Rethinking false spring risk

Leaf gene expression trajectories during the growing season are consistent between sites and years in American beech

Transcriptomics provides a versatile tool for ecological monitoring. Here, through genome-guided profiling of transcripts mapping to 33 042 gene models, expression differences can be discerned among multi-year and seasonal leaf samples collected from American beech trees at two latitudinally separated sites. Despite a bottleneck due to post-Columbian deforestation, the single nucleotide polymorphism-based population genetic background analysis has yielded sufficient variation to account for differences between populations and among individuals. Our expression analyses during spring-summer and summer-autumn transitions for two consecutive years involved 4197 differentially expressed protein coding genes. Using Populus orthologues we reconstructed a protein-protein interactome representing leaf physiological states of trees during the seasonal transitions. Gene set enrichment analysis revealed gene ontology terms that highlight molecular functions and biological processes possibly influenced by abiotic forcings such as recovery from drought and response to excess precipitation. Further, based on 324 co-regulated transcripts, we focused on a subset of GO terms that could be putatively attributed to late spring phenological shifts. Our conservative results indicate that extended transcriptome-based monitoring of forests can capture diverse ranges of responses including air quality, chronic disease, as well as herbivore outbreaks that require activation and/or downregulation of genes collectively tuning reaction norms maintaining the survival of long living trees such as the American beech.

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.

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.

The role of genotypic and climatic variation at the range edge: A case study in winegrapes

PREMISE

Changes in habitat suitability due to climate change are causing range shifts, with new habitat potentially available at cold range edges. We must predict these range shifts, but forecasters have limited knowledge of how genetic differences in plant physiological tolerances influence range shifts. Here, we focus on a major determinant of species ranges-physiological tolerance to extreme cold-to ask how warming over recent decades and genetic variation shape expansion across complex landscapes.

METHODS

We examined how genotypes vary in maximum cold tolerance from 9 years of cold hardiness data across 18 genotypes from 13 sites, using winegrapes (Vitis vinifera subsp. vinifera) as a case study. Combining a Bayesian hierarchical dose-response model with gridded climate data, we then project changes in climatic suitability near winegrapes' current cold range-edge between 1949 and 2016.

RESULTS

Plants increased maximum cold hardiness non-linearly with decreasing air temperature (maximum cold hardiness: -23.6°C), but with substantial (by 2°C) variation across genotypes. Our results suggest, since the 1980s, decreasing freeze injury risk has made conditions more favorable for all genotypes at the cold range-edge, but conditions remained more favorable for more cold hardy genotypes and in warmer areas. There was substantial spatial variation in habitat suitability, with the majority of suitably warm habitat located in a narrow north-south oriented strip.

CONCLUSIONS

We highlight the importance of genotypic differences in physiological tolerances when assessing range shift potential with climate change. Habitat improvements were unevenly distributed over the spatially complex landscape, though, emphasizing the importance of dispersal in range expansion.

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.

Immediate and carry-over effects of late-spring frost and growing season drought on forest gross primary productivity capacity in the Northern Hemisphere

Forests are increasingly exposed to extreme global warming-induced climatic events. However, the immediate and carry-over effects of extreme events on forests are still poorly understood. Gross primary productivity (GPP) capacity is regarded as a good proxy of the ecosystem's functional stability, reflecting its physiological response to its surroundings. Using eddy covariance data from 34 forest sites in the Northern Hemisphere, we analyzed the immediate and carry-over effects of late-spring frost (LSF) and growing season drought on needle-leaf and broadleaf forests. Path analysis was applied to reveal the plausible reasons behind the varied responses of forests to extreme events. The results show that LSF had clear immediate effects on the GPP capacity of both needle-leaf and broadleaf forests. However, GPP capacity in needle-leaf forests was more sensitive to drought than in broadleaf forests. There was no interaction between LSF and drought in either needle-leaf or broadleaf forests. Drought effects were still visible when LSF and drought coexisted in needle-leaf forests. Path analysis further showed that the response of GPP capacity to drought differed between needle-leaf and broadleaf forests, mainly due to the difference in the sensitivity of canopy conductance. Moreover, LSF had a more severe and long-lasting carry-over effect on forests than drought. These results enrich our understanding of the mechanisms of forest response to extreme events across forest types.

Phenological physiology: seasonal patterns of plant stress tolerance in a changing climate

The physiological challenges posed by climate change for seasonal, perennial plants include increased risk of heat waves, postbudbreak freezing ('false springs'), and droughts. Although considerable physiological work has shown that the traits conferring tolerance to these stressors - thermotolerance, cold hardiness, and water deficit stress, respectively - are not static in time, they are frequently treated as such. In this review, I synthesize the recent literature on predictable seasonal - and therefore, phenological - patterns of acclimation and deacclimation to heat, cold, and water-deficit stress in perennials, focusing on woody plants native to temperate climates. I highlight promising, high-throughput techniques for quantifying thermotolerance, cold hardiness, and drought tolerance. For each of these forms of stress tolerance, I summarize the current balance of evidence regarding temporal patterns over the course of a year and suggest a characteristic temporal scale in these responses to environmental stress. In doing so, I offer a synthetic framework of 'phenological physiology', in which understanding and leveraging seasonally recurring (phenological) patterns of physiological stress acclimation can facilitate climate change adaptation and mitigation.

Genome-wide identification and expression analysis of DREB genes in alfalfa (Medicago sativa) in response to cold stress

Dehydration-responsive element-binding proteins (DREBs) belong to members of the AP2/ERF transcription factor superfamily, which has been reported to involve various abiotic-stress responses and tolerance in plants. However, research on the DREB-family is still limited in alfalfa (Medicago sativa L.), a forage legume cultivated worldwide. The recent genome-sequence release of the alfalfa cultivar "XinJiangDaYe" allowed us to identify 172 DREBs by a multi-step homolog search. The phylogenetic analysis indicated that such MsDREBs could be classified into 5 groups, namely A-1 (56 members), A-2 (39), A-3 (3), A-4 (61) and 13 (A-5 (13), thus adding substantial new members to the DREB-family in alfalfa. Furthermore, a comprehensive survey in silico of conserved motif, gene structure, molecular weight, and isoelectric point (pI) as well as gene expression was conducted. The resulting data showed that, for cold-stress response, 33 differentially expressed MsDREBs were identified with a threshold of Log2-fold > 1, and most of which were transcriptionally upregulated within 48 h during a cold treatment(s). Moreover, the expression profiling of MsDREBs from two ecotypes of alfalfa subspecies i.e. M. sativa ssp. falcata (F56, from a colder region of Central Asia) and M. sativa ssp. sativa (B47, from Near East) revealed that most of the cold-stress responsive MsDREBs exhibited a significantly lower expression in F56, leading to a proposal of the existence of a distinct mechanism(s) for cold tolerance regulated by DREB-related action, which would have been evolved in alfalfa with a genotypic specificity. Additionally, by examining the transcriptome of a freezing-tolerance species (M. sativa cv. Zhaodong), eight DREBs were found to be implicated in a long-term freezing-stress adaptation with a great potential. Taken together, the current genome-wide identification in alfalfa points to the importance of some MsDREBs in the cold-stress response, providing some promising molecular targets to be functionally characterized for the improvement of cold tolerance in crops including alfalfa.

Genome-Wide Identification of bHLH Transcription Factor in Medicago sativa in Response to Cold Stress

Alfalfa represents one of the most important legume forages, and it is also applied as an organic fertilizer to improve soil quality. However, this perennial plant is native to warmer temperate regions, and its valuable cold-acclimation-related regulatory mechanisms are still less known. In higher plants, the bHLH transcription factors play pleiotropic regulatory roles in response to abiotic stresses. The recently released whole genome sequencing data of alfalfa allowed us to identify 469 MsbHLHs by multi-step homolog search. Herein, we primarily identified 65 MsbHLH genes that significantly upregulated under cold stress, and such bHLHs were classified into six clades according to their expression patterns. Interestingly, the phylogenetic analysis and conserved motif screening of the cold-induced MsbHLHs showed that the expression pattern is relatively varied in each bHLH subfamily, this result indicating that the 65 MsbHLHs may be involved in a complex cold-responsive regulatory network. Hence, we analyzed the TFBSs at promoter regions that unraveled a relatively conserved TFBS distribution with genes exhibiting similar expression patterns. Eventually, to verify the core components involved in long-term cold acclimation, we examined transcriptome data from a freezing-tolerant species (cv. Zhaodong) in the field and compared the expression of cold-sensitive/tolerant subspecies of alfalfa, giving 11 bHLH as candidates, which could be important for further cold-tolerance enhancement and molecular breeding through genetic engineering in alfalfa.

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.

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.

Peach [Prunus persica (L.) Batsch] Cultivars Differ in Apparent Base Temperature and Growing Degree Hour Requirement for Floral Bud Break

Bud break timing in peach [Prunus persica (L.) Batsch] is determined by the sequential fulfillment of a chilling requirement (CR) and a heat requirement (HR) for development. Genotypic variation in CR has been well characterized in peach. Adapting peaches to low chilling environment through reduced CR can make them susceptible to crop destroying spring frosts, if bloom occurs too early. Potential variation in HR between accessions has received less attention due to the methodological difficulty in assessing HR independently of CR. HR could vary in the magnitude of growing degree hours (GDHs) and/or the base temperature at which GDH accumulation begins. Characterizing HR traits in peach accessions could allow improved bloom time modeling and selection of phenotypes with improved spring frost avoidance through delayed bloom. We estimated GDH and apparent base temperature for floral bud break by observing time to floral bud break at several constant forcing temperatures. We evaluated 54 peach accessions (representing a range of CR) in which chilling had been saturated after >1,700 h at 3°C. Accessions differed widely in both the GDH requirement (2,015 to 11,191°C⋅h) and apparent base temperature (-1.85 to 8.69°C) for GDH accumulation. GDH and apparent base temperature were negatively correlated. A simulation exercise was performed to assess relative importance of varying base temperature vs. GDH for delaying bloom at different chilling accumulations at three locations in the southeastern United States using 30 years of historical weather data. The aim of this study was to determine whether there may be unrecognized diversity in peach germplasm for two HR traits (base temperature and thermal time) to enable breeding efforts to delay floral bud break and reduce the frost exposure risk of developing flowers and fruits. Our results suggest that selecting cultivars for increased GDH would be a safer, more reliable strategy for delaying bloom than increasing base temperature for GDH accumulation.

Seasonal changes in carbohydrates and water content predict dynamics of frost hardiness in various temperate tree species

Predicting tree frost tolerance is critical to select adapted species according to both the current and predicted future climate. The relative change in water to carbohydrate ratio is a relevant trait to predict frost acclimation in branches from many tree species. The objective of this study is to demonstrate the interspecific genericity of this approach across nine tree species. In the studied angiosperm species, frost hardiness dynamics were best correlated to a decrease in water content at the early stage of acclimation (summer and early autumn). Subsequently, frost hardiness dynamics were more tightly correlated to soluble carbohydrate contents until spring growth resumption. Based on different model formalisms, we predicted frost hardiness at different clade levels (angiosperms, family, genus and species) with high to moderate accuracy (1.5-6.0 °C root mean squared error (RMSE)) and robustness (2.8-6.1 °C prediction RMSE). The TOT model, taking all soluble carbohydrate and polyols into account, was more effective and adapted for large scale studies aiming to explore frost hardiness across a wide range of species. The ISC model taking the individual contribution of each soluble carbohydrate molecule into account was more efficient at finer scale such as family or species. The ISC model performance also suggests that the role of solutes cannot be reduced to a 'bulk' osmotic effect as could be computed if all of them were located in a single, common, compartment. This study provides sets of parameters to predict frost hardiness in a wide range of species, and clues for targeting specific carbohydrate molecules to improve frost hardiness.

Late spring freezes coupled with warming winters alter temperate tree phenology and growth

Spring phenology is advancing with warming but late spring freezes may not advance at the same rate, potentially leading to an increase in freezes after trees initiate budburst. Research suggests warming winters may delay budburst through reduced chilling, which may cause plants to leafout more slowly, thus decreasing spring freeze tolerance. Here, we assessed the effects of late spring freezes and reduced over-winter chilling on sapling phenology, growth and tissue traits, across eight temperate tree and shrub species in a laboratory experiment. We found that spring freezes delayed leafout - extending the period of greatest risk for freeze damage - increased damage to the shoot apical meristem, and decreased leaf toughness and leaf thickness. Longer chilling accelerated budburst and leafout, even under spring freeze conditions. Thus, chilling compensated for the adverse effects of late spring freezes on phenology. Despite the effects of spring freezes and chilling on phenology, we did not see any major reordering in the sequence of species leafout. Our results suggest climate change may impact forest communities not through temporal reassembly, but rather through impacts on phenology and growth from the coupled effects of late spring freezes and decreased over-winter chilling under climate change.

Phenological sensitivity to temperature mediates herbivory

Species interactions drive ecosystem processes and are a major focus of global change research. Among the most consequential interactions expected to shift with climate change are those between insect herbivores and plants, both of which are highly sensitive to temperature. Insect herbivores and their host plants display varying levels of synchrony that could be disrupted or enhanced by climate change, yet empirical data on changes in synchrony are lacking. Using evidence of herbivory on herbarium specimens collected from the northeastern United States and France from 1900 to 2015, we provide evidence that plant species with temperature-sensitive phenologies experience higher levels of insect damage in warmer years, while less temperature-sensitive, co-occurring species do not. While herbivory might be mediated by interactions between warming and phenology through multiple pathways, we suggest that warming might lengthen growing seasons for phenologically sensitive plant species, exposing their leaves to herbivores for longer periods of time in warm years. We propose that elevated herbivory in warm years may represent a previously underappreciated cost to phenological tracking of climate change over longer timescales.

The direct and indirect effects of extreme climate events on insects

Extreme climate events are predicted to increase in the future, which will have significant effects on insect biodiversity. Research into this area has been rapidly expanding, but knowledge gaps still exist. We conducted a review of the literature to provide a synthesis of extreme climate events on insects and identify future areas of research. In our review, we asked the following questions: 1) What are the direct and indirect mechanisms that extreme climate events affect individual insects? 2) What are the effects of extreme climate events on insect populations and demography? 3) What are the implications of the extreme climate events effects on insect communities? Drought was among the most frequently described type of extreme climate event affecting insects, as well as the effects of temperature extremes and extreme temperature variation. Our review explores the factors that determine the sensitivity or resilience to climate extremes for individuals, populations, and communities. We also identify areas of future research to better understand the role of extreme climate events on insects including effects on non-trophic interactions, alteration of population dynamics, and mediation of the functional the trait set of communities. Many insect species are under threat from global change and extreme climate events are a contributing factor. Biologists and policy makers should consider the role of extreme events in their work to mitigate the loss of biodiversity and delivery of ecosystem services by insects.

Endodormancy release in Norway spruce grafts representing trees of different ages

Studies addressing endodormancy release in adult trees are usually carried out using twigs detached from the trees in the experiments. Potential problems caused by cutting the root-shoot connection when detaching the twigs can be avoided by using grafts as the experimental material. We studied the effects of chilling on the endodormancy release in Norway spruce (Picea abies (L.) Karst.) grafts where twigs of 16-, 32- and 80-year-old trees were used as the scions. The grafts were first exposed to chilling in natural conditions and then samples of them were transferred at intervals to a regrowth test in forcing conditions in a greenhouse. The bud burst percentage, BB%, in the forcing conditions generally increased from zero to near 100% with increasing previous chilling accumulation from mid-October until mid-November, indicating that endodormancy was released in almost all of the grafts by mid-November. The days to bud burst, DBB, decreased in the forcing conditions with successively later transfers until the next spring. Neither BB% nor DBB was dependent on the age of the scion. However, in the early phase of ecodormancy release, the microscopic internal development of the buds was more advanced in the grafts representing the 16-year-old than in those representing the 32- or 80-year-old trees. In conclusion, our findings suggest that no major change in the environmental regulation of endodormancy release in Norway spruce takes place when the trees get older. Taken together with earlier findings with Norway spruce seedlings, our results suggest that regardless of the seedling or tree age, the chilling requirement of endodormancy release is met in late autumn. The implications of our findings for Norway spruce phenology under climatic warming and the limitations of our novel method of using grafts as a proxy of trees of different ages are discussed.

Extending the Cultivation Area of Pecan (Carya illinoinensis) Toward the South in Southeastern Subtropical China May Cause Increased Cold Damage

Pecan (Carya illinoinensis) is an important nut tree species in its native areas in temperate and subtropical North America, and as an introduced crop in subtropical southeastern China as well. We used process-based modeling to assess the effects of climatic warming in southeastern China on the leaf-out phenology of pecan seedlings and the subsequent risk of "false springs," i.e., damage caused by low temperatures occurring as a result of prematurely leafing out. In order to maximize the biological realism of the model used in scenario simulations, we developed the model on the basis of experiments explicitly designed for determining the responses modeled. The model showed reasonable internal accuracy when calibrated against leaf-out observations in a whole-tree chamber (WTC) experiment with nine different natural-like fluctuating temperature treatments. The model was used to project the timing of leaf-out in the period 2022-2099 under the warming scenarios RCP4.5 and RCP8.5 in southeastern China. Two locations in the main pecan cultivation area in the northern subtropical zone and one location south of the main cultivation area were addressed. Generally, an advancing trend of leaf-out was projected for all the three locations under both warming scenarios, but in the southern location, a delay was projected under RCP8.5 in many years during the first decades of the 21st century. In the two northern locations, cold damage caused by false springs was projected to occur once in 15-26 years at most, suggesting that pecan cultivation can be continued relatively safely in these two locations. Paradoxically, more frequent cold damage was projected for the southern location than for the two northern locations. The results for the southern location also differed from those for the northern locations in that more frequent cold damage was projected under the RCP4.5 warming scenario (once in 6 years) than under the RCP8.5 scenario (once in 11 years) in the southern location. Due to the uncertainties of the model applied, our conclusions need to be re-examined in an additional experimental study and further model development based on it; but on the basis of our present results, we do not recommend starting large-scale pecan cultivation in locations south of the present main pecan cultivation area in southeastern subtropical China.

Champions of winter survival: cold acclimation and molecular regulation of cold hardiness in evergreen conifers

Evergreen conifers are champions of winter survival, based on their remarkable ability to acclimate to cold and develop cold hardiness. Counterintuitively, autumn cold acclimation is triggered not only by exposure to low temperature, but also by a combination of decreasing temperature, decreasing photoperiod and changes in light quality. These environmental cues control a network of signaling pathways that coordinate cold acclimation and cold hardiness in overwintering conifers, leading to cessation of growth, bud dormancy, freezing tolerance and changes in energy metabolism. Advances in genomic, transcriptomic and metabolomic tools for conifers have improved our understanding of how trees sense and respond to changes in temperature and light during cold acclimation and the development of cold hardiness, but there remain considerable gaps deserving further research in conifers. In the first section of this review, we focus on the physiological mechanisms used by evergreen conifers to adjust metabolism seasonally and to protect overwintering tissues against winter stresses. In the second section, we review how perception of low temperature and photoperiod regulate the induction of cold acclimation. Finally, we explore the evolutionary context of cold acclimation in conifers and evaluate challenges imposed on them by changing climate and discuss emerging areas of research in the field.

Late-spring frost risk between 1959 and 2017 decreased in North America but increased in Europe and Asia

Late-spring frosts (LSFs) affect the performance of plants and animals across the world's temperate and boreal zones, but despite their ecological and economic impact on agriculture and forestry, the geographic distribution and evolutionary impact of these frost events are poorly understood. Here, we analyze LSFs between 1959 and 2017 and the resistance strategies of Northern Hemisphere woody species to infer trees' adaptations for minimizing frost damage to their leaves and to forecast forest vulnerability under the ongoing changes in frost frequencies. Trait values on leaf-out and leaf-freezing resistance come from up to 1,500 temperate and boreal woody species cultivated in common gardens. We find that areas in which LSFs are common, such as eastern North America, harbor tree species with cautious (late-leafing) leaf-out strategies. Areas in which LSFs used to be unlikely, such as broad-leaved forests and shrublands in Europe and Asia, instead harbor opportunistic tree species (quickly reacting to warming air temperatures). LSFs in the latter regions are currently increasing, and given species' innate resistance strategies, we estimate that ∼35% of the European and ∼26% of the Asian temperate forest area, but only ∼10% of the North American, will experience increasing late-frost damage in the future. Our findings reveal region-specific changes in the spring-frost risk that can inform decision-making in land management, forestry, agriculture, and insurance policy.

Contrasting resistance and resilience to extreme drought and late spring frost in five major European tree species

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.