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The phenological timing of leaf out in temperate forests is a critical transition point each year that alters the global climate system, which in turn, feeds back to plants, driving leaf out to occur nearly 3 d earlier per decade as temperatures rise. To improve predictions of leaf out timing, urban heat islands (UHIs) or densely developed areas that are hotter than surrounding undeveloped regions are often used to approximate warming via space-for-time substitutions (i.e., rural-to-urban temperature gradients). However, more than just environment changes along these gradients—urban regions are highly managed systems with limited-to-no within species diversity. We demonstrate here that recent observations that UHI gradients underpredict leaf out response to temperature when compared to temperature gradients through time is likely because both genetics and environment are changing across rural-to-urban gradients, whereas only environment is changing through time. We tested this hypothesis using genomic, phenological, and temperature data of northern red oak (Quercus rubra) over several years between an urban and rural site. Across our gradient, models that included just temperature predicted moderate advancement of leaf out. However, if we account for the genetic diversity of our trees in our model, leaf out phenology is predicted to advance significantly more in response to temperature. We demonstrate that this stronger relationship between phenological timing and climate is because urban trees have reduced genetic diversity as they are planted from limited stock by humans and, moreover, are most closely related to individuals at the rural site that leaf out later on average. 

Abstract Selection causes local adaptation across populations within species and simultaneously divergence between species. However, it is unclear if either the force of or the response to selection is similar across these scales. We show that natural selection drives divergence between closely related species in a pattern that is distinct from local adaptation within species. We use reciprocal transplant experiments across three species ofPhloxwildflowers to characterize widespread adaptive divergence. Using provenance trials, we also find strong local adaptation between populations within a species. Comparing divergence and selection between these two scales of diversity we discover that one suite of traits predicts fitness differences between species and that an independent suite of traits predicts fitness variation within species. Selection drives divergence between species, contributing to speciation, while simultaneously favoring extensive diversity that is maintained across populations within a species. Our work demonstrates how the selection landscape is complex and multidimensional. 

Abstract Interactions between water and carbon dynamics underlie drought-related tree mortality. While whole-tree water relations have been shown to play a key role in the response to and recovery from drought, the role of nonstructural carbohydrates (NSC) and how their storage and allocation changes surrounding drought events deserves further attention and is critical for understanding tree survival. Here, we quantified in situ NSC responses of temperate forest trees to the 2016 drought in the northeastern United States. Sugar and starch concentrations were measured in the stemwood of five tree species from 2014 to 2019, which allowed us to monitor NSCs in relation to climatic conditions before, during, and after the natural drought. We found that immediately following the drought, measured stemwood NSC concentrations decreased. However, NSC concentrations rebounded quickly within three years. Notably, trees allocated proportionally more to starch than to sugars following the 2016 drought. In winter 2017, starch comprised 45% of total stemwood stores, whereas starch made up only 1–2% in other years. Further, we modeled and assessed the climatic drivers of total NSC concentrations in the stem. Variation in total NSC concentrations was significantly predicted by the previous year’s temperature, precipitation, and standardized precipitation-evapotranspiration index (SPEI), with stemwood concentrations decreasing following hotter, drier periods and increasing following cooler, wetter periods. Overall, our work provides insight into the climatic drivers of NSC storage and highlights the important role that a tree’s carbon economy may play in its response and recovery to environmental stress. 

Summary 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. 

Abstract Woody plant species store nonstructural carbohydrates (NSCs) for many functions. While known to buffer against fluctuations in photosynthetic supply, such as at night, NSC stores are also thought to buffer against environmental extremes, such as drought or freezing temperatures by serving as either back‐up energy reserves or osmolytes. However, a clear picture of how NSCs are shaped by climate is still lacking. Here, we update and leverage a unique global database of seasonal NSC storage measurements to examine whether maximum total NSC stores and the amount of soluble sugars are associated with clinal patterns in low temperatures or aridity, indicating they may confer a benefit under freezing or drought conditions. We examine patterns using the average climate at each study site and the unique climatic conditions at the time and place in which the sample was taken. Altogether, our results support the idea that NSC stores act as critical osmolytes. Soluble Sugars increase with both colder and drier conditions in aboveground tissues, indicating they can plastically increase a plants' tolerance of cold or arid conditions. However, maximum total NSCs increased, rather than decreased, with average site temperature and had no relationship to average site aridity. This result suggests that the total amount of NSC a plant stores may be more strongly determined by its capacity to assimilate carbon than by environmental stress. Thus, NSCs are unlikely to serve as reservoir of energy. This study is the most comprehensive synthesis to date of global NSC variation in relation to climate and supports the idea that NSC stores likely serve as buffers against environmental stress. By clarifying their role in cold and drought tolerance, we improve our ability to predict plant response to environment. 

Abstract Trees'totalamount of non‐structural carbohydrate (NSC) stores and theproportionof these stores residing as insoluble starch are vital traits for individuals living in variable environments. However, our understanding of how stores vary in response to environmental stress is poorly understood as the genetic component of storage is rarely accounted for in studies. Here, we quantified variation in NSC traits in branch samples taken from over 600 clonally transplanted black cottonwood (Populus trichocarpa)trees grown in two common gardens. We found heritable variation in both total NSC stores and the proportion of stores in starch (H²_TNC = 0.19, H²_PropStarch = 0.31), indicating a substantial genetic component of variation. In addition, we found high amounts of plasticity in both traits in response to cold temperatures and significant genotype‐by‐environment (GxE) interactions in the total amount of NSC stored (54% of P is GxE). This finding of high GxE indicates extensive variation across trees in their response to environment, which may explain why previous studies of carbohydrate stores' responses to stress have failed to converge on a consistent pattern. Overall, we found high amounts of environmental and genetic variation in NSC storage concentrations, which may bolster species against future climate change. 

Summary Trade‐offs among carbon sinks constrain how trees physiologically, ecologically, and evolutionarily respond to their environments. These trade‐offs typically fall along a productive growth to conservative, bet‐hedging continuum. How nonstructural carbohydrates (NSCs) stored in living tree cells (known as carbon stores) fit in this trade‐off framework is not well understood.We examined relationships between growth and storage using both within species genetic variation from a common garden, and across species phenotypic variation from a global database.We demonstrate that storage is actively accumulated, as part of a conservative, bet‐hedging life history strategy. Storage accumulates at the expense of growth both within and across species. Within the speciesPopulus trichocarpa, genetic trade‐offs show that for each additional unit of wood area growth (in cm² yr⁻¹) that genotypes invest in, they lose 1.2 to 1.7 units (mg g⁻¹NSC) of storage. Across species, for each additional unit of area growth (in cm² yr⁻¹), trees, on average, reduce their storage by 9.5% in stems and 10.4% in roots.Our findings impact our understanding of basic plant biology, fit storage into a widely used growth‐survival trade‐off spectrum describing life history strategy, and challenges the assumptions of passive storage made in ecosystem models today.