Temperature and photoperiod drive spring phenology across all species in a temperate forest community

Long-term species-level observations indicate the critical role of soil moisture in regulating China's grassland productivity relative to phenological and climatic factors

As a sensitive indicator of climate change and a key variable in ecosystem surface-atmosphere interaction, vegetation phenology, and the growing season length, as well as climatic factors (i.e., temperature, precipitation, and sunshine duration) are widely recognized as key factors influencing vegetation productivity. Recent studies have highlighted the importance of soil moisture in regulating grassland productivity. However, the relative importance of phenology, climatic factors, and soil moisture to plant species-level productivity across China's grasslands remains poorly understood. Here, we use nearly four decades (1981 to 2018) of in situ species-level observations from 17 stations distributed across grasslands in China to examine the key mechanisms that control grassland productivity. The results reveal that soil moisture is the strongest determinant of the interannual variability in grassland productivity. In contrast, the spring/autumn phenology, the length of vegetation growing season, and climate factors have relatively minor impacts. Generally, annual aboveground biomass increases by 3.9 to 25.3 g∙m2 (dry weight) with a 1 % increase in growing season mean soil moisture across the stations. Specifically, the sensitivity of productivity to moisture in wetter and colder environments (e.g., alpine meadows) is significantly higher than that in drier and warmer environments (e.g., temperate desert steppes). In contrast, the sensitivity to the precipitation of the latter is greater than the former. The effect of soil moisture is the most pronounced during summer. Dominant herb productivity is more sensitive to soil moisture than the others. Moreover, multivariate regression analyses show that the primary climatic factors and their attributions to variations in soil moisture differ among the stations, indicating the interaction between climate and soil moisture is very complex. Our study highlights the interspecific difference in the soil moisture dependence of grassland productivity and provides guidance to climate change impact assessments in grassland ecosystems.

Monitoring the phenology of plant pathogenic fungi: why and how?

Phenology is a key adaptive trait of organisms, shaping biotic interactions in response to the environment. It has emerged as a critical topic with implications for societal and economic concerns due to the effects of climate change on species' phenological patterns. Fungi play essential roles in ecosystems, and plant pathogenic fungi have significant impacts on global food security. However, the phenology of plant pathogenic fungi, which form a huge and diverse clade of organisms, has received limited attention in the literature. This diversity may have limited the use of a common language for comparisons and the integration of phenological data for these taxonomic groups. Here, we delve into the concept of 'phenology' as applied to plant pathogenic fungi and explore the potential drivers of their phenology, including environmental factors and the host plant. We present the PhenoFun scale, a phenological scoring system suitable for use with all fungi and fungus-like plant pathogens. It offers a standardised and common tool for scientists studying the presence, absence, or predominance of a particular phase, the speed of phenological phase succession, and the synchronism shift between pathogenic fungi and their host plants, across a wide range of environments and ecosystems. The application of the concept of 'phenology' to plant pathogenic fungi and the use of a phenological scoring system involves focusing on the interacting processes between the pathogenic fungi, their hosts, and their biological, physical, and chemical environment, occurring during the life cycle of the pathogen. The goal is to deconstruct the processes involved according to a pattern orchestrated by the fungus's phenology. Such an approach will improve our understanding of the ecology and evolution of such organisms, help to understand and anticipate plant disease epidemics and their future evolution, and make it possible to optimise management models, and to encourage the adoption of cropping practices designed from this phenological perspective.

Plant species phenology differs between climate and land-use scenarios and relates to plant functional traits

Phenological shifts due to changing climate are often highly species and context specific. Land-use practices such as mowing or grazing directly affect the phenology of grassland species, but it is unclear if plants are similarly affected by climate change in differently managed grassland systems such as meadows and pastures. Functional traits have a high potential to explain phenological shifts and might help to understand species-specific and land-use-specific phenological responses to changes in climate. In the large-scale field experiment Global Change Experimental Facility (GCEF), we monitored the first flowering day, last flowering day, flowering duration, and day of peak flowering, of 17 herbaceous grassland species under ambient and future climate conditions, comparing meadows and pastures. Both climate and land use impacted the flowering phenology of plant species in species-specific ways. We did not find evidence for interacting effects of climate and land-use type on plant phenology. However, the data indicate that microclimatic and microsite conditions on meadows and pastures were differently affected by future climate, making differential effects on meadows and pastures likely. Functional traits, including the phenological niche and grassland utilization indicator values, explained species-specific phenological climate responses. Late flowering species and species with a low mowing tolerance advanced their flowering more strongly under future climate. Long flowering species and species following an acquisitive strategy (high specific leaf area, high mowing tolerance, and high forage value) advanced their flowering end more strongly and thus more strongly shortened their flowering under future climate. We associated these trait-response relationships primarily with a phenological drought escape during summer. Our results provide novel insights on how climate and land use impact the flowering phenology of grassland species and we highlight the role of functional traits in mediating phenological responses to climate.

Time to budbreak is not enough: cold hardiness evaluation is necessary in dormancy and spring phenology studies

BACKGROUND

Dormancy of buds is an important phase in the life cycle of perennial plants growing in environments where unsuitable growth conditions occur seasonally. In regions where low temperature defines these unsuitable conditions, the attainment of cold hardiness is also required for survival. The end of the dormant period culminates in budbreak and flower emergence, or spring phenology, one of the most appreciated and studied phenological events - a time also understood to be most sensitive to low-temperature damage. Despite this, we have a limited physiological and molecular understanding of dormancy, which has negatively affected our ability to model budbreak. This is also true for cold hardiness.

SCOPE

Here we highlight the importance of including cold hardiness in dormancy studies that typically only characterize time to budbreak. We show how different temperature treatments may lead to increases in cold hardiness, and by doing so also (potentially inadvertently) increase time to budbreak.

CONCLUSIONS

We present a theory that describes evaluation of cold hardiness as being key to clarifying physiological changes throughout the dormant period, delineating dormancy statuses, and improving both chill and phenology models. Erroneous interpretations of budbreak datasets are possible by not phenotyping cold hardiness. Changes in cold hardiness were very probably present in previous experiments that studied dormancy, especially when those included below-freezing temperature treatments. Separating the effects between chilling accumulation and cold acclimation in future studies will be essential for increasing our understanding of dormancy and spring phenology in plants.

Stable water isotopes reveal the onset of bud dormancy in temperate trees, whereas water content is a better proxy for dormancy release

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.

Weak evidence of provenance effects in spring phenology across Europe and North America

Forecasting the biological impacts of climate change requires understanding how species respond to warmer temperatures through interannual flexible variation vs through adaptation to local conditions. Yet, we often lack this information entirely or find conflicting evidence across studies, which is the case for spring phenology. We synthesized common garden studies across Europe and North America that reported spring event dates for a mix of angiosperm and gymnosperm tree species in the northern hemisphere, capturing data from 384 North American and 101 European provenances (i.e. populations) with observations from 1962 to 2019, alongside autumn event data when provided. Across continents, we found no evidence of provenance effects in spring phenology, but strong clines with latitude and mean annual temperature in autumn. These effects, however, appeared to diverge by continent and species type (gymnosperm vs angiosperm), with particularly pronounced clines in North America in autumn events. Our results suggest flexible, likely plastic responses, in spring phenology with warming, and potential limits - at least in the short term - due to provenance effects for autumn phenology. They also highlight that, after over 250 yr of common garden studies on tree phenology, we still lack a holistic predictive model of clines across species and phenological events.

Feasting on the ordinary or starving for the exceptional in a warming climate: Phenological synchrony between spongy moth (Lymantria dispar) and budburst of six European tree species

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.

Forest understorey flowering phenology responses to experimental warming and illumination

Species are altering their phenology to track warming temperatures. In forests, understorey plants experience tree canopy shading resulting in light and temperature conditions, which strongly deviate from open habitats. Yet, little is known about understorey phenology responses to forest microclimates. We recorded flowering onset, peak, end and duration of 10 temperate forest understorey plant species in two mesocosm experiments to understand how phenology is affected by sub-canopy warming and how this response is modulated by illumination, which is related to canopy change. Furthermore, we investigated whether phenological sensitivities can be explained by species' characteristics, such as thermal niche. We found a mean advance of flowering onset of 7.1 d per 1°C warming, more than previously reported in studies not accounting for microclimatic buffering. Warm-adapted species exhibited greater advances. Temperature sensitivity did not differ between early- and later-flowering species. Experimental illumination did not significantly affect species' phenological temperature sensitivities, but slightly delayed flowering phenology independent from warming. Our study suggests that integrating sub-canopy temperature and light availability will help us better understand future understorey phenology responses. Climate warming together with intensifying canopy disturbances will continue to drive phenological shifts and potentially disrupt understorey communities, thereby affecting forest biodiversity and functioning.

Daylength predominates the bud growth initiation of winter deciduous forest trees in the monsoon region of China

Climate warming has induced significant shifts in spring phenology over both temperate and boreal forests. The timing of bud growth resuming from dormancy is crucial for predicting spring phenology. However, the mechanisms by which environmental cues, other than chilling accumulation, initiate bud growth remains unclear. By constructing a revised process-based spring phenology model incorporating photoperiod and temperature triggers of bud growth, we simulated the first leaf unfolding and first flowering dates of four deciduous forest trees during 1981-2014 at 102 stations across China's monsoon regions. Then, we revealed spatial patterns of the two triggers. Moreover, we compared fitting precision and robustness of the revised model with three mainstream models. Results show that the revised models can effectively simulate all spring phenology time series. Growth initiation of foliar and floral buds was induced by photoperiod lengthening in 80.8% and 77.7% of time series, and by temperature increasing in remaining 19.2% and 22.3% of time series, respectively. The proportions of time series with photoperiod- and temperature-initiated bud growth significantly increase and decrease from northern to southern climatic zones, respectively. Chilling exposure controls the predominant bud growth triggers in different climate zones. Specifically, in regions with long and severe winters where chilling requirement is easily fulfilled, rising temperature in spring alleviates the cold constraint and initiate bud growth. Conversely, in regions with short and mild winters, prolonged daylength in spring compensates the lack of chilling exposure to initiate bud growth. These findings suggest that photoperiod may limit spring phenology response to temperature in low-latitudes. Overall, our model slightly outperforms other models in terms of efficiency, accuracy, and robustness in modeling leaf unfolding and flowering dates. Therefore, this study deepens our understanding of the mechanisms of spring phenology, and improves the predicting capability of spring phenology models in the face of ongoing global warming.

Nonstructural carbohydrate dynamics' relationship to leaf development under varying environments

Leaf-out in temperate forests is a critical transition point each spring and advancing with global change. The mechanism linking phenological variation to external cues is poorly understood. Nonstructural carbohydrate (NSC) availability may be key. Here, we use branch cuttings from northern red oak (Quercus rubra) and measure NSCs throughout bud development in branch tissue. Given genes and environment influence phenology, we placed branches in an arrayed factorial experiment (three temperatures × two photoperiods, eight genotypes) to examine their impact on variation in leaf-out timing and corresponding NSCs. Despite significant differences in leaf-out timing between treatments, NSC patterns were much more consistent, with all treatments and genotypes displaying similar NSC concentrations across phenophases. Notably, the moderate and hot temperature treatments reached the same NSC concentrations and phenophases at the same growing degree days (GDD), but 20 calendar days apart, while the cold treatment achieved only half the GDD of the other two. Our results suggest that NSCs are coordinated with leaf-out and could act as a molecular clock, signaling to cells the passage of time and triggering leaf development to begin. This link between NSCs and budburst is critical for improving predictions of phenological timing.

Sex-dependent phenological responses to climate vary across species' ranges

Anthropogenic climate change has significantly altered the flowering times (i.e., phenology) of plants worldwide, affecting their reproduction, survival, and interactions. Recent studies utilizing herbarium specimens have uncovered significant intra- and inter-specific variation in flowering phenology and its response to changes in climate but have mostly been limited to animal-pollinated species. Thus, despite their economic and ecological importance, variation in phenological responses to climate remain largely unexplored among and within wind-pollinated dioecious species and across their sexes. Using both herbarium specimens and volunteer observations of cottonwood (Populus) species, we examined how phenological sensitivity to climate varies across species, their ranges, sexes, and phenophases. The timing of flowering varied significantly across and within species, as did their sensitivity to spring temperature. In particular, male flowering generally happened earlier in the season and was more sensitive to warming than female flowering. Further, the onset of flowering was more sensitive to changes in temperature than leaf out. Increased temporal gaps between male and female flowering time and between the first open flower date and leaf out date were predicted for the future under two climate change scenarios. These shifts will impact the efficacy of sexual reproduction and gene flow among species. Our study demonstrates significant inter- and intra-specific variation in phenology and its responses to environmental cues, across species' ranges, phenophases, and sex, in wind-pollinated species. These variations need to be considered to predict accurately the effects of climate change and assess their ecological and evolutionary consequences.

Poleward shifts in the maximum of spring phenological responsiveness of Ginkgo biloba to temperature in China

Global warming is advancing the timing of spring leaf-out in temperate and boreal plants, affecting biological interactions and global biogeochemical cycles. However, spatial variation in spring phenological responsiveness to climate change within species remains poorly understood. Here, we investigated variation in the responsiveness of spring phenology to temperature (RSP; days to leaf-out at a given temperature) in 2754 Ginkgo biloba twigs of trees distributed across subtropical and temperate regions in China from 24°N to 44°N. We found a nonlinear effect of mean annual temperature on spatial variation in RSP, with the highest response rate at c. 12°C and lower response rates at warmer or colder temperatures due to declines in winter chilling accumulation. We then predicted the spatial maxima in RSP under current and future climate scenarios, and found that trees are currently most responsive in central China, which corresponds to the species' main distribution area. Under a high-emission scenario, we predict a 4-degree latitude shift in the responsiveness maximum toward higher latitudes over the rest of the century. The identification of the nonlinear responsiveness of spring phenology to climate gradients and the spatial shifts in phenological responsiveness expected under climate change represent new mechanistic insights that can inform models of spring phenology and ecosystem functioning.

Warming increases the differences among spring phenology models under future climate change

Phenological models are built upon an understanding of the influence of environmental factors on plant phenology, and serve as effective tools for predicting plant phenological changes. However, the differences in phenological model predictive performance under different climate change scenarios have been rarely studied. In this study, we parameterized thirteen spring phenology models, including six one-phase models and seven two-phase models, by combining phenological observations and meteorological data. Using climatic data from two Shared Socioeconomic Pathways (SSP) scenarios, namely SSP126 (high mitigation and low emission) and SSP585 (no mitigation and high emission), we predicted spring phenology in Germany from 2021 to 2100, and compared the impacts of dormancy phases and driving factors on model predictive performance. The results showed that the average correlation coefficient between the predicted start of growing season (SOS) by the 13 models and the observed values exceeded 0.72, with the highest reaching 0.80. All models outperformed the NULL model (Mean of SOS), and the M1 model (driven by photoperiod and forcing temperature) performed the best for all the tree species. In the SSP126 scenario, the average SOS advanced initially and then gradually shifted towards a delay starting around 2070. In the SSP585 scenario, the average SOS advanced gradually at a rate of approximately 0.14 days per year. Moreover, the standard deviation of the simulated SOS by the 13 spring phenology models exhibited a significant increase at a rate of 0.04 days per year. On average, two-phase models exhibited larger standard deviations than one-phase models after approximately 2050. Models driven solely by temperature showed larger standard deviations after 2060 compared to models driven by both temperature and photoperiod. Our findings suggest investigating the release mechanisms of endodormancy phase and incorporating new insights into future phenological models to better simulate the changes in plant phenology.

Accelerating effects of growing-season warming on tree seasonal activities are progressively disappearing

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.

Climatic drivers and ecological implications of variation in the time interval between leaf-out and flowering

Leaf-out and flowering in any given species have evolved to occur in a predetermined sequence, with the inter-stage time interval optimized to maximize plant fitness. Although warming-induced advances of both leaf-out and flowering are well documented, it remains unclear whether shifts in these phenological phases differ in magnitudes and whether changes have occurred in the length of the inter-stage intervals. Here, we present an extensive synthesis of warming effects on flower-leaf time intervals, using long-term (1963-2014) and in situ data consisting of 11,858 leaf-out and flowering records for 183 species across China. We found that the timing of both spring phenological events was generally advanced, indicating a dominant impact of forcing conditions compared with chilling. Stable time intervals between leaf-out and flowering prevailed for most of the time series despite increasing temperatures; however, some of the investigated cases featured significant changes in the time intervals. The latter could be explained by differences in the temperature sensitivity (ST) between leaf and flower phenology. Greater ST for flowering than for leaf-out caused flowering times to advance faster than leaf emergence. This shortened the inter-stage intervals in leaf-first species and lengthened them in flower-first species. Variation in the time intervals between leaf-out and flowering events may have far-reaching ecological and evolutionary consequences, with implications for species fitness, intra/inter-species interactions, and ecosystem structure, function, and stability.

Climate-driven shifts in leaf senescence are greater for boreal species than temperate species in the Acadian Forest region in contrast to leaf emergence shifts

The Acadian Forest Region is a temperate-boreal transitional zone in eastern North America which provides a unique opportunity for understanding the potential effects of climate change on both forest types. Leaf phenology, the timing of leaf life cycle changes, is an important indicator of the biological effects of climate change, which can be observed with stationary timelapse cameras known as phenocams. Using four growing seasons of observations for the species Acer rubrum (red maple), Betula papyrifera (paper/white birch) and Abies balsamea (balsam fir) from the Acadian Phenocam Network as well as multiple growing season observations from the North American PhenoCam Network we parameterized eight leaf emergence and six leaf senescence models for each species which span a range in process and driver representation. With climate models from the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5) we simulated future leaf emergence, senescence and season length (senescence minus emergence) for these species at sites within the Acadian Phenocam Network. Model performances were similar across models and leaf emergence model RMSE ranged from about 1 to 2 weeks across species and models, while leaf senescence model RMSE ranged from about 2 to 4 weeks. The simulations suggest that by the late 21st century, leaf senescence may become continuously delayed for boreal species like Betula papyrifera and Abies balsamea, though remain relatively stable for temperate species like Acer rubrum. In contrast, the projected advancement in leaf emergence was similar across boreal and temperate species. This has important implications for carbon uptake, nutrient resorption, ecology and ecotourism for the Acadian Forest Region. More work is needed to improve predictions of leaf phenology for the Acadian Forest Region, especially with respect to senescence. Phenocams have the potential to rapidly advance process-based model development and predictions of leaf phenology in the context of climate change.

How consistently do species leaf-out or flower in the same order? Understanding the factors that shape this characteristic of plant communities

Plant species are frequently reported to undergo leaf-out and flowering in a consistent order from 1 year to the next; however, only a limited number of these findings arise from studies encompassing many species or sites. Here, we evaluate the consistency in the order species leafed out in the northeastern United States using observations contributed to the USA National Phenology Network's Nature's Notebook platform. We repeated this analysis for flowering, evaluating a total of 132 species across 84 sites. We documented a relatively high degree of consistency in the order of both events among individual plants, with higher consistency in flowering. A small number of species pairs exhibited very high consistency in phenological order across several sites. The majority of species pairs exhibited variability in how consistently they underwent either leaf-out or flowering from site to site, which could be the result of either plastic or locally adaptive responses. Our investigation revealed that neither functional type nor seasonal position played a major role in shaping how consistently species leafed out or flowered in the same order. Instead, we found the number of days separating the events and interannual variability in timing to be the most influential factors driving the consistency in ordering.

A stronger advance of urban spring vegetation phenology narrows vegetation productivity difference between urban settings and natural environments

Climate change is posing dramatic effects on terrestrial vegetation dynamics. The links between vegetation phenology or vegetation activity (growth) and climate change have been widely reported, yet, less is known about the impacts of phenological shifts on vegetation growth. Urban settings characterized by urban heat island and CO₂ dome are often used as ideal natural laboratories to understand how vegetation responds to global climate change. Here we assessed the impacts of phenology changes on vegetation growth in China using satellite phenology metrics and gross primary production (GPP) data from 2003 to 2018 and urban-natural contrast analysis. Compared with natural environments, phenological metrics (e.g., start/end of growing season (SOS/EOS), and the length of growing season (GSL), etc.) were observed to change more dramatically in urban environments. Furthermore, we found that GPP in both settings increased over time but with a higher increment in the urban environments, and the urban-natural vegetation productivity gap had been diminishing at a rate of 16.9 ± 6.76 g C m^-2 y^-1. The narrowing of the urban-natural GPP difference over time can be attributed to a more advanced SOS and extended GSL in urban settings than their natural counterparts, particularly SOS shift. These findings suggested that the distinct urban phenological shifts would become increasingly important in offsetting the loss of vegetation productivity induced by urbanization.

Mechanisms driving interspecific variation in regional synchrony of trees reproduction

Seed production in many plants is characterized by large interannual variation, which is synchronized at subcontinental scales in some species but local in others. The reproductive synchrony affects animal migrations, trophic responses to resource pulses and the planning of management and conservation. Spatial synchrony of reproduction is typically attributed to the Moran effect, but this alone is unable to explain interspecific differences in synchrony. We show that interspecific differences in the conservation of seed production-weather relationships combine with the Moran effect to explain variation in reproductive synchrony. Conservative timing of weather cues that trigger masting allows populations to be synchronized at distances >1000 km. Conversely, if populations respond to variable weather signals, synchrony cannot be achieved. Our study shows that species vary in the extent to which their weather cueing is spatiotemporally conserved, with important consequences, including an interspecific variation of masting vulnerability to climate change.

Acclimation of phenology relieves leaf longevity constraints in deciduous forests

Leaf phenology is key for regulating total growing-season mass and energy fluxes. Long-term temporal trends towards earlier leaf unfolding are observed across Northern Hemisphere forests. Phenological dates also vary between years, whereby end-of-season (EOS) dates correlate positively with start-of-season (SOS) dates and negatively with growing-season total net CO₂ assimilation (A_net). These associations have been interpreted as the effect of a constrained leaf longevity or of premature carbon (C) sink saturation-with far-reaching consequences for long-term phenology projections under climate change and rising CO₂. Here, we use multidecadal ground and remote-sensing observations to show that the relationships between A_net and EOS are opposite at the interannual and the decadal time scales. A decadal trend towards later EOS persists in parallel with a trend towards increasing A_net-in spite of the negative A_net-EOS relationship at the interannual scale. This finding is robust against the use of diverse observations and models. Results indicate that acclimation of phenology has enabled plants to transcend a constrained leaf longevity or premature C sink saturation over the course of several decades, leading to a more effective use of available light and a sustained extension of the vegetation CO₂ uptake season over time.

Limits on phenological response to high temperature in the Arctic

Tundra plants are widely considered to be constrained by cool growing conditions and short growing seasons. Furthermore, phenological development is generally predicted by daily heat sums calculated as growing degree days. Analyzing over a decade of seasonal flower counts of 23 plant species distributed across four plant communities, together with hourly canopy-temperature records, we show that the timing of flowering of many tundra plants are best predicted by a modified growing degree day model with a maximum temperature threshold. Threshold maximums are commonly employed in agriculture, but until recently have not been considered for natural ecosystems and to our knowledge have not been used for tundra plants. Estimated maximum temperature thresholds were found to be within the range of daily temperatures commonly experienced for many species, particularly for plants at the colder, high Arctic study site. These findings provide an explanation for why passive experimental warming-where moderate changes in mean daily temperatures are accompanied by larger changes in daily maximum temperatures-generally shifts plant phenology less than ambient warming. Our results also suggest that many plants adapted to extreme cold environments may have limits to their thermal responsiveness.

Fuzzy clustering for the within-season estimation of cotton phenology

Crop phenology is crucial information for crop yield estimation and agricultural management. Traditionally, phenology has been observed from the ground; however Earth observation, weather and soil data have been used to capture the physiological growth of crops. In this work, we propose a new approach for the within-season phenology estimation for cotton at the field level. For this, we exploit a variety of Earth observation vegetation indices (derived from Sentinel-2) and numerical simulations of atmospheric and soil parameters. Our method is unsupervised to address the ever-present problem of sparse and scarce ground truth data that makes most supervised alternatives impractical in real-world scenarios. We applied fuzzy c-means clustering to identify the principal phenological stages of cotton and then used the cluster membership weights to further predict the transitional phases between adjacent stages. In order to evaluate our models, we collected 1,285 crop growth ground observations in Orchomenos, Greece. We introduced a new collection protocol, assigning up to two phenology labels that represent the primary and secondary growth stage in the field and thus indicate when stages are transitioning. Our model was tested against a baseline model that allowed to isolate the random agreement and evaluate its true competence. The results showed that our model considerably outperforms the baseline one, which is promising considering the unsupervised nature of the approach. The limitations and the relevant future work are thoroughly discussed. The ground observations are formatted in an ready-to-use dataset and will be available at https://github.com/Agri-Hub/cotton-phenology-dataset upon publication.

The influence of climate warming on flowering phenology in relation to historical annual and seasonal temperatures and plant functional traits

Climate warming has the potential to influence plant flowering phenology which in turn can have broader ecological consequences. Herbarium collections offer a source of historical plant data that makes possible the ability to document and better understand how warming climate can influence long-term shifts in flowering phenology. We examined the influence of annual, winter, and spring temperatures on the flowering phenology of herbarium specimens for 36 species collected from 1884-2015. We then compared the response to warming between native and non-native, woody and herbaceous, dry and fleshy fruit, and spring vs summer blooming species. Across all species, plants flowered 2.26 days earlier per 1 °C increase in annual average temperatures and 2.93 days earlier per 1 °C increase in spring onset average temperatures. Winter temperatures did not significantly influence flowering phenology. The relationship of temperature and flowering phenology was not significantly different between native and non-native species. Woody species flowered earlier than herbaceous species only in response to increasing annual temperatures. There was no difference in the phenological response between species with dry fruits and those fleshy fruits for any of the temperature periods. Spring blooming species exhibited a significantly greater phenological response to warming yearly average temperatures than summer blooming species. Although herbarium specimens can reveal climate change impacts on phenology, it is also evident that the phenological responses to warming vary greatly among species due to differences in functional traits such as those considered here, as well as other factors.

Early spring onset increases carbon uptake more than late fall senescence: modeling future phenological change in a US northern deciduous forest

In deciduous forests, spring leaf development and fall leaf senescence regulate the timing and duration of photosynthesis and transpiration. Being able to model these dates is therefore critical to accurately representing ecosystem processes in biogeochemical models. Despite this, there has been relatively little effort to improve internal phenology predictions in widely used biogeochemical models. Here, we optimized the phenology algorithms in a regionally developed biogeochemical model (PnET-CN) using phenology data from eight mid-latitude PhenoCam sites in eastern North America. We then performed a sensitivity analysis to determine how the optimization affected future predictions of carbon, water, and nitrogen cycling at Bartlett Experimental Forest, New Hampshire. Compared to the original PnET-CN phenology models, our new spring and fall models resulted in shorter season lengths and more abrupt transitions that were more representative of observations. The new phenology models affected daily estimates and interannual variability of modeled carbon exchange, but they did not have a large influence on the magnitude or long-term trends of annual totals. Under future climate projections, our new phenology models predict larger shifts in season length in the fall (1.1-3.2 days decade^-1) compared to the spring (0.9-1.5 days decade^-1). However, for every day the season was longer, spring had twice the effect on annual carbon and water exchange totals compared to the fall. These findings highlight the importance of accurately modeling season length for future projections of carbon and water cycling.

Low temperature and short daylength interact to affect the leaf senescence of two temperate tree species

Temperature and photoperiod are two major environmental cues shaping the leaf senescence of temperate tree species. However, how the control of leaf senescence is split between photoperiod and temperature is unknown for many ecologically important species. Here, we conducted a growth chamber experiment to test the effects of temperature (6, 9, 18 and 21°C) and photoperiod (8 and 16 h daylength) on leaf senescence of two temperate tree species (Quercus mongolica Fisch. and Larix principis-rupprechtii Mayr.) distributed in montane forest of China. The results showed that low temperature (LT) alone could induce leaf senescence of both species under long daylength (LD) conditions, but the leaf senescence of L. principis-rupprechtii was more sensitive to the decrease in temperature than that of Q. mongolica under the LD condition. Short daylength (SD) alone could only induce the leaf senescence of L. principis-rupprechtii, suggesting that the photoperiod sensitivity varies between species. SD could accelerate the LT-induced senescence, but the effect of SD reduced with the decrease in temperature. Based on these findings, we developed a new autumn phenology model by incorporating interspecific differences in the photoperiod sensitivity of leaf senescence. Compared with the three existing process-based autumn phenology models, the new model was more robust in simulating the experimental data. When employing these models to available long-term phenological data, our new model also performed best in reproducing the observed leaf senescence date of two closely related species (Quercus robur L. and Larix decidua Mill.). These results enhance our understanding of how LT and SD control leaf senescence. The prediction of the climate change impacts on forest carbon uptake could be improved by incorporating this new autumn phenological model into the terrestrial biosphere models.

Spring phenology and pathogen infection affect multigenerational plant attackers throughout the growing season

Climate change has been shown to advance spring phenology, increase the number of insect generations per year (multivoltinism) and increase pathogen infection levels. However, we lack insights into the effects of plant spring phenology and the biotic environment on the preference and performance of multivoltine herbivores and whether such effects extend into the later part of the growing season. To this aim, we used a multifactorial growth chamber experiment to examine the influence of spring phenology on plant pathogen infection, and how the independent and interactive effects of spring phenology and plant pathogen infection affect the preference and performance of multigenerational attackers (the leaf miner Tischeria ekebladella and the aphid Tuberculatus annulatus) on the pedunculate oak in the early, mid and late parts of the plant growing season. Pathogen infection was highest on late phenology plants, irrespective of whether inoculations were conducted in the early, mid or late season. The leaf miner consistently preferred to oviposit on middle and late phenology plants, as well as healthy plants, during all parts of the growing season, whereas we detected an interactive effect between spring phenology and pathogen infection on the performance of the leaf miner. Aphids preferred healthy, late phenology plants during the early season, healthy plants during the mid season, and middle phenology plants during the late season, whereas aphid performance was consistently higher on healthy plants during all parts of the growing season. Our findings highlight that the impact of spring phenology on pathogen infection and the preference and performance of insect herbivores is not restricted to the early season, but that its imprint is still present - and sometimes equally strong - during the peak and end of the growing season. Plant pathogens generally negatively affected herbivore preference and performance, and modulated the effects of spring phenology. We conclude that spring phenology and pathogen infection are two important factors shaping the preference and performance of multigenerational plant attackers, which is particularly relevant given the current advance in spring phenology, pathogen outbreaks and increase in voltinism with climate change.

The early bud gets the cold: Diverging spring phenology drives exposure to late frost in a Picea mariana [(Mill.) BSP] common garden

Under climate change, the increasing occurrence of late frost combined with advancing spring phenology can increase the risk of frost damage in trees. In this study, we tested the link between intra-specific variability in bud phenology and frost exposure and damages. We analysed the effects of the 2021 late frost event in a black spruce (Picea mariana (Mill.) BSP) common garden in Québec, Canada. We hypothesised that the timing of budbreak drives the exposure of vulnerable tissues and explains differences in frost damage. Budbreak was monitored from 2015 to 2021 in 371 trees from five provenances originating between 48° and 53° N and planted in a common garden at 48° N. Frost damages were assessed on the same trees through the proportion of damaged buds per tree and related to the phenological phases by ordinal regressions. After an unusually warm spring, minimum temperatures fell to -1.9°C on May 28 and 29, 2021. At this moment, trees from the northern provenances were more advanced in their phenology and showed more frost damage. Provenances with earlier budbreak had a higher probability of damage occurrence according to ordinal regression. Our study highlights the importance of intra-specific variability of phenological traits on the risk of frost exposure. We provide evidence that the timings of bud phenology affect sensitivity to frost, leading to damages at temperatures of -1.9°C. Under the same conditions, the earlier growth reactivation observed in the northern provenances increases the risks of late frost damage on the developing buds.

Climate warming shifts the time interval between flowering and leaf unfolding depending on the warming period

The timing of flowering (FL) and leaf unfolding (LU) determine plants' reproduction and vegetative growth. Global warming has substantially advanced FL and LU of temperate and boreal plants, but their responses to warming differ, which may influence the time interval between FL and LU (∆LU-FL), thereby impacting plant fitness and intraspecific physiological processes. Based on twigs collected from two flowering-first tree species, Populus tomentosa and Amygdalus triloba, we conducted a manipulative experiment to investigate the effects of winter chilling, spring warming and photoperiod on the ∆LU-FL. We found that photoperiod did not affect the ∆LU-FL of Amygdalus triloba, but shortened ∆LU-FL by 5.1 d of Populus tomentosa. Interestingly, spring warming and winter chilling oppositely affected the ∆LU-FL of both species. Specifically, low chilling accumulation extended the ∆LU-FL by 3.8 and 9.4 d for Populus tomentosa and Amygdalus triloba, but spring warming shortened the ∆LU-FL by 4.1 and 0.2 d °C^-1. Our results indicate that climate warming will decrease or increase the ∆LU-FL depending on the warming periods, i.e., spring or winter. The shifted time interval between flowering and leaf unfolding may have ecological effects including affecting pollen transfer efficiency and alter the structure and functioning of terrestrial ecosystem.

Winter warming offsets one half of the spring warming effects on leaf unfolding

Winter temperature-related chilling and spring temperature-related forcing are two major environmental cues shaping the leaf-out date of temperate species. To what degree insufficient chilling caused by winter warming would slow phenological responses to spring warming remains unclear. Using 27,071 time series of leaf-out dates for 16 tree species in Europe, we constructed a phenological model based on the linear or exponential function between the chilling accumulation (CA) and forcing requirements (FR) of leaf-out. We further used the phenological model to quantify the relative contributions of chilling and forcing on past and future spring phenological change. The results showed that the delaying effect of decreased chilling on the leaf-out date was prevalent in natural conditions, as more than 99% of time series exhibited a negative relationship between CA and FR. The reduction in chilling linked to winter warming from 1951 to 2014 could offset about one half of the spring phenological advance caused by the increase in forcing. In future warming scenarios, if the same model is used and a linear, stable correlation between CA and FR is assumed, declining chilling will continuously offset the advance of leaf-out to a similar degree. Our study stresses the importance of assessing the antagonistic effects of winter and spring warming on leaf-out phenology.

Integrating experiments to predict interactive cue effects on spring phenology with warming

Climate change has advanced plant phenology globally 4-6 d °C^-1 on average. Such shifts are some of the most reported and predictable biological impacts of rising temperatures. Yet as climate change has marched on, phenological shifts have appeared muted over recent decades - failing to match simple predictions of an advancing spring with continued warming. The main hypothesis for these changing trends is that interactions between spring phenological cues - long-documented in laboratory environments - are playing a greater role in natural environments due to climate change. Here, we argue that accurately linking shifts observed in long-term data to underlying phenological cues is slowed by biases in observational studies and limited integration of insights from laboratory studies. We synthesize seven decades of laboratory experiments to quantify how phenological cue-space has been studied and how treatments compare with shifts caused by climate change. Most studies focus on one cue, limiting our ability to make accurate predictions, but some well-studied forest species offer opportunities to advance forecasting. We outline how greater integration of controlled-environment studies with long-term data could drive a new generation of laboratory experiments, built on physiological insights, that would transform our fundamental understanding of phenology and improve predictions.

Accurate phenology analyses require bud traits and energy budgets

Spring phenology is mainly driven by temperature in extratropical ecosystems. Recent evidence highlighted the key role of micrometeorology and bud temperature on delaying or advancing leaf unfolding. Yet, phenology studies, either using ground-based or remote sensing observations, always substitute plant tissue temperature by air temperature. In fact, temperatures differ substantially between plant tissues and the air because plants absorb and lose energy. Here, we build on recent observations and well-established energy balance theories to discuss how solar radiation, wind and bud traits might affect our interpretation of spring phenology sensitivity to warming. We show that air temperature might be an imprecise and biased predictor of bud temperature. Better characterizing the plants' phenological response to warming will require new observations of bud traits and temperature for accurately quantifying their energy budget. As consistent micrometeorology datasets are still scarce, new approaches coupling energy budget modelling and plant traits could help to improve phenology analyses across scales.

Climate warming-driven phenological shifts are species-specific in woody plants: evidence from twig experiment in Kashmir Himalaya

Experimental evidences in support of climate warming-driven phenological shifts are still scarce, particularly from the developing world. Here, we investigated the effect of experimental warming on flowering phenology of selected woody plants in Kashmir Himalaya. We selected the twigs of four congeneric pairs of temperate woody species (Prunus, Populus, Ulmus, Viburnum)-typical spring-flowering plants in the region. Using randomised block design, we monitored these winter dormant twigs in controlled growth chambers to study the effect of different temperature regimes (9, 17, 20 and 23 °C) and species identity on the patterns of phenological shifts. We observed a significant phenological shift in all the species showing preponement in the first flower out and senescence phases ranging from 0.56 to 3.0 and 0.77 to 4.04 days per degree increase in temperature, respectively. The duration of flowering phase in all the species showed a corresponding decrease along the gradient of increasing temperature, which was more driven by preponement of the flower senescence than the start of flowering. The patterns of phenological shifts were highly species-specific, and the magnitude of these shifts significantly varied in all the four pairs of congeneric species despite their phylogenetic similarity. Our study provides experimental support to the previous long-term observation and herbarium-based studies showing that the patterns of phenological shifts in response to global climate warming are likely to vary between species, even those belonging to same evolutionary stock. Our findings highlight that a one-size-fits-all strategy to manage the likely impacts of climate warming-induced phenological shifts will seldom succeed, and should instead be designed for the specific phenological responses of species and regions.

Leaf out time correlates with wood anatomy across large geographic scales and within local communities

There is a long-standing idea that the timing of leaf production in seasonally cold climates is linked to xylem anatomy, specifically vessel diameter because of the hydraulic requirements of expanding leaves. We tested for a relationship between the timing of leaf out and vessel diameter in 220 plants in three common gardens accounting for species' phylogenetic relationships. We investigated how vessel diameter related to wood porosity, plant height and leaf length. We also used dye perfusion tests to determine whether plants relied on xylem produced during the previous growing season at the time of leaf out. In all three gardens, there was later leaf out in species with wider vessels. Ring-porous species had the widest vessels, exhibited latest leaf out and relied less on xylem made during the previous growing season than diffuse-porous species. Wood anatomy and leaf phenology did not exhibit a phylogenetic signal. The timing of leaf out is correlated with wood anatomy across species regardless of species' geographic origin and phylogenetic relationships. This correlation could be a result of developmental and physiological links between leaves and wood or tied to a larger safety efficiency trade-off.

Drivers of phenological changes in southern Europe

The life cycle of plants is largely determined by climate, which renders phenological responses to climate change a highly suitable bioindicator of climate change. Yet, it remains unclear, which are the key drivers of phenological patterns at certain life stages. Furthermore, the varying responses of species belonging to different plant functional types are not fully understood. In this study, the role of temperature and precipitation as environmental drivers of phenological changes in southern Europe is assessed. The trends of the phenophases leaf unfolding, flowering, fruiting, and senescence are quantified, and the corresponding main environmental drivers are identified. A clear trend towards an earlier onset of leaf unfolding, flowering, and fruiting is detected, while there is no clear pattern for senescence. In general, the advancement of leaf unfolding, flowering and fruiting is smaller for deciduous broadleaf trees in comparison to deciduous shrubs and crops. Many broadleaf trees are photoperiod-sensitive; therefore, their comparatively small phenological advancements are likely the effect of photoperiod counterbalancing the impact of increasing temperatures. While temperature is identified as the main driver of phenological changes, precipitation also plays a crucial role in determining the onset of leaf unfolding and flowering. Phenological phases advance under dry conditions, which can be linked to the lack of transpirational cooling leading to rising temperatures, which subsequently accelerate plant growth.

Spectrum of artificial light at night drives impact of a diurnal species in insect food web

Artificial light at night (ALAN) has become a profound form of global anthropogenic environmental change differing in from natural light regimes in intensity, duration, distribution and spectra. It is clear that ALAN impacts individual organisms, however, population level effects, particularly of spectral changes, remain poorly understood. Here we exposed experimental multigenerational aphid-parasitoid communities in the field to seven different light spectra at night ranging from 385 to 630 nm and compared responses to a natural day-night light regime. We found that while aphid population growth was initially unaffected by ALAN, parasitoid efficiency declined under most ALAN spectra, leading to reduced top-down control and higher aphid densities. These results differ from those previously found for white light, showing a strong impact on species' daytime performance. This highlights the importance of ALAN spectra when considering their environmental impact. ALAN can have large impacts on the wider ecological community by influencing diurnal species.

Winter warming stimulates vegetative growth and alters fruit quality of blackcurrant (Ribes nigrum)

The rate of global warming varies in magnitude between seasons, with warming being more pronounced in winter and spring than in summer and autumn at high latitudes. Winter warming can have profound effects on dormancy release and spring phenology of perennial fruit crops, but potential follow-on impacts on growth, fruit yield or quality have only rarely been investigated. We studied the effects of mild winter warming on spring phenology, current year shoot growth, cropping performance and fruit quality in four field-grown cultivars of blackcurrant with different chilling requirements. Plants were exposed to ambient or slightly elevated (+ 0.5 °C) temperatures from early October to mid-April the following year. Winter warming had few effects on spring phenology and fruit yield, but caused significant changes in berry contents of phenolic compounds and a reduction in soluble sugars. Increased vegetative growth of warmed plants likely accounts for the changes in berry quality. The results demonstrate a persistent effect of winter warming on shoot growth, which indirectly changes fruit quality.

Stronger Spring Phenological Advance in Future Warming Scenarios for Temperate Species With a Lower Chilling Sensitivity

Spring warming could induce earlier leaf-out or flowering of temperate plant species, and decreased chilling in winter has a delaying effect on spring phenology. However, the relative contribution of the decreased chilling and increased forcing on spring phenological change is unclear. Here, we analyzed the experimental data for 14 temperate woody species in Beijing, China and quantified the forcing requirements (FR) of spring phenology and chilling sensitivity (the ratio of the FR at the low chilling condition to the FR at the high chilling condition) for each species. Furthermore, using species-specific functions between the amount of chilling and FR, we established a phenological model to simulate the annual onset dates of spring events during the past 69 years (1952-2020) and in the future (2021-2099) under RCP 4.5 and RCP 8.5 climate scenarios. We also developed a novel method to quantitatively split the predicted phenological change into the effects caused by changes in forcing and those caused by changes in chilling. The results show that the FR of spring events decreased with the increase in the amount of chilling, and this relationship could be described as an exponential decay function. The basic FR (the FR at the high chilling condition) and chilling sensitivity varied greatly among species. In the 1952-2020 period, the advancing effect of increased forcing was stronger than the effect of chilling, leading to earlier spring events with a mean trend of -1.96 days/decade. In future climate scenarios, the spring phenology of temperate species would continue to advance but will be limited by the decreased chilling. The species with lower chilling sensitivities showed stronger trends than those with high chilling sensitivities. Our results suggested that the delaying effect of declining chilling could only slow down the spring phenological advance to a certain extent in the future.

Woody species do not differ in dormancy progression: Differences in time to budbreak due to forcing and cold hardiness

Budbreak is one of the most observed and studied phenological phases in perennial plants, but predictions remain a challenge, largely due to our poor understanding of dormancy. Two dimensions of exposure to temperature are generally used to model budbreak: accumulation of time spent at low temperatures (chilling) and accumulation of heat units (forcing). These two effects have a well-established negative correlation; with more chilling, less forcing is required for budbreak. Furthermore, temperate plant species are assumed to vary in chilling requirements for dormancy completion allowing proper budbreak. Here, dormancy is investigated from the cold hardiness standpoint across many species, demonstrating that it should be accounted for to study dormancy and accurately predict budbreak. Most cold hardiness is lost prior to budbreak, but rates of cold hardiness loss (deacclimation) vary among species, leading to different times to budbreak. Within a species, deacclimation rate increases with accumulation of chill. When inherent differences between species in deacclimation rate are accounted for by normalizing rates throughout winter by the maximum rate observed, a standardized deacclimation potential is produced. Deacclimation potential is a quantitative measurement of dormancy progression based on responsiveness to forcing as chill accumulates, which increases similarly for all species, contradicting estimations of dormancy transition based on budbreak assays. This finding indicates that comparisons of physiologic and genetic control of dormancy require an understanding of cold hardiness dynamics. Thus, an updated framework for studying dormancy and its effects on spring phenology is suggested where cold hardiness in lieu of (or in addition to) budbreak is used.

Understorey light quality affects leaf pigments and leaf phenology in different plant functional types

Forest understorey plants receive most sunlight in springtime before canopy closure, and in autumn following leaf-fall. We hypothesised that plant species must adjust their phenological and photoprotective strategies in response to large changes in the spectral composition of the sunlight they receive. Here, we identified how plant species growing in northern deciduous and evergreen forest understoreys differ in their response to blue light and ultraviolet (UV) radiation according to their functional strategy. We installed filters in a forest understorey in southern Finland, to create the following treatments attenuating: UV radiation below 350 nm, all UV radiation (< 400 nm), all blue light and UV radiation (< 500 nm), and a transparent control. In eight species, representing different functional strategies, we assessed leaf optical properties, phenology, and epidermal flavonoid contents over two years. Blue light accelerated leaf senescence in all species measured in the understorey, apart from Quercus robur seedlings, whereas UV radiation only accelerated leaf senescence in Acer platanoides seedlings. More light-demanding species accumulated flavonols in response to seasonal changes in light quality compared to shade-tolerant and wintergreen species and were particularly responsive to blue light. Reduction of blue and UV radiation under shade reveals an important role for microclimatic effects on autumn phenology and leaf photoprotection. An extension of canopy cover under climate change, and its associated suppression of understorey blue light and UV radiation, may delay leaf senescence for understorey species with an autumn niche.

Contrasting Effects of Nitrogen Addition on Vegetative Phenology in Dry and Wet Years in a Temperate Steppe on the Mongolian Plateau

Changes in spring and autumn phenology and thus growing season length (GSL) pose great challenges in accurately predicting terrestrial primary productivity. However, how spring and autumn phenology in response to land-use change and nitrogen deposition and underlying mechanisms remain unclear. This study was conducted to explore the GSL and its components [i.e., the beginning of growing season and ending of growing season (EGS)] in response to mowing and nitrogen addition in a temperate steppe on the Mongolia Plateau during 2 years with hydrologically contrasting condition [dry (2014) vs. wet (2015)]. Our results demonstrated that mowing advanced the BGS only by 3.83 days, while nitrogen addition advanced and delayed the BGS and EGS by 2.85 and 3.31 days, respectively, and thus prolonged the GSL by 6.16 days across the two growing seasons from 2014 to 2015. When analyzed by each year, nitrogen addition lengthened the GSL in the dry year (2014), whereas it shortened the GSL in the wet year (2015). Further analyses revealed that the contrasting impacts of nitrogen on the GSL were attributed to monthly precipitation regimes and plant growth rate indicated by the maximum of normalized difference vegetation index (NDV_max). Moreover, changes in the GSL and its two components had divergent impacts on community productivity. The findings highlight the critical role of precipitation regimes in regulating the responses of spring and autumn phenology to nutrient enrichment and suggest that the relationships of ecosystem productivity with spring and autumn phenology largely depend on interannual precipitation fluctuations under future increased nitrogen deposition scenarios.

Artificial light at night: an underappreciated effect on phenology of deciduous woody plants

Artificial light at night (ALAN), an increasing anthropogenic driver, is widespread and shows rapid expansion with potential adverse impact on the terrestrial ecosystem. However, whether and to what extent does ALAN affect plant phenology, a critical factor influencing the timing of terrestrial ecosystem processes, remains unexplored due to limited ALAN observation. Here, we used the Black Marble ALAN product and phenology observations from USA National Phenology Network to investigate the impact of ALAN on deciduous woody plants phenology in the conterminous United States. We found that (1) ALAN significantly advanced the date of breaking leaf buds by 8.9 ± 6.9 days (mean ± SD) and delayed the coloring of leaves by 6.0 ± 11.9 days on average; (2) the magnitude of phenological changes was significantly correlated with the intensity of ALAN (P < 0.001); and (3) there was an interaction between ALAN and temperature on the coloring of leaves, but not on breaking leaf buds. We further showed that under future climate warming scenarios, ALAN will accelerate the advance in breaking leaf buds but exert a more complex effect on the coloring of leaves. This study suggests intensified ALAN may have far-reaching but underappreciated consequences in disrupting key ecosystem functions and services, which requires an interdisciplinary approach to investigate. Developing lighting strategies that minimize the impact of ALAN on ecosystems, especially those embedded and surrounding major cities, is challenging but must be pursued.

A global increase in tree cover extends the growing season length as observed from satellite records

Plant phenology provides information on the seasonal dynamics of plants, and changes herein are important for understanding the impact of climate change and human management on the biosphere. Land surface phenology is the study of plant phenology across large spatial scales estimated by satellite observations. However, satellite observations (pixels) are often composed of a mixture of vegetation types, like woody vegetation and herbaceous vegetation, having different phenological characteristics. Therefore, any changes in tree cover presumably impact land surface phenology, as trees usually have a different seasonal cycle compared to herbaceous vegetation. On the other hand, changes in land surface phenology are often interpreted as a result of climate change-induced impacts on the photosynthetic activity of vegetation. Therefore, it is important to better understand the role of changes in vegetation cover (here, the proportion between tree and short vegetation cover) in satellite-derived land surface phenology analysis. We studied the impact of changes in tree cover on satellite observed land surface phenology at a global scale over the past three decades. We found an extension of the growing season length in 36.6% of the areas where tree cover increased, whereas only 20.1% of the areas where tree cover decreased showed an increase in growing season length. Furthermore, the ratio between tree cover and short vegetation cover was found to affect changes in the length of the growing season, with the denser tree cover showing a more pronounced extension of the growing season length (especially in boreal forests). These results highlight the importance of changes in tree cover when analyzing the impact of climate change on vegetation phenology. Our study thereby addresses a critical knowledge gap for an improved understanding of changes in land surface phenology during recent decades in the context of climate and human-induced global land cover change.

Precipitation Dominates the Relative Contributions of Climate Factors to Grasslands Spring Phenology on the Tibetan Plateau

Temperature and precipitation are the primary regulators of vegetation phenology in temperate zones. However, the relative contributions of each factor and their underlying combined effect on vegetation phenology are much less clear, especially for the grassland of the Tibetan Plateau To quantify the contribution of each factor and the potential interactions, we conducted redundancy analysis for grasslands spring phenology on the Tibetan Plateau during 2000–2017. Generally, the individual contribution of temperature and precipitation to grasslands spring phenology (the start of growing season (SOS)) was lower, despite a higher correlation coefficient, which further implied that these factors interact to affect the SOS. The contributions of temperature and precipitation to the grasslands spring phenology varied across space on the Tibetan Plateau, and these spatial heterogeneities can be mainly explained by the spatial gradient of long-term average precipitation during spring over 2000–2017. Specifically, the SOS for meadow was dominated by the mean temperature in spring (Tspring) in the eastern wetter ecoregion, with an individual contribution of 24.16% (p < 0.05), while it was strongly negatively correlated with the accumulated precipitation in spring (Pspring) in the western drier ecoregion. Spatially, a 10 mm increase in long-term average precipitation in spring resulted in an increase in the contribution of Tspring of 2.0% (p < 0.1) for meadow, while it caused a decrease in the contribution of Pspring of −0.3% (p < 0.05). Similarly, a higher contribution of Pspring for steppe was found in drier ecoregions. A spatial decrease in precipitation of 10 mm increased the contribution of Pspring of 1.4% (p < 0.05). Considering these impacts of precipitation on the relative contribution of warming and precipitation to the SOS, projected climate change would have a stronger impact on advancing SOS in a relatively moist environment compared to that of drier areas. Hence, these quantitative interactions and contributions must be included in current ecosystem models, mostly driven by indicators with the direct and the overall effect in response to projected climate warming.

Direct Phenological Responses but Later Growth Stimulation upon Spring and Summer/Autumn Warming of Prunus spinosa L. in a Common Garden Environment

Future predictions of forest ecosystem responses are a challenge, as global temperatures will further rise in the coming decades at an unprecedented rate. The effect of elevated temperature on growth performance and phenology of three Prunus spinosa L. provenances (originating from Belgium, Spain, and Sweden) in a common garden environment was investigated. One-year-old seedlings were grown in greenhouse conditions and exposed to ambient and elevated temperatures in the spring (on average 5.6 °C difference) and in the late summer/autumn of 2018 (on average 1.9 °C difference), while they were kept hydrated, in a factorial design. In the following years, all plants experienced the same growing conditions. Bud burst, leaf senescence, height, and diameter growth were recorded. Height and radial growth were not affected in the year of the treatments (2018) but were enhanced the year after (2019), whereas phenological responses depended on the temperature treatments in the year of the treatments (2018) with little carry-over effects in the succeeding years. Spring warming enhanced more height growth in the succeeding year, whereas summer/autumn warming stimulated more radial growth. Spring warming advanced bud burst and shortened the leaf opening process whereas summer/autumn warming delayed leaf senescence and enlarged the duration of this phenophase. These results can help predict the putative shifts in species composition of future forests and woody landscape elements.

Delaying effect of humidity on leaf unfolding in Europe

Understanding the drivers of plant phenology is critical to predict the impact of future warming on terrestrial ecosystem carbon cycling and feedbacks to climate. Using indoor growth chambers, air humidity is reported to influence spring phenology in temperate trees. However, previous studies have not investigated the effect of air humidity on the spring phenology using long-term and large-scale ground observations. Therefore, the role of humidity in spring phenology in temperate trees still remains poorly understood. Here, we synthesized 229,588 records of leaf unfolding dates in eight temperate tree species, including four early-successional and four late-successional species, at 1716 observation sites during 1951-2015 in Europe, and comprehensively analyzed the effect of humidity on the spring phenology. We found that rising humidity significantly delayed spring leaf unfolding for all eight temperate tree species. Leaf unfolding was more sensitive to humidity in early-successional species compared to late-successional species. In addition, the delaying effect of humidity on leaf unfolding increased as temperature warmed over the past 65 years. Our results provide evidence that spring leaf unfolding of temperate trees was significantly delayed by rising humidity. The delaying effect of humidity may restrict earlier spring phenology induced by warming, especially for early-successional species, under future climate warming scenarios in temperate forests.

Green with phenology

[Figure: see text].

Specific Drivers and Responses to Land Surface Phenology of Different Vegetation Types in the Qinling Mountains, Central China

Land surface phenology (LSP), as a precise bio-indicator that responds to climate change, has received much attention in fields concerned with climate change and ecology. Yet, the dynamics of LSP changes in the Qinling Mountains (QMs)—A transition zone between warm-temperate and north subtropical climates with complex vegetation structure—under significant climatic environmental evolution are unclear. Here, we analyzed the spatiotemporal dynamics of LSP for different vegetation types in the QMs from 2001 to 2019 and quantified the degree of influence of meteorological factors (temperature, precipitation, and shortwave radiation), and soil (temperature and moisture), and biological factors (maximum of NDVI and middle date during the growing season) on LSP changes using random forest models. The results show that there is an advanced trend (0.15 days/year) for the start of the growing season (SOS), a delayed trend (0.24 days/year) for the end of the growing season (EOS), and an overall extended trend (0.39 days/year) for the length of the growing season (LOS) in the QMs over the past two decades. Advanced SOS and delayed EOS were the dominant patterns leading to a lengthened vegetation growing season, followed by a joint delay of SOS and EOS, and the latter was particularly common in shrub and evergreen broadleaved forests. The growth season length increased significantly in western QMs. Furthermore, we confirmed that meteorological factors are the main factors affecting the interannual variations in SOS and EOS, especially the meteorological factor of preseason mean shortwave radiation (SWP). The grass and crop are most influenced by SWP. The soil condition has, overall, a minor influence the regional LSP. This study highlighted the specificity of different vegetation growth in the QMs under warming, which should be considered in the accurate prediction of vegetation growth in the future.

Contrasting temporal variations in responses of leaf unfolding to daytime and nighttime warming

Earlier spring phenological events have been widely reported in plants under global warming. Recent studies reported a slowdown in the warming-induced advanced spring phenology in temperate regions. However, previous research mainly focused on daily mean temperature, thus neglecting the asymmetric phenological responses to daytime and nighttime temperature. Using long-term records of leaf unfolding in eight deciduous species at 1300 sites across central Europe, we assessed and compared the effects of daytime temperature, nighttime temperature, and photoperiod on leaf unfolding during 1951-1980 and 1981-2013. Although leaf unfolding was advanced by daytime warming during 1951-2013, the advancing responses of leaf unfolding significantly decreased from 1951-1980 to 1981-2013 due to a lower accumulation of chilling units by daytime warming. Nighttime warming delayed leaf unfolding during 1951-1980 but advanced it during 1981-2013 due to a higher accumulation of chilling units by nighttime warming. In contrast, critical daylength and plasticity of leaf unfolding dates remained unchanged between 1951 and 2013. Our study provided evidence that daytime warming instead of nighttime warming accounts for the slowdown in the advancing spring phenology and implied that nighttime warming-induced earlier spring phenology may be buffering the slowdown of the advanced spring phenology by daytime warming. The response of spring phenology to nighttime temperature may override that to daytime temperature under the actual trends in global warming.