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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. Summary statement: Our stable isotope labelling study demonstrates that tree saplings take up soil and atmospheric water to hydrate above‐ground tissues before budburst. However, no carbon uptake was found for the investigated deciduous species during this period. [ABSTRACT FROM AUTHOR]

Tree phenology, the timing of periodic biological events in trees, is highly sensitive to climate change. Previous studies have indicated that forest greening can impact the local climate by modifying the seasonal surface energy budget. However, the understanding of tree phenological responses to forest greening at large spatial scales remains limited. Utilizing satellite‐derived phenological and leaf area index data spanning from 2001 to 2021, herein we show that forest greening led to earlier spring and autumn phenology in both temperate and boreal forests. Our findings demonstrated that forest greening during winter and spring contributed to a reduction in surface albedo, resulting in biophysical warming and consequently advancing spring leaf phenology. Conversely, forest greening in summer and autumn induced biophysical cooling through increased evapotranspiration, leading to an earlier onset of autumn leaf phenology. Our findings highlight the significant impact of forest greening‐induced local seasonal climate changes on shaping tree phenology in temperate and boreal forests. It is crucial to consider these greening‐induced alterations in microclimate conditions when modeling changes in tree phenology under future climate warming scenarios. Plain Language Summary: Tree phenology, such as spring leaf‐out and autumn leaf senescence in trees, influences various biochemical processes and functions in forest ecosystems. Although forest greening‐induced local warming or cooling has been reported, potential effect of greening on tree phenology remains poorly understood. Here we show that forest greening could significantly alter tree phenology through biophysical warming and cooling. Our findings provide novel insights into understanding the drivers of the shifts in tree phenology under climate change. Key Points: Forest greening significantly influences tree phenology through seasonal biophysical warming and cooling in temperate and boreal forestsForest greening in winter and spring reduces surface albedo, resulting in biophysical warming and consequently advancing spring phenologyForest greening in summer and autumn induces biophysical cooling by enhancing evapotranspiration, resulting in earlier autumn phenology [ABSTRACT FROM AUTHOR]

Invasive tree species pose major threats to various ecosystems, and their accurate identification and mapping are vital to the development of effective management strategies. Many invasive species demonstrate unique phenological characteristics that are visually identifiable through remote sensing. Previous species identification research relies heavily on the fusion of multiple remote sensing data sources, and are highly constricted in spatial scale. Here, we used high resolution, single instance, 4-band aerial imagery acquired during the blooming season to identify and map Callery pear (Pyrus calleryana) across all five New York City Boroughs (~ 1.188 km2) in urban, suburban, and non-developed environments. We compared traditional pixel-based filtering models against U-Net based convolutional neural networks (CNN). The U-Net CNN greatly outperformed the traditional pixel-based models, achieving a precision, recall, and F1 score of 86.9%, 89.5%, and 88.2% respectively compared to a performance of 47.2%, 52.7%, and 49.8% for the best of the pixel-based models. We also greatly improved CNN performance through the introduction of negative training data, specifically in non-urban areas. We show an effective deep learning strategy for identifying and mapping canopy coverage of Callery pear, which provides a base map for monitoring and management of Callery pear in the Greater New York City Metropolitan Area. More importantly, the method can be readily applicable to the mapping of Callery pear in other regions or other invasive species with unique phenological characteristics given the availability of punctual, high-resolution, multispectral imagery. [ABSTRACT FROM AUTHOR]

Key message: Despite the better performance in wood growth compared with Siberian larch, Siberian spruce may be more vulnerable to ongoing climate change. Understanding the species-specific wood formation kinetics is critical to assess forest growth and carbon sequestration under climate warming; however, such knowledge is still scarce in the fragile forest ecosystems of arid and semi-arid central Asia. Here, we monitored wood formation in two dominant tree species Siberian spruce (Picea obovata Ledeb.) and Siberian larch (Larix sibirica Ledeb.) during 2018 and 2019 at two sites in the southern Altai Mountains, northwest China, and aimed to describe the intra-annual dynamics of wood formation and to understand the annual variation in growth and carbon sequestration of these two species. Results show that the differences in wood production between species were remarkable. Siberian spruce produced 68.5–87.6 cells year−1 at an average rate of 0.82–0.95 cells day−1, about twofold higher than Siberian larch. The differences between species in terms of the onset, cessation timing, and duration of cell production were marginal. Cell production of both species started from mid-May to early June, ceased from early to mid-August, and lasted for about 64 to 70 days. The higher growth rate of spruce led to greater wood production compared to larch. The longer growing seasons induced by climate warming may not result in increased forest growth, thereby failing to enhance carbon sequestration in arid and semi-arid taiga of Central Asia. [ABSTRACT FROM AUTHOR]

Abstract Tree phenology, the timing of periodic biological events in trees, is highly sensitive to climate change. Previous studies have indicated that forest greening can impact the local climate by modifying the seasonal surface energy budget. However, the understanding of tree phenological responses to forest greening at large spatial scales remains limited. Utilizing satellite‐derived phenological and leaf area index data spanning from 2001 to 2021, herein we show that forest greening led to earlier spring and autumn phenology in both temperate and boreal forests. Our findings demonstrated that forest greening during winter and spring contributed to a reduction in surface albedo, resulting in biophysical warming and consequently advancing spring leaf phenology. Conversely, forest greening in summer and autumn induced biophysical cooling through increased evapotranspiration, leading to an earlier onset of autumn leaf phenology. Our findings highlight the significant impact of forest greening‐induced local seasonal climate changes on shaping tree phenology in temperate and boreal forests. It is crucial to consider these greening‐induced alterations in microclimate conditions when modeling changes in tree phenology under future climate warming scenarios.

Fractal dimension (D) can be used to characterise temporal changes of crown architecture of individual trees. Our goal in this study was to analyse seasonal changes in tree crown fractal dimension of two species of deciduous oaks (Quercus castanea and Q. obtusata) coexisting in a natural forest in central Mexico using low cost sampling, and relate these changes to morphological attributes and environmental variables. Every two months, from May 2017 to September 2018, for each oak species, we photographed fixed portions of the crowns of individual trees, measured their trunk diameters, and obtained average temperature and accumulated precipitation data recorded for the sampling date. From the obtained images, we calculated D values by the semivariogram method using three different variability estimators (square increment, isotropic, and transect variation). We identified a positive correlation between D and temperature, and a negative correlation between temperature and crown cover. The fractal dimension (D) of crowns of two deciduous oak species changes according to the tree's phenological stage. D values varied through time in relation to tree crown phenological variation, but not with crown cover dimension. We propose a model of annual D value fluctuation in deciduous trees, characterised by two high complexity peaks and two low complexity valleys, corresponding to the effects on crown cover of annual periods of leaf abscission and development. [ABSTRACT FROM AUTHOR]

The editorial "Dynamics of Asia's and Australasia's forests in a changing world" published in Frontiers in Forests & Global Change discusses the pressures faced by forests due to climate change, anthropogenic disturbances, and biological invasions. The research topic explores various aspects of forest science in Asia and Australasia, focusing on carbon sequestration, ecosystem management, biodiversity conservation, and policy implications. The articles highlight the importance of interdisciplinary approaches to address the challenges of climate change and ecological degradation, aiming to inform policy and promote sustainable forest management practices in the region. [Extracted from the article]

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. [ABSTRACT FROM AUTHOR]

Climate change is expected to increase the frequency and intensity of winter thaws, which could affect leaf phenology. Phenology could either be advanced through the acceleration of forcing accumulation or chilling completion, or be postponed through a reduction in chilling associated with warming air temperature. We tested the influence of winter thaws on budburst phenology by exposing 300 tree cuttings of sugar maple and yellow birch trees to five different frequencies and durations of winter thaws in the lab. In spring, half of the cuttings were exposed to air temperature in two cities representing an air temperature gradient of + 2.0 °C to mimic the ongoing climate warming and bud phenology was monitored three times a week. Irrespective of thaw treatment, yellow birch bud phenology occurred earlier in the warmer city, showing the importance of spring temperature in triggering budburst. The treatment with the highest frequency and duration of thawing increased bud mortality and delayed the onset of spring budburst whereas low frequency treatments did not, thereby identifying a tipping point (3 days twice a month) in the impact of winter thaws on bud phenology. Past this point, winter thaws could slow down bud phenology induced by warmer spring temperature and limit carbon uptake by delaying the closure of the canopy. Climate change simulations projected by the CMIP6 Canadian downscaled climate scenario show that winter thaws will increase in frequency Hence the expected advance in spring leaf emergence associated with warmer spring is not necessarily as straightforward as previously thought.

In temperate and boreal regions, perennial plants adapt their annual growth cycle to the change of seasons. In natural forests, juvenile seedlings usually display longer growth seasons compared to adult trees to ensure their establishment and survival under canopy shade. However, how trees adjust their annual growth according to their age is not known. In this study, we show that age-dependent seasonal growth cessation is genetically controlled and found that the miR156-SPL3/5 module, a key regulon of vegetative phase change (VPC), also triggers age-dependent growth cessation in Populus trees. We show that miR156 promotes shoot elongation during vegetative growth, and its targets SPL3/5s function in the same pathway but as repressors. We find that the miR156-SPL3/5s regulon controls growth cessation in both leaves and shoot apices and through multiple pathways, but with a different mechanism compared to how the miR156-SPL regulon controls VPC in annual plants. Taken together, our results reveal an age-dependent genetic network in mediating seasonal growth cessation, a key phenological process in the climate adaptation of perennial trees. [ABSTRACT FROM AUTHOR]

Functional traits influence plant distributions along broad environmental gradients leading species to occupy communities where their traits enable them to successfully establish and compete for resources. Trait differences are also expected to influence plant distributions and diversity at finer spatial scales within communities. However, relatively few empirical studies have examined the extent to which interspecific trait variation predicts local species-environment distributions. We surveyed herbaceous plants within a heterogeneous acid bedrock glade in south-central Wisconsin to elucidate how traits influence local plant distributions and diversity. Using quadrat-scale environmental covariates (soil depth, canopy openness, and neighborhood tree phenology) and species functional trait means (SLA, vegetative height, and seed mass), we modeled variation in local species distributions as well as plant diversity across 361 quadrats distributed evenly in a 1 ha study plot. Functional traits predictably mediated individualistic species distributions along local gradients in soil depth and canopy openness as well as differential plant responses to variation in canopy leaf phenology. Small-seeded herbs occurred in shallow soil microsites while the prevalence of large-statured plants increased with canopy openness. Local species richness and functional trait dispersion were greatest in microsites near canopy gaps where sun-adapted and shade-adapted plant species co-occur and in microsites surrounded by later-leafing trees. Interspecific trait differences influence local species distributions and shape spatial patterns of diversity within heterogeneous plant communities like bedrock glades. The parallels between local plant distributions within this heterogeneous community and regional plant distributions across the landscape suggest trait-mediated ecological sorting influences plant distributions along environmental gradients similarly across spatial scales. [ABSTRACT FROM AUTHOR]

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.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.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.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. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Autumn leaf phenology marks the end of the growing season, during which trees assimilate atmospheric CO2. Since autumn leaf phenology responds to climatic conditions, climate change affects the length of the growing season. Thus, autumn leaf phenology is often modelled to assess possible climate change effects on future CO2 mitigating capacities and species compositions of forests. Projected trends have been mainly discussed with regards to model performance and climate change scenarios. However, there has been no systematic and thorough evaluation of how performance and projections are affected by the calibration approach. Here, we analyzed >2.3 million performances and 39 million projections across 21 models, 5 optimization algorithms, =7 sampling procedures, and 26 climate model chains from two representative concentration pathways. Calibration and validation were based on >45 000 observations for beech, oak, and larch from 500 Central European sites each. Phenology models had the largest influence on model performance. The best performing models were (1) driven by daily temperature, day length, and partly by seasonal temperature or spring leaf phenology and (2) calibrated with the Generalized Simulated Annealing algorithm (3) based on systematically balanced or stratified samples. Assuming an advancing spring phenology, projected autumn phenology shifts between -13 and +20 days by 2080-2099, resulting in a lengthening of the growing season by 7-40 days. Climate scenarios and sites explained more than 80% of the variance in these shifts and thus had eight to 22 times the influence of phenology models. Warmer climate scenarios and better performing models predominantly extended the growing season more than cooler scenarios and poorer models. Our results justify inferences from comparisons of process-oriented phenology models to phenology-driving processes and we advocate species-specific models for such analyses and subsequent projections. For sound calibration, we recommend a combination of cross-validations and independent tests, using randomly selected sites from stratified bins based on mean annual temperature and average autumn phenology, respectively. Poor performance and little influence of phenology models on autumn phenology projections suggest that the models are overlooking relevant drivers. While the uncertain projections indicate an extension of the growing season, further studies are needed to develop models that adequately consider the relevant processes for autumn phenology. [ABSTRACT FROM AUTHOR]

Winter chilling, spring forcing temperature and photoperiod are the most important drivers explaining the spatial and temporal variability of spring phenology in temperate trees. However, how these factors interact with each other on dormancy release and spring budburst date remains unclear and varies greatly depending on species. Our knowledge is also limited as to whether heat accumulation of forcing temperatures that trigger bud break in spring is a linear or non-linear process. Here, we aimed at experimentally quantifying the effect of chilling, forcing, photoperiod and their interactions on the budburst dates of nine different temperate tree species from East Asia (near Beijing, China) and Central Europe (near Zurich, Switzerland), including six phylogenetically related species (same genus). We conducted a full factorial experiment in climate chambers using two chilling (low and high, i.e., 0 vs. 56 days at 2°C after sampling at the end of December), four forcing (5, 10, 15, and 20°C), and two photoperiod (8 vs. 16 h) treatments simultaneously in Beijing and Zurich. We found that species growing near Beijing responded more readily to forcing conditions than species of the same genus growing near Zurich regardless of chilling treatment. Budburst timing of most species but European beech was marginally, if at all, affected by photoperiod. Furthermore, our results suggest that linear heat accumulation, as commonly used with the growing degree hours (GDH) model, could result in accurate prediction of budburst date depending on the temperature threshold used as a basis for heat accumulation. Our results also demonstrate the important role of chilling in shaping the sensitivity and rate of forcing accumulation to trigger budburst and suggest that species-specific sigmoid relationship for accumulating heat that accounts for prior chilling exposure may yield better predictions of budburst dates. Our results suggest that deciduous trees may have adapted their chilling and forcing requirements in regards to the predictability of winter-spring transition and late spring frosts. A less predictable winter-spring transition, as observed in Central Europe, could have driven species evolution towards higher chilling and forcing requirements compared to species growing in a more predictable climate of Northeastern Asia. Our cross-continental experiment therefore suggests that the spring phenology of East Asian species is tighter coupled to spring forcing temperature than Central European forests.

Urban forests are important for ameliorating urban heat loads and the urban microclimate may have an impact on tree phenology. Understanding these interactions is important to urban planners to select suitable trees for the urban forest. For one year we tracked leaf cover, flowers and fruit of 22 tree species in Bang Kachao Peninsula and explored correlations between phenology and climatic factors. Tropical lowland trees, with exception of mangroves, had reduced leaf cover in the dry season. Mangroves flowered and fruited throughout the year as compared to 1-3 times a year for tropical lowland forests. Positive correlations were found between flowering of Diospyros decandra with temperature; flowering of Aglaia cucullata and Elaeocarpus hygrophilus with rainfall; and fruiting of Dillenia indica and Diospyros malabarica with rainfall and relative humidity. We intend to apply tree phenology results to urban forest restoration and to monitor any adaptation to climate change for future mitigation strategies in tropical urban forests in Thailand.

Large-scale and long-term baselines on climate-sensitive phenology of widespread tree species are lacking in the Indian subcontinent. Citizen scientists can help bridge this information gap by contributing simple, technology-based data. Here we describe an India-wide initiative called SeasonWatch, with preliminary insights into contributor behaviour and species phenology. Between 2011 and 2019, cumulative contributor numbers have increased every year, although consistent contribution remains constant and low. We describe seasonal and spatial phenological patterns in most-observed species based on repeated monitoring and one-time 'bioblitz' events. We study in detail the flowering phenology of one particular species, Cassia fistula, which appears to show aberrant phenology, reflecting a potential shift away from culturally known flowering dates. We conclude that citizen science-contributed information can be a valuable reference database to compare future changes in tree phenology. [ABSTRACT FROM AUTHOR]

Autumn senescence regulates multiple aspects of ecosystem function, along with associated feedbacks to the climate system. Despite its importance, current understanding of the drivers of senescence is limited, leading to a large spread in predictions of how the timing of senescence, and thus the length of the growing season, will change under future climate conditions. The most commonly held paradigm is that temperature and photoperiod are the primary controls, which suggests a future extension of the autumnal growing season as global temperatures rise. Here, using two decades of ground‐ and satellite‐based observations of temperate deciduous forest phenology, we show that the timing of autumn senescence is correlated with the timing of spring budburst across the entire eastern United States. On a year‐to‐year basis, an earlier/later spring was associated with an earlier/later autumn senescence, both for individual species and at a regional scale. We use the observed relationship to develop a novel model of autumn phenology. In contrast to current phenology models, this model predicts that the potential response of autumn phenology to future climate change is strongly limited by the impact of climate change on spring phenology. Current models of autumn phenology therefore may overpredict future increases in the length of the growing season, with subsequent impacts for modeling future CO₂ uptake and evapotranspiration.

Summary: Over the last decades, spring leaf‐out of temperate and boreal trees has substantially advanced in response to global warming, affecting terrestrial biogeochemical fluxes and the Earth's climate system. However, it remains unclear whether leaf‐out will continue to advance with further warming because species' effective chilling temperatures, as well as the amount of chilling time required to break dormancy, are still largely unknown for most forest tree species.Here, we assessed the progress of winter dormancy and quantified the efficiency of different chilling temperatures in six dominant temperate European tree species by exposing 1170 twig cuttings to a range of temperatures from −2°C to 10°C for 1, 3, 6 or 12 wk.We found that freezing temperatures were most effective for half of the species or as effective as chilling temperatures up to 10°C, that is, leading to minimum thermal time to and maximum success of budburst. Interestingly, chilling duration had a much larger effect on dormancy release than absolute chilling temperature.Our experimental results challenge the common assumption that optimal chilling temperatures range c. 4–6°C, instead revealing strong sensitivity to a large range of temperatures. These findings are valuable for improving phenological models and predicting future spring phenology in a warming world. [ABSTRACT FROM AUTHOR]

Urban forests are important for ameliorating urban heat loads and the urban microclimate may have an impact on tree phenology. Understanding these interactions is important to urban planners to select suitable trees for the urban forest. For one year we tracked leaf cover, flowers and fruit of 22 tree species in Bang Kachao Peninsula and explored correlations between phenology and climatic factors. Tropical lowland trees, with exception of mangroves, had reduced leaf cover in the dry season. Mangroves flowered and fruited throughout the year as compared to 1-3 times a year for tropical lowland forests. Positive correlations were found between flowering of Diospyros decandra with temperature; flowering of Aglaia cucullata and Elaeocarpus hygrophilus with rainfall; and fruiting of Dillenia indica and Diospyros malabarica with rainfall and relative humidity. We intend to apply tree phenology results to urban forest restoration and to monitor any adaptation to climate change for future mitigation strategies in tropical urban forests in Thailand. [ABSTRACT FROM AUTHOR]

Examining whether a phenophase occurrence date in the current year affects the same phenophase occurrence date in the following year is crucial for developing cross-year phenological prediction models. Here, we carried out correlation analyses between leaf unfolding start (LUS)/leaf fall end (LFE) dates in the current and following years for four dominant tree species in temperate northern China from 1981 to 2012. Then, we calculated the recurrence intervals of LUS and LFE between two adjacent years for each species. Moreover, we investigated temperature effects on LUS/LFE dates, growing season and non-growing season lengths. Results show that correlation coefficients between LUS/LFE dates in the current and following years are nonsignificant at most stations. The recurrence interval of a phenophase has slight interannual variation and correlates significantly (and negatively) with the phenophase occurrence date of the current year. Further analyses indicate that LUS dates correlate significantly (and negatively) with spring mean temperatures, while LFE dates correlate significantly (and positively) with autumn mean temperatures, but negatively with growing season mean temperatures. In addition, spring mean temperatures can influence growing season length by controlling LUS date but cannot influence the following non-growing season length. Similarly, autumn mean temperatures and growing season mean temperatures can influence the subsequent non-growing season length but cannot influence the growing season length of the following year. Our study highlights that recurrence interval and time restrictions in the effects of seasonal temperatures on phenophase dates are the main environmental causes of nonsignificant correlations between phenophase occurrence dates in the current and following years. [ABSTRACT FROM AUTHOR]

• Whether overwintered nuts have a rest period (endodormancy) remains unexplored. • The study of such phenomena progresses through our unique experimental design with potted trees. • Nuts exhibit and a chilling requirement similar to those of leaf and flower buds. • All these three organs, however, have different ranges of chilling and forcing temperatures. • Research on the global change calls for organ-specific phenological experiments and models. Winter rest and chilling accumulation play a crucial role in regulating the leaf-out and flowering of extratropical woody plants. Still, there is a lack of understanding regarding the potential rest condition of overwintered immature cones of coniferous trees and how the dormancy of different organ types responds to environmental cues. To address this gap, we conducted a unique experimental design using 8–10-year-old potted adult Torreya grandis trees. We examined the rest condition in leaf buds, flower buds, and overwintered immature nuts (cones) simultaneously, and we summarized our findings with process-based models. Our results revealed that overwintered nuts, too, exhibit a rest condition and a chilling requirement similar to those of leaf and flower buds. The effects of photoperiod on rest break were minor in general, so that we did not include photoperiod in our process-based models. The upper limit of an effective chilling temperature was lower for the nuts (<13 °C) than for the flower and leaf buds (<17 °C). During the quiescence period after rest completion, a low forcing temperature of 10 °C enabled ontogenetic development in the nuts and the flower buds but not in the leaf buds. Despite these differences, our models predicted an almost identical change rate for the spring phenology under climate warming in the three organ types. Organ-specific phenological responses provide valuable insights into the impact of environmental factors on plant growth and development. [ABSTRACT FROM AUTHOR]

Extreme climate events (ECEs) such as severe droughts, heat waves, and late spring frosts are rare but exert a paramount role in shaping tree species distributions. The frequency of such ECEs is expected to increase with climate warming, threatening the sustainability of temperate forests. Here, we analyzed 2,844 tree‐ring width series of five dominant European tree species from 104 Swiss sites ranging from 400 to 2,200 m a.s.l. for the period 1930–2016. We found that (a) the broadleaved oak and beech are sensitive to late frosts that strongly reduce current year growth; however, tree growth is highly resilient and fully recovers within 2 years; (b) radial growth of the conifers larch and spruce is strongly and enduringly reduced by spring droughts—these species are the least resistant and resilient to droughts; (c) oak, silver fir, and to a lower extent beech, show higher resistance and resilience to spring droughts and seem therefore better adapted to the future climate. Our results allow a robust comparison of the tree growth responses to drought and spring frost across large climatic gradients and provide striking evidence that the growth of some of the most abundant and economically important European tree species will be increasingly limited by climate warming. These results could serve for supporting species selection to maintain the sustainability of forest ecosystem services under the expected increase in ECEs. [ABSTRACT FROM AUTHOR]

Graphical abstract We calibrated and implemented a new phenology module for evergreen conifers in the global model ORCHIDEE with an explicit representation of both needle budburst and senescence. The new phenology module now allows to represent the seasonality of observed leaf area for evergreen conifers at the regional scale. Sensitivity tests in ORCHIDEE show a strong impact on the simulated carbon cycle for which we highlighted a 15% increase of the growth primary productivity and a doubling of the needle turnover compared to default simulations. We argue that global ecosystem models have to simulate explicit phenological processes for evergreen species if we want to improve future projections of the carbon budget. Highlights • A new module with explicit budburst and senescence for coniferous is developed. • It reproduces observed budburst and leaf area index dynamics. • Budburst models calibrated on sites lose predictability at a coarser resolution. • Future projections showed an advancement of the budburst of 12.3 ± 4.1 days in 2100. • Explicit phenology increases the simulated carbon assimilation of conifers by 15%. Abstract Global ecosystem models lack an explicit representation of budburst and senescence for evergreen conifers despite their primordial role in the carbon cycle. In this study we evaluated eight different budburst models, combining forcing, chilling and photoperiod, for their ability to describe spring budburst, and one model of needle senescence for temperate evergreen coniferous forests. The models' parameters were optimized against field observations from a national forest monitoring network in France. The best fitting budburst model was determined according to a new metrics which accounts for both temporal and spatial variabilities of budburst events across sites. The best model could reproduce observed budburst dates both at the site scale (±5 days) and at regional scale (±12 days). We also showed that the budburst models parameterized at site scale lose some predictive capability when applied at coarser spatial resolution, e.g., in grid-based simulations. The selected budburst model was then coupled to a senescence function defined from needle survivorship observations in order to describe the full phenology cycle of coniferous forests. Implemented in the process-driven ecosystem model ORCHIDEE, this new conifer phenology module represented accurately the intra and inter-annual dynamics of leaf area index at both the local and regional scales when compared against MODIS remote sensing observations. A sensitivity analysis showed only a small impact of the new budburst model on the timing of the seasonal cycle of photosynthesis (GPP). Yet, due to the faster renewal of needles compared to the standard version of ORCHIDEE, we simulated an increase in the GPP by on average 15% over France, while the simulated needle turnover was doubled. Compared to 1970–2000, projections indicated an advancement of the budburst date of 10.3 ± 2.8 and 12.3 ± 4.1 days in average over the period 2060–2100 with the best forcing and chilling-forcing models respectively. Our study suggests that including an explicit simulation of needle budburst and senescence for evergreen conifers in global terrestrial ecosystem models may significantly impact future projections of carbon budgets. [ABSTRACT FROM AUTHOR]

We investigated the water use of Caatinga vegetation, the largest seasonally dry forest in South America. We identified and analysed the environmental phenological drivers in woody species and their relationship with transpiration. To monitor the phenological evolution, we used remote sensing indices at different spatial and temporal scales: normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), and green chromatic coordinate (GCC). To represent the phenology, we used the GCC extracted from in-situ automated digital camera images; indices calculated based on sensors included NDVI, SAVI and GCC from Sentinel-2A and B satellites images, and NDVI products MYD13Q1 and MOD13Q1 from a moderate-resolution imaging spectroradiometer (MODIS). Environmental drivers included continuously monitored rainfall, air temperature, soil moisture, net radiation, and vapour pressure deficit. To monitor soil water status and vegetation water use, we installed soil moisture sensors along three soil profiles and sap flow sensors for five plant species. Our study demonstrated that the near-surface GCC data played an important role in permitting individual monitoring of species, whereas the species’ sap flow data correlated better with NDVI, SAVI, and GCC than with species’ near-surface GCC. The wood density appeared to affect the transpiration cessation times in the dry season, given that species with the lowest wood density reach negligible values of transpiration earlier in the season than those with high woody density. Our results show that soil water availability was the main limiting factor for transpiration during more than 80% of the year, and that both the phenological response and water use are directly related to water availability when relative saturation of the soil profile fell below 0.25.

Here, we reviewed both endogenous and exogenous factors involved in the processes of flower bud formation and flower development in peach, analyzing how they can be affected by climatic change in temperate zones, explored the expansion of peach to tropical or subtropical zones. The process of flower bud formation in peach differs between low winter chilling and temperate conditions. Although the main steps of flower development are maintained, the timing in which each one occurs is different, and some processes can be altered under low winter chilling conditions, with a great impact on fruit production and crop management. Further studies on flower bud induction and differentiation under warmer conditions are fundamental for addressing the alterations in flower bud development that negatively impact on next season’s harvest. In the future, horticulturalists and scientists will face several challenges, mainly how high temperatures affect the expression of the main genes regulating flower formation and how to improve crop management in these conditions.

In temperate and boreal regions, perennial plants adapt their annual growth cycle to the change of seasons. In natural forests, juvenile seedlings usually display longer growth seasons compared to adult trees to ensure their establishment and survival under canopy shade. However, how trees adjust their annual growth according to their age is not known. In this study, we show that age-dependent seasonal growth cessation is genetically controlled and found that the miR156-SPL3/5 module, a key regulon of vegetative phase change (VPC), also triggers age-dependent growth cessation in Populus trees. We show that miR156 promotes shoot elongation during vegetative growth, and its targets SPL3/5s function in the same pathway but as repressors. We find that the miR156-SPL3/5s regulon controls growth cessation in both leaves and shoot apices and through multiple pathways, but with a different mechanism compared to how the miR156-SPL regulon controls VPC in annual plants. Taken together, our results reveal an age-dependent genetic network in mediating seasonal growth cessation, a key phenological process in the climate adaptation of perennial trees., Competing Interests: Competing interests statement:The authors declare no competing interest.

Highlights • Accelerometer data matched the expected seasonal change in tree mass. • Estimates from accelerometers correlated with ground observations of leaf emergence. • Sample frequency had a substantial impact on detecting a phenology signal. Abstract Shifts in forest tree phenology are one of the most important and conspicuous plant responses to climate variability. However, systematically documenting changes in phenology of individual trees across large areas at high temporal frequency is often prohibitively labor- and resource-intensive. Here we present a new method that uses accelerometers to overcome challenges of measuring high-frequency tree phenology in the field. Accelerometers are small, portable devices that can be attached to trees to record movement due to forcing by wind. Time series analyses of tree movement data recorded by accelerometers can yield an approximation of tree mass. Because leaf emergence and leaf drop alter aboveground tree mass, these phenological events are expected to be detectable from accelerometer data. To test how well accelerometers can be used to measure phenological dates, we deployed 20 accelerometers on balsam poplar (Populus balsamifera) trees across a variety of sites during the 2016 growing season and assessed how well phenology derived from accelerometers matched visual observation of phenology recorded by citizen scientists. We found that accelerometer measurements fit the theoretical expectation for the seasonal change in tree mass associated with leaf phenology; specifically, an increase in tree mass in the spring, and a decline in the autumn. Furthermore, we found that accelerometer-derived phenology matched visual observations for leaf emergence, with a strong correlation between the dates of first observed full leaves and accelerometer-derived phenology (r = 0.82, p < 0.01). Estimates of leaf drop from accelerometers and visual observations, however, were not significantly correlated (r = 0.16, p = 0.69). Our work shows that accelerometers can reliably be used to detect spring phenology of forest trees, and have the potential to overcome some of the challenges related to documenting spring tree phenology at high spatial and temporal resolution in the field. [ABSTRACT FROM AUTHOR]

Trees have evolved to time flowering to maximize outcrossing, minimize exposure to damaging frosts, and synchronize development with soil moisture and nutrient availability. Understanding the environmental cues that influence the timing of reproductive budburst will be important for predicting how flowering phenology of trees will change with a changing climate, and aid in the time-sensitive management of seed orchards. We examined how temperature influenced the timing of female flowering of coastal Douglas-fir with over 4500 flowering observations of trees from 12 sites across western Oregon and Washington. We predicted flowering dates by modifying chilling and forcing effectiveness functions from a model of vegetative budburst of Douglas-fir. We also examined whether genetic variation in Douglas-fir influenced the relationships between chilling and forcing accumulations using flowering observations from two common-garden experiments with trees from 60 populations from a diverse range of climates. Our reproductive budburst model predicted the day of flowering within an average of five days of the observed flowering dates across all sites and years. Fewer hours of forcing temperatures were required for flowering on sites that had experienced high chilling. Warmer temperatures in the future will likely result in earlier flowering on sites which currently have high chilling; however, sites which currently experience low chilling may display no change or possibly even a delay in flowering. Douglas-fir genotypes from different geographic regions flowered in the same order from year to year in common gardens, indicating that both temperature and genetic variation influence flowering. Genetic variation in flowering dates was more strongly related to summer drought of seed source locations than to cold winters. Knowledge of the environmental cues and genetic variation in timing of flowering can help predict how future changes in temperature under various climate models could change flowering time across the range of coastal Douglas-fir. [ABSTRACT FROM AUTHOR]

The phenological changes induced by climate warming have profound effects on water, energy, and carbon cycling in forest ecosystems. In addition to pre-season warming, growing-season warming may drive tree phenology by altering photosynthetic carbon uptake. It has been reported that the effect of pre-season warming on tree phenology is decreasing. However, temporal change in the effect of growing-season warming on tree phenology is not yet clear. Combining long-term ground observations and remote-sensing data, here we show that spring and autumn phenology were advanced by growing-season warming, while the accelerating effects of growing-season warming on tree phenology were progressively disappearing, manifesting as phenological events converted from being advanced to being delayed, in the temperate deciduous broadleaved forests across the Northern Hemisphere between 1983 and 2014. We further observed that the effect of growing-season warming on photosynthetic productivity showed a synchronized decline over the same period. The responses of phenology and photosynthetic productivity had a strong linear relationship with each other, and both showed significant negative correlations with the elevated temperature and vapor pressure deficit during the growing season. These findings indicate that warming-induced water stress may drive the observed decline in the responses of tree phenology to growing-season warming by decelerating photosynthetic productivity. Our results not only demonstrate a close link between photosynthetic carbon uptake and tree seasonal activities but also provide a physiological perspective of the nonlinear phenological responses to climate warming. • Acceleration of tree seasonal activities due to growing-season warming slows down • Positive effect of growing-season warming on productivity is declining • Warming responses of tree phenology and productivity are linearly correlated • Temperature sensitivities of phenology and productivity are linked to water stress Temporal changes in the effects of growing-season warming on trees' spring and autumn phenology are not yet clear. Qiao et al. find that growing-season warming leads to earlier tree phenological events in spring and autumn, but the accelerating effects on phenology are disappearing in the deciduous broadleaved forests in the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

Bud phenology identifies the growing period of trees and determines the pattern of mass and energy exchanges between forest and atmosphere over time and space. Canopy color metrics derived from phenocams have been widely used to investigate tree phenology. However, it remains unclear which color-based index better tracks the seasonal variations of tree phenology in evergreen forest ecosystems. Herein, we compared four color metrics (red chromatic coordinate (RCC), green chromatic coordinate (GCC), vegetation contrast index (VCI) and excess green index (ExG)) derived from phenocam images with bud phenological phases recorded in black spruce [ Picea mariana (Mill.) B.S·P] during 2017–2020 at a boreal forest site in Quebec, Canada. Canopy redness (RCC) and greenness (GCC, ExG, and VCI) showed a bimodal and bell-shaped seasonal pattern, respectively. The phases of bud burst and bud set lasted from end-May to end-June and from mid-July to end-September, respectively. The neural network model indicated that GCC had the best predictive ability in capturing the sequential phases of bud phenology. Bud phenological phases predicted by GCC showed the highest correlation with actual bud phenological phases among four indices, with R 2 above 0.9 and RMSE lower than 0.5. Overall, color indices performed better when representing bud burst than bud set. Our findings improve the efficiency and confidence of the phenocam greenness index to characterize the growing season of evergreen forests. • Phenocam color indices are compared with phenological observations in black spruce. • GCC describes better bud phenology than ExG, VCI and RCC. • Color indices track better bud burst than bud set. • GCC can be used to represent large-scale evergreen forest phenology. [ABSTRACT FROM AUTHOR]

Understanding tree mortality after fires can be used to improve planning of prescribed burning and post-burning salvage logging. This study aims to assess the effects of burning season, fire severity and tree size on the evolution of post-burning mortality of pine species with different fire resistance traits. In 2013, two paired plots (1 ha each) were burned, one in spring and the other in fall, in two mixed-stands of P. sylvestris and P. nigra, and the fire severity and tree size variables were measured. Mortality was assessed individually in a total of 933 pines during the following 3 years post-burning. We modeled the initial (1 year post-burning) and delayed (3 years post-burning) tree mortality using Firth logistic regression and Conditional Inference trees. The probability of dying after burning was lower in P. nigra than in P. sylvestris confirming that P. nigra has higher fire-resistance compared to the moderate fire-resistance of P. sylvestris . Initial mortality was slightly higher for pines burned in spring than for those burned in fall; however, after 3 years the proportion of pines that had died after the fall burns was higher than that after the spring burns. Nevertheless, for the two periods modeled, the likelihood of a pine dying was significantly higher in spring than in fall given a certain level of crown scorch. This suggests that pines with damaged crowns were more likely to die when burning occurred during periods of active growth. However, the higher fire severities at the base of the trees that occurred during the fall burns compared to the spring burns are probably the cause of the higher rates of delayed tree mortality. Higher likelihood of mortality in trees burned in fall, with higher percentages of white ashes in the stem base suggest basal stem girdling as a cause of delayed pine mortality. We can conclude that crown damage is more detrimental when burning occurs during the growth season, while longer combustion times during fall burns may have caused the higher delayed pine mortality in comparison with spring burns. [ABSTRACT FROM AUTHOR]

Climate change is already driving shifts in phenology, the timing of life-history events such as flowering, fruiting, egg-laying, birth, and migration, and this is set to increase. Although climate change is happening, and will continue to happen, globally, most of our ecological knowledge around its potential impacts on phenology is derived from temperate areas and ecosystems in the Northern Hemisphere, and information from the Southern Hemisphere is greatly lacking. This would not be a problem if biomes, ecosystems, species assemblages and species were the same in the Northern and Southern Hemispheres, but as they, in fact, differ across many factors and scales, understanding gained from one hemisphere is not necessarily applicable to the other. Our knowledge of the impacts of climate change on biodiversity is strongly skewed. Northern Hemisphere and temperate system bias dominate understanding of phenology. Conservation plans that do not consider phenological changes may be ineffective. [ABSTRACT FROM AUTHOR]

13 CO2 pulse-labelling experiments were performed in situ on adult beeches ( Fagus sylvatica) and pines ( Pinus pinaster) at different phenological stages to study seasonal and interspecific short-term dynamics and partitioning of recently assimilated carbon (C) in leaves., Polar fraction ( PF, including soluble sugars, amino acids and organic acids) and starch were purified from foliage sampled during a 10-d chase period. C contents, isotopic compositions and 13C dynamics parameters were determined in bulk foliage, PF and starch., Decrease in 13C amount in bulk foliage followed a two-pool exponential model highlighting 13C partitioning between 'mobile' and 'stable' pools, the relative proportion of the latter being maximal in beech leaves in May. Early in the growing season, new foliage acted as a strong C sink in both species, but although young leaves and needles were already photosynthesizing, the latter were still supplied with previous-year needle photosynthates 2 months after budburst. Mean 13C residence times ( MRT) were minimal in summer, indicating fast photosynthate export to supply perennial organ growth in both species. In late summer, MRT differed between senescing beech leaves and overwintering pine needles., Seasonal variations of 13C partitioning and dynamics in field-grown tree foliage are closely linked to phenological differences between deciduous and evergreen trees. [ABSTRACT FROM AUTHOR]

In tropical forests, deciduousness is an outcome of integrated effect of drought, tree characteristics and soil moisture conditions and thus it is a reliable indicator of seasonal drought experienced by different tree species. Variations in the deciduousness are associated with several ecophysiological characteristics, such as varying allocation pattern of metabolic products, resource capture and conservation, water relations and stem water storages, annual carbon sequestration, timing of reproductive event initiation, extent of separation of vegetative and reproductive events and leaf strategies, and it helps in maintenance of water balance and protection of tree organs during the seasonal drought. Tropical forests support mosaics of tree functional types showing marked differences in the duration of deciduousness (from leaf exchanging to >8 months deciduous), as a result of varying degree of water stress experienced by physiognomy, distribution and wood anatomy of tropical trees. Wide variations in deciduousness in the same species growing at different sites suggest the high sensitivity of tropical trees to small changes in growing habitat. In the present review we have explored the ecological significance of deciduousness in tropical trees with emphasis on: (a) inter- and intraspecies plasticity in deciduousness, (b) various capacity adaptations related with the duration of deciduousness, (c) relationship between tree stem water status and deciduousness, and (d) probable effect of impending climate change on tropical trees. An attempt has also been made to establish deciduousness as climate change indicator in the dry tropics. There is need to develop capabilities to detect and predict the impact of climate change on deciduousness through long-term phenological network in tropics. Remote sensing techniques can generate valuable ecological information such as leaf level drought response and phenological patterns. Deciduousness has the potential to emerge as an important focus for ecological research to address critical questions in global modeling, monitoring, and climate change. [ABSTRACT FROM AUTHOR]

• The process-based models performed well in height-growth phenology simulation. • Height-growth onset was predicted to advance but more synchronous across elevations. • Increased heat requirement would potentially delay the cessation of height-growth. • Future warming would enhance Smith fir forest productivity and carbon sequestration. Accurate projections of growing-season duration of trees are crucial in evaluating the capacity of forests to mitigate climate warming through growth and CO 2 uptake. However, little is known on how and to what extent the growing season of tree height growth will change under future warming. Herein, we projected the height-growth phenology of Smith fir (Abies georgei var. smithii) along two altitudinal gradients on the southeastern Tibetan Plateau under CMIP6 climate scenarios by using process-based phenology models. The models performed well when simulating timings of height-growth onset and cessation on the plot level, with root mean square errors of less than 5.8 and 3.8 days, respectively. From the 2020s to 2090s, the onset of height growth (i.e. bud swelling date) was predicted to advance by 10 ± 6 days under four warming scenarios. The elevational gradients of onset of bud swelling would significantly decrease from 3–4 to 1–2 days 100m−1 under the warmest climate scenario, suggesting future warming may result in more uniform onset of height growth along elevational gradients. Given the temporal variation of thermal requirements, the projected cessation of height growth was delayed by 3 ± 1 days in most plots. Overall, the height-growth duration was predicted to lengthen by 4–22 days by the end of the century. The extended duration of height growth predicted for Smith fir could enhance primary growth, forest productivity, and CO 2 sequestration on the southeastern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Plant resistance strongly depends on plant characteristics that affect herbivore preference or performance. Different plant genotypes may express specific characteristics that lead to differential resistance to herbivores. Few studies have investigated these aspects for weevils, a particularly speciose taxon of insect herbivores, on fruit tree cultivars, which are vegetatively propagated., We studied the influence of apple cultivar on preference and performance of a fruit tree herbivore, the flower-feeding apple blossom weevil Anthonomus pomorum ( Coleoptera: Curculionidae). We quantified field infestation of five different apple genotypes by the weevil at different collection dates as measure of preference, as well as the mass and emergence time of weevils developing on these cultivars as measures of performance., Infestation of apple flower buds by A. pomorum differed significantly between cultivars. The collection date affected the number of infested flowers in a cultivar-specific way., Weevil mass differed significantly between apple cultivars. The average mass of weevils from Ariwa, the cultivar producing the heaviest weevils, was 40% higher than the mass of weevils from Rewena, the cultivar producing the lightest weevils., Weevil emergence time from infested flowers differed significantly between cultivars. Weevils emerged earliest from cultivars Ariwa and Retina, at an intermediate time from Discovery and Florina, and latest from Rewena., The present study demonstrates that preference and performance of florivores feeding on fruit trees can strongly differ between apple cultivars representing specific tree genotypes. These differences may directly and indirectly affect herbivore and antagonist populations and can be considered for pest management. [ABSTRACT FROM AUTHOR]

Seasonally dry tropical forests (SDTF) are located in regions with alternating wet and dry seasons, with dry seasons that last several months or more. By the end of the 21st century, climate models predict substantial changes in rainfall regimes across these regions, but little is known about how individuals, species, and communities in SDTF will cope with the hotter, drier conditions predicted by climate models. In this review, we explore different rainfall scenarios that may result in ecological drought in SDTF through the lens of two alternative hypotheses: 1) these forests will be sensitive to drought because they are already limited by water and close to climatic thresholds, or 2) they will be resistant/resilient to intra- and inter-annual changes in rainfall because they are adapted to predictable, seasonal drought. In our review of literature that spans microbial to ecosystem processes, a majority of the available studies suggests that increasing frequency and intensity of droughts in SDTF will likely alter species distributions and ecosystem processes. Though we conclude that SDTF will be sensitive to altered rainfall regimes, many gaps in the literature remain. Future research should focus on geographically comparative studies and well-replicated drought experiments that can provide empirical evidence to improve simulation models used to forecast SDTF responses to future climate change at coarser spatial and temporal scales.

Recent temperature increases have elicited strong phenological shifts in temperate tree species, with subsequent effects on photosynthesis. Here, we assess the impact of advanced leaf flushing in a winter warming experiment on the current year's senescence and next year's leaf flushing dates in two common tree species: Quercus robur L. and Fagus sylvatica L. Results suggest that earlier leaf flushing translated into earlier senescence, thereby partially offsetting the lengthening of the growing season. Moreover, saplings that were warmed in winter-spring 2009-2010 still exhibited earlier leaf flushing in 2011, even though the saplings had been exposed to similar ambient conditions for almost 1 y. Interestingly, for both species similar trends were found in mature trees using a long-term series of phenological records gathered from various locations in Europe. We hypothesize that this longterm legacy effect is related to an advancement of the endormancy phase (chilling phase) in response to the earlier autumnal senescence. Given the importance of phenology in plant and ecosystem functioning, and the prediction of more frequent extremely warm winters, our observations and postulated underlying mechanisms should be tested in other species. [ABSTRACT FROM AUTHOR]

The impact of global warming on phenology has been widely studied, and almost consistently advancing spring events have been reported. Especially in alpine regions, an extraordinary rapid warming has been observed in the last decades. However, little is known about phenological phases over the whole vegetation period at high elevations. We observed 12 phenological phases of seven tree species and measured air temperature at 42 sites along four transects of about 1000 m elevational range in the years 2010 and 2011 near Garmisch- Partenkirchen, Germany. Site- and species-specific onset dates for the phenological phases were determined and related to elevation, temperature lapse rates and site-specific temperature sums. Increasing temperatures induced advanced spring and delayed autumn phases, in which both yielded similar magnitudes. Delayed leaf senescence could therefore have been underestimated until now in extending the vegetation period. Not only the vegetation period, but also phenological periods extended with increasing temperature. Moreover, sensitivity to elevation and temperature strongly depends on the specific phenological phase. Differences between species and groups of species (deciduous, evergreen, high elevation) were found in onset dates, phenological response rates and also in the effect of chilling and forcing temperatures. Increased chilling days highly reduced forcing temperature requirements for deciduous trees, but less for evergreen trees. The problem of shifted species associations and phenological mismatches due to species-specific responses to increasing temperature is a recent topic in ecological research. Therefore, we consider our findings from this novel, dense observation network in an alpine area of particular importance to deepen knowledge on phenological responses to climate change. [ABSTRACT FROM AUTHOR]

Highlights: [•] We propose a model evaluation procedure for selecting models for extrapolation. [•] We compared 15 models of budburst using Swedish data for five tree species. [•] Most models are not able to track the inter-annual variation in temperature. [•] Simple temperature sum models are better for birch, oak, beech and Norway spruce. [•] Due to limitations, most models are not suitable for climate change assessments. [Copyright &y& Elsevier]

Using first leaf unfolding data of Salix matsudana, Populus simonii, Ulmus pumila, and Prunus armeniaca, and daily mean temperature data during the 1981-2005 period at 136 stations in northern China, we fitted unified forcing and chilling phenology models and selected optimum models for each species at each station. Then, we examined performances of each optimum local species-specific model in predicting leaf unfolding dates at all external stations within the corresponding climate region and selected 16 local species-specific models with maximum effective predictions as the regional unified models in different climate regions. Furthermore, we validated the regional unified models using leaf unfolding and daily mean temperature data beyond the time period of model fitting. Finally, we substituted gridded daily mean temperature data into the regional unified models, and reconstructed spatial patterns of leaf unfolding dates of the four tree species across northern China during 1960-2009. At local scales, the unified forcing model shows higher simulation efficiency at 83% of data sets, whereas the unified chilling model indicates higher simulation efficiency at 17% of data sets. Thus, winter temperature increase so far has not yet significantly influenced dormancy and consequent leaf development of deciduous trees in most parts of northern China. Spatial and temporal validation confirmed capability and reliability of regional unified species-specific models in predicting leaf unfolding dates in northern China. Reconstructed leaf unfolding dates of the four tree species show significant advancements by 1.4-1.6 days per decade during 1960-2009 across northern China, which are stronger for the earlier than the later leaf unfolding species. Our findings suggest that the principal characteristics of plant phenology and phenological responses to climate change at regional scales can be captured by phenological and climatic data sets at a few representative locations. [ABSTRACT FROM AUTHOR]