damaging late spring frost (LSF) and severe summer drought during the same growing season. However, the interactive effects of these two stresses are barely explored. We investigated the physiological and growth responses of Acer campestre, Fagus sylvatica, Quercus robur and Quercus petraea saplings to artificially induced LSF and drought, focusing on stomatal gas exchange, carbon partitioning, nonstructural carbohydrates (NSCs), phenology and tree growth. LSF depleted NSCs and changed carbon allocation patterns 1 month after the event. Additionally, LSF decreased diameter increment and root growth of A. campestre and F. sylvatica in the current year. Drought affected gas exchange of all species, decreased NSCs of A. campestre, reduced biomass of Q. robur, and exacerbated the detrimental LSF effect on Q. robur's NSCs. Our findings indicate that saplings prioritized canopy restoration immediately after LSF, and favored reserve replenishment before growth until the end of the growing season. Furthermore, we highlight the risk that LSF and drought in the same year could push tree species beyond their physiological limits and we emphasize the importance of studying multiple stressors' interactions to better understand threshold effects that could profoundly alter forest ecosystems.

1. The concept of growing degree days (GDDs) is commonly used to predict phenological events in plants, assuming that plants develop proportionally to the accumulated temperature. Two species-specific parameters, T_Base and t₀ (minimum temperature above which and start date when GDDs begin to accumulate), are considered for the calculation. However, species-specific optimised thresholds of wild herbaceous species remain sparse, and therefore the reliability of the models is questionable.
2. By employing several modelling approaches using phenological records of leaf unfolding and flowering onset of 87 wild herbaceous species collected in six European botanical gardens between 2019 and 2024, we assessed the reliability of GDD models across a diverse array of species. We further examined whether thresholds of T_Base and t₀ for calculating GDD can be optimised for a large set of species and for single species. We aimed to estimate and evaluate these thresholds and the reliability of GDD models using species' temporal niche and bud traits to see whether for specific groups of species, specific GDD models work better.
3. Our analyses revealed that GDD models for leaf unfolding and flowering onset performed better than the null model (i.e. mean date across years and species) for 84% and 70% of the species, respectively. Our results showed that species with intermediate temporal niches were less dependent on the selection of T_Base and t₀. Overall, we found better performance of the models using a T_Base around 4°C for most of the species. By considering optimised thresholds, we found that predictions of leaf unfolding dates were more accurate in early-growing species, and regarding the start date for temperature accumulation, we found that larger values for t₀ are suitable for predictions for species with later leaf unfolding or flowering onset.
4. Our results emphasise that simple temperature accumulating GDD models can be optimised by using the temporal niches of the studied species to approximate the underlying model parameters or by applying thresholds that are valid for many species. The use of simple but optimised GDD models can be advantageous for small datasets that would otherwise be overfitted with more complex models.

Purpose of Review: This review synthesizes recent advancements and identifies knowledge gaps in the tree growth phenology of both belowground and aboveground organs in extra-tropical forest ecosystems. Phenology, the study of periodic plant life cycle events, is crucial for understanding tree fitness, competition for resources, and the impacts of climate change on ecosystems. By examining the phenological processes of various tree organs, the review aims to provide a comprehensive understanding of how these processes are interconnected and how they influence overall tree growth and ecosystem dynamics. The review aims to provide a comprehensive overview of current knowledge, highlight recent technological advancements, and identify critical areas where further research is needed.
Recent Findings: The review highlights significant progress in monitoring leaf and canopy phenology, thanks to advancements in remote sensing and automated observation systems. These technologies have enhanced our ability to track seasonal changes in leaf development and canopy dynamics more accurately and over larger areas. There has also been a substantial increase in research on wood formation in stems, expanding beyond northern hemisphere conifers to include a broader range of functional groups. However, despite these efforts, identifying the precise drivers of wood formation remains challenging, necessitating further integration of molecular and eco-physiological insights. A critical area of focus is root phenology, encompassing both primary and secondary growth. Despite the fundamental role of roots in tree physiology and ecosystem dynamics, our understanding of root phenology remains limited, primarily due to the inherent difficulties in monitoring root growth. The review emphasizes the need for more detailed studies on root growth processes and the development of new methodologies and technologies to improve root phenology assessments.
Summary: The review highlights the importance of incorporating eco-physiological insights into phenological assessments. Leaf and canopy phenology would benefit from more studies focusing on autumnal events. Indeed, compared to the onset of the growing season, much less is known about its end, despite its critical importance for understanding processes such as carbon uptake and nutrient cycle. Advancing knowledge of wood growth phenology will require greater focus on angiosperms, as research on xylogenesis has historically been centered on gymnosperms. This will likely necessitate the development of new, tailored methodologies to address the characteristics of angiosperm wood formation. Similarly, further exploration of phloem phenology is essential to better understand the links between phenological processes across different organs. Finally, compared to other organs, root growth remains less well understood, underscoring the need for deepening the investigation on root phenology in the coming years.

A variety of phenology process-based models have been developed to simulate environmental influences on the timing of spring and autumn phenophases. Similar performances between different types of mechanistic models have raised questions about reliability of their predictions. To assess the biological relevance of phenology models, we used a seven-decade dataset of five species across 170 sites and 1700 m elevation in Switzerland. We evaluated nine leaf emergence and ten senescence models over time and space. We explored how optimal parameter values and influences vary, reflecting transitions in model aptitude and phenology responses to drivers. Leaf emergence models showed improved predictions at external sites over time, while emergence dates converged across Switzerland. In contrast, leaf senescence models often failed to outperform the null model predicting the mean date of training data and showed divergent performance trends. Trends in optimal parameters indicated species-specific responses to emergence drivers, with cold-climate suited species favouring earlier thresholds for warmth accumulation in spring, while the trends were opposite for warm-climate suited species, except for beech showing stable parameters likely due to strong photoperiod constraints. Warming increased the importance of chilling-related parameters for leaf emergence, while senescence parameter sensitivities remained stable. Spatial analyses revealed that complex models were less robust to training and validation at different elevations than simple models, and that phenological responses may vary non-linearly with elevation, likely due to local adaptations. Senescence models performed better with validation at high elevations, where climatic variables such as cooling temperatures play a large role, while predictions were more challenging at other elevations. These findings highlight the need for further refinement of process-based models to account for all driving influences on plant phenology, particularly for leaf senescence models. Our work demonstrates the potential for process-based modelling techniques to better understand phenology responses to climate change.

• Climate change could reduce dioecious plant fitness if the phenology of males and females responds differently to temperature. However, the extent to which spring phenological responses to climate differ between sexes in wind-pollinated dioecious trees remains poorly understood.
• Here, we combined ground observations with climate-controlled experiments to investigate sexual differences in spring budburst in Ginkgo biloba, Fraxinus chinensis, and Eucommia ulmoides.
• In 96% of in situ cases, male trees initiated budburst earlier than females, on average by 3.0 ± 0.4 d. This disparity was more pronounced in warmer regions. The experiment indicated that background climate is a key predictor of sexual disparity in budburst, with the largest differences observed in twigs originating from regions with higher mean annual temperatures and precipitation. However, these disparities declined in areas where mean annual temperatures exceeded 17.1°C, indicating nonlinear trends. This pattern aligns with the warming treatments, where sexual disparities decreased under spring warming of 2–10°C.
• These results suggest that while sexual disparities can be larger in warmer climates, dioecious trees possess stabilizing mechanisms, including photoperiod and chilling requirements, to maintain synchrony under warming conditions. Our findings enhance understanding of sex-specific phenological responses to climate change, with important implications for future species conservation and ecosystem management.

Long-term phenological data in alpine regions are often limited to a few locations and thus, little is known about climate-change-induced plant phenological shifts above the treeline. Because plant growth initiation in seasonally snow-covered regions is largely driven by snowmelt timing and local temperature, it is essential to simultaneously track phenological shifts, snowmelt, and near-ground temperatures. In this study, we make use of ultrasonic snow height sensors installed at climate stations in the Swiss Alps to reveal the phenological advance of grassland ecosystems and relate them to climatic changes over 25 years (1998–2023). When snow is absent, these snow height sensors additionally provide information on plant growth at a uniquely fine temporal scale. We applied a two-step machine learning algorithm to separate snow- from plant-height measurements, allowing us to determine melt-out for 122 stations between 1560 and 2950 m a.s.l., and to extract seasonal plant growth signals for a subset of 40 stations used for phenological analyses. We identified the start of growth and calculated temperature trends, focusing particularly on thermal conditions between melt-out and growth initiation. We observed an advance of green-up by −2.4 days/decade coinciding with strong warming of up to +0.8°C/decade. Although the timing of snowmelt has not changed significantly over the study period in this focal region, phenological responses to early melt-out years varied due to differing influences of photoperiodic and thermal constraints, which were not equally important across elevations and communities. Phenological shifts of alpine grasslands are thus likely to become even more pronounced if snowmelt timing advances in the future as predicted. As climate change continues to reshape mountain ecosystems, understanding the interplay between phenological changes and species turnover will be essential for predicting future biodiversity patterns and informing conservation strategies in alpine regions.

Progressively warmer and drier climatic conditions impact tree phenology and carbon cycling with large consequences for forest carbon balance. However, it remains unclear how individual impacts of warming and drier soils differ from their combined effects and how species interactions modulate tree responses. Using mesocosms, we assessed the multiyear impact of continuous air warming and lower soil moisture alone or in combination on phenology, leaf-level photosynthesis, nonstructural carbohydrate concentrations, and aboveground growth of young European beech (Fagus sylvatica L.) and Downy oak (Quercus pubescens Willd.) trees. We further tested how species interactions (in monocultures and in mixtures) modulated these effects. Warming prolonged the growing season of both species but reduced growth in oak. In contrast, lower moisture did not impact phenology but reduced carbon assimilation and growth in both species. Combined impacts of warming and drier soils did not differ from their single effects. Under warmer and drier conditions, performances of both species were enhanced in mixtures compared to monocultures. Our work revealed that higher temperature and lower soil moisture have contrasting impacts on phenology vs. leaf-level assimilation and growth, with the former being driven by temperature and the latter by moisture. Furthermore, we showed a compensation in the negative impacts of chronic heat and drought by tree species interactions.

1. Plant phenology is crucial for understanding plant growth and climate feedback. It affects canopy structure, surface albedo, and carbon and water fluxes. While the influence of environmental factors on phenology is well-documented, the role of plant intrinsic factors, particularly internal physiological processes and their interaction with external conditions, has received less attention.
2. Non-structural carbohydrates (NSC), which include sugars and starch essential for growth, metabolism and osmotic regulation, serve as indicators of carbon availability in plants. NSC levels reflect the carbon balance between photosynthesis (source activity) and the demands of growth and respiration (sink activity), making them key physiological traits that potentially influence phenology during critical periods such as spring leaf-out and autumn leaf senescence. However, the connections between NSC concentrations in various organs and phenological events are poorly understood.
3. This review synthesizes current research on the relationship between leaf phenology and NSC dynamics. We qualitatively delineate seasonal NSC variations in deciduous and evergreen trees and propose testable hypotheses about how NSC may interact with phenological stages such as bud break and leaf senescence. We also discuss how seasonal variations in NSC levels, align with existing conceptual models of carbon allocation.
4. Accurate characterization and simulation of NSC dynamics are crucial and should be incorporated into carbon allocation models. By comparing and reviewing the development of carbon allocation models, we highlight the shortcomings in current methodologies and recommend directions to address these gaps in future research. Understanding the relationship between NSC, source–sink relationships, and leaf phenology poses challenges due to the difficulty of characterizing NSC dynamics with high temporal resolution. We advocate for a multi-scale approach that combines various methods, which include deepening our mechanistic understanding through manipulative experiments, integrating carbon sink and source data from multiple observational networks with carbon allocation models to better characterize the NSC dynamics, and quantifying the spatial pattern and temporal trends of the NSC-phenology relationship using remote sensing and modelling. This will enhance our comprehension of how NSC dynamics impact leaf phenology across different scales and environments. Read the free Plain Language Summary for this article on the Journal blog.

Aim: Climate change challenges temperate forest trees by increasingly irregular precipitation and rising temperatures. Due to long generation cycles, trees cannot quickly adapt genetically. Hence, the persistence of tree populations in the face of ongoing climate change depends largely on phenotypic variation, that is the capability of a genotype to express variable phenotypes under different environmental conditions, known as plasticity. We aimed to quantify phenotypic variation of central Europe's naturally dominant forest tree across various intraspecific scales (individuals, mother trees (families), populations) to evaluate its potential to respond to changing climatic conditions.
Location: Europe.
Time Period: 2016–2019.
Major Taxa Studied: European beech (Fagus sylvatica L.).
Methods: We conducted a fully reciprocal transplantation experiment with more than 9000 beech seeds from seven populations across a Europe-wide gradient. We compared morphological (Specific Leaf Area), phenological (leaf unfolding) and fitness-related (growth, survival) traits across various biological scales: within single mother trees, within populations and across different populations under the contrasting climates of the translocation sites. Results: The experiment revealed significant phenotypic variation within the offspring of each mother tree, regardless of geographic origin. Initially, seedling height growth varied among mother trees and populations, likely due to maternal effects. However, the growth performance successively aligned after the first year. In summary, we observed a consistent growth response in different beech populations to diverse environments after initial maternal effects.
Main conclusions: The study strikingly demonstrates the importance of considering intraspecific variation. Given the surprisingly broad spectrum of phenotypes each mother tree holds within its juvenile offspring, we conclude that Fagus sylvatica might have the potential for medium-term population persistence in face of climate change, provided that this pattern persists into later life stages. Hence, we also suggest further investigating the inclusion of passive adaptation and natural dynamics in the adaptive management of forests.

Global warming is affecting the phenological cycles of plants and animals, altering the complex synchronization that has co-evolved over thousands of years between interacting species and trophic levels. Here, we examined how warmer winter conditions affect the timing of budburst in six common European trees and the hatching of a generalist leaf-feeding insect, the spongy moth Lymantria dispar, whose fitness depends on the synchrony between egg hatch and leaf emergence of the host tree. We applied four different temperature treatments to L. dispar eggs and twig cuttings, that mimicked warmer winters and reduced chilling temperatures that are necessary for insect diapause and bud dormancy release, using heated open-top chambers (ambient or +3.5°C), and heated greenhouses (maintained at >6°C or >10°C). In addition, we conducted preference and performance tests to determine which tree species the larvae prefer and benefit from the most. Budburst success and twig survival were highest for all tree species at ambient temperature conditions, whereas it declined under elevated winter temperature for Tilia cordata and Acer pseudoplatanus, likely due to a lack of chilling. While L. dispar egg hatch coincided with budburst in most tree species within 10 days under ambient conditions, it coincided with budburst only in Quercus robur, Carpinus betulus, and, to a lesser extent, Ulmus glabra under warmer conditions. With further warming, we, therefore, expect an increasing mismatch in trees with high chilling requirements, such as Fagus sylvatica and A. pseudoplatanus, but still good synchronization with trees having low chilling requirements, such as Q. robur and C. betulus. Surprisingly, first instar larvae preferred and gained weight faster when fed with leaves of F. sylvatica, while Q. robur ranked second. Our results suggest that spongy moth outbreaks are likely to persist in oak and hornbeam forests in western and central Europe.

Despite considerable experimental effort, the physiological mechanisms governing temperate tree species' water and carbon dynamics before the onset of the growing period remain poorly understood. We applied ²H-enriched water during winter dormancy to the soil of four potted European tree species. After 8 weeks of chilling, hydrogen isotopes in stem, twig and bud water were measured six times during 2 consecutive weeks of forcing conditions (Experiment 1). Additionally, we pulse-labelled above-ground plant tissues using ²H-enriched water vapour and ¹³C-enriched CO₂ 7 days after exposure to forcing conditions to trace atmospheric water and carbon uptake (Experiment 2). Experiment 1 revealed soil water incorporation into the above-ground organs of all species during the chilling phase and significant species-specific differences in water allocation during the forcing conditions, which we attributed to differences in structural traits. Experiment 2 illustrated water vapour incorporation into all above-ground tissue of all species. However, the incorporation of carbon was found for evergreen saplings only. Our results suggest that temperate trees take up and reallocate soil water and absorb atmospheric water to maintain sufficient above-ground tissue hydration during winter. Therefore, our findings provide new insights into the water allocation dynamics of temperate trees during early spring.

Earlier spring growth onset in temperate forests is a visible effect of global warming that alters global water and carbon cycling. Consequently, it becomes crucial to accurately predict the future spring phenological shifts in vegetation under different climate warming scenarios. However, current phenological models suffer from a lack of physiological insights of tree dormancy and are rarely experimentally validated. Here, we sampled twig cuttings of five deciduous tree species at two climatically different locations (270 and 750 m a.s.l., ~ 2.3 °C difference) throughout the winter of 2019–20. Twig budburst success, thermal time to budburst, bud water content and short-term ²H-labelled water uptake into buds were quantified to link bud dormancy status with vascular water transport efficacy, with the objective of establishing connections between the dormancy status of buds and their effectiveness in vascular water transport. We found large differences in the dormancy status between species throughout the entire investigation period, likely reflecting species-specific environmental requirements to initiate and release dormancy, whereas only small differences in the dormancy status were found between the two studied sites. We found strong ²H-labelled water uptake into buds during leaf senescence, followed by a sharp decrease, which we ascribed to the initiation of endodormancy. However, surprisingly, we did not find a progressive increase in ²H-labelled water uptake into buds as winter advanced. Nonetheless, all examined tree species exhibited a consistent relationship between bud water content and dormancy status. Our results suggest that short-term ²H-labelled water uptake may not be a robust indicator of dormancy release, yet it holds promise as a method for tracking the induction of dormancy in deciduous trees. By contrast, bud water content emerges as a cost-effective and more reliable indicator of dormancy release.

1. Phenological shifts in response to changing climatic conditions is a key acclimation process for the persistence of perennial plants in temperate and boreal climates. The optimal time to leaf-out is the result of an evolutionary process determined by the trade-off between minimizing the risk of freezing damages and herbivory pressure while maximizing resource uptake to increase competitiveness against the other plants.
2. We quantified the penalty exerted by frost damage at the time of leaf emergence on plant development (reduction in leaf area, canopy duration and growth) over the potential gains without frost (increased biomass and non-structural carbohydrate reserves), depending on when leaf-out occurs. To this purpose, we exposed 960 saplings of four temperate deciduous tree species with contrasting cold hardiness to two frost intensities shortly after leaf emergence, which was artificially induced at four occasions to reflect the whole range of natural leaf-out dates.
3. One year above-ground biomass (AGB) increments following the frost revealed a clear ranking among the species depending on their strategy to cope with damaging frosts. Prunus avium (−41% of AGB-increment compared to control saplings) resprouted from the stem base, Quercus robur (−62%) rapidly produced new leaves from dormant reserve buds, Fagus sylvatica (−98%) showed highest chlorophyll content in autumn and delayed senescence together with Carpinus betulus (−105%), which overcompensated NSC reserves after the growing season but showed highest mortality (up to 32%). In all species, NSC reserves recovered rapidly to control levels at the expense of growth.
4. The timing of leaf-out (advanced and delayed artificially) significantly affected the performance and recovery (regreening and growth) of both frozen and non-frozen saplings, with the lowest performance found at the most delayed leaf-out date. We propose that the potential to recover from frost damages is an important component of a tree's performance, particularly at the juvenile stage. The ability to recover may become even more decisive in the future with the predicted increase of false springs in many extratropical regions.

Radial stem growth is a key ecosystem process resulting in long-term carbon sequestration. Despite recognition of its importance to global carbon cycling, high uncertainties remain regarding how radial growth phenology (e.g., the onset, mid, and cessation of radial growth) will be affected by climate change. In this study, we evaluated to what extent high spatially (3 × 3 m) and temporally (up to daily) resolved satellite imagery from PlanetScope can be used to monitor stem growth phenology. For this, we made use of detailed stem growth phenological observations of six common European tree species measured by automated point dendrometers at 14 distinct sites across Switzerland between 2017 and 2021. These growth phenological observations were then linked through multiple regression modeling with metrics extracted from spectral index time series. Our results show that the remote sensing-based models enable monitoring the onset (root mean squared deviation (RMSD) ranges from 5.96 to 27.04 days) and mid-stages of stem growth (RMSD ranges from 10.20 to 36.34 days) with reasonable accuracy as opposed to the cessation of stem growth that showed low accuracy (RMSD ranges from 16.02 to 153.63 days). The accuracy of the remote sensing-based prediction models and their optimal suite of predictors varied across species. The latter has important implications for the remote sensing of stem growth phenology in mixed forests, suggesting that it is important for satellite sensors to resolve individual tree crowns. Overall, our results suggest the need for novel spectral indices that capture the spectral components of mechanistic linkages between stem growth and canopy properties that go beyond the mere detection of leaf phenology. When employing such spectral indices, remote sensing could make it possible to detect not only shifts in leaf phenology caused by climate change but also those in stem growth on a broad spatial scale.

Over the past decades, global warming has led to a lengthening of the time window during which temperatures remain favorable for carbon assimilation and tree growth, resulting in a lengthening of the green season. The extent to which forest green seasons have tracked the lengthening of this favorable period under climate warming, however, has not been quantified to date. Here, we used remote sensing data and long-term ground observations of leaf-out and coloration for six dominant species of European trees at 1773 sites, for a total of 6060 species-site combinations, during 1980–2016 and found that actual green season extensions (GS: 3.1 ± 0.1 day decade^-1) lag four times behind extensions of the potential thermal season (TS: 12.6 ± 0.1 day decade^-1). Similar but less pronounced differences were obtained using satellite-derived vegetation phenology observations, that is, a lengthening of 4.4 ± 0.13 and 7.5 ± 0.13 day decade^-1 for GS and TS, respectively. This difference was mainly driven by the larger advance in the onset of the thermal season compared to the actual advance of leaf-out dates (spring mismatch: 7.2 ± 0.1 day decade^-1), but to a less extent caused by a phenological mismatch between GS and TS in autumn (2.4 ± 0.1 day decade^-1). Our results showed that forest trees do not linearly track the new thermal window extension, indicating more complex interactions between winter and spring temperatures and photoperiod and a justification of demonstrating that using more sophisticated models that include the influence of chilling and photoperiod is needed to accurately predict spring phenological changes under warmer climate. They urge caution if such mechanisms are omitted to predict, for example, how vegetative health and growth, species distribution and crop yields will change in the future.