In 2023, more than half of olive harvests ( Olea europaea ) across Spain, Greece, and Türkiye were lost to drought. The same year late freeze destroyed 90% of the peach crop ( Prunus persica ) on the Georgia Piedmont and the apple crop ( Malus domestica ) in central New York, Vermont, and southern Quebec. Climate extremes now rank with the costliest threats to agriculture, but their role in forest recovery from diebacks that are happening globally is unknown for lack of tree fecundity estimates in forests. Tolerance of climate extremes could depend on past exposure but constrained by phylogenetic conservatism. We report a continental scale analysis of climate extremes and forest fecundity across North America and Europe showing that responses to late freeze and drought are happening now. Species differences are not explained by the traits typically included in ecological studies and they are weakly associated with phylogeny. Late freeze, that is, freezing temperatures that follow the onset of flower development in spring, is shown to be “normal” in North America, but not Europe, potentially explaining failed seed production due to delayed onset and the resultant shorter growing period by North American transplants dating back at least to the 18th century. Drought has thus far had the greatest impacts in dry forested regions, but here too, species differences are not explained by traditional trait values. If responses have been buffered from drought and late freeze by past exposure, acclimation and local adaptation prove inadequate as extremes intensify.

To survive climate change, forest trees will have to shift seed production poleward. However, warming will not stimulate tree fecundity in the north if it is limited by other habitat variables. We evaluated the responses of tree fecundity to climate change for 292 tree species in North America and Europe, using response velocity, defined as (climate sensitivity) × (climate-change rate). The sensitivities to climate were estimated for each species and combined with rates of climate change to quantify how temperature, moisture deficits, and late freeze are influencing biogeographic shifts in tree reproduction. The results show that moisture deficit and late freeze, not annual temperature, drive changing seed production. Unlike annual temperature, which is increasing generally, change in these climate variables is not driving poleward shifts in seed production. These findings do not challenge the expectation that forests might eventually shift poleward. Rather, they show why current efforts offer divergent interpretations. The changes happening now are not consistent with annual temperature trends. As warming continues, fecundity changes can best be anticipated from temperature interactions with precipitation and extremes that impact flowering and fruiting in winter and spring.

Hotter droughts have resulted in widespread tree die-off events globally, frequently leading to regeneration failure. Dry forest recovery often depends on the growth and survival of extant juvenile trees. However, it is unclear how microenvironmental changes following overstorey tree die-off affect juvenile trees, particularly in dryland systems where tree recruitment is typically limited by water availability and heat stress. We simulated an overstorey tree die-off event by girdling trees in pi & ntilde;on-juniper woodlands across the south-western United States. We sampled juvenile pi & ntilde;on pine growth from live and dead overstorey treatments across six study sites spanning a regional latitudinal gradient and local elevational gradients. We examined how juvenile branch and needle growth differed between live and dead overstorey treatments, and whether responses varied with weather conditions and juvenile tree size following overstorey mortality. We found greater juvenile branch and needle growth under dead compared with live overstorey trees for 2 years following overstorey mortality at mid- and high-elevation sites which are typically cooler and wetter than the other sites. These observed growth releases were contingent on favourable post-mortality weather conditions. Higher growth under dead overstorey occurred at sites experiencing near-average climatic water deficits compared with sites experiencing above-average climatic water deficits. Growth at all sites increased from the first to second year after overstorey mortality. Across sites, growth was unrelated to juvenile tree size. Synthesis. Our results underscore differentiation in juvenile responses to overstorey tree die-off driven by local site conditions and weather across the range of Pinus edulis. Overstorey mortality resulted in consistently higher juvenile growth only at climatically favourable sites and during favourable weather, while unmeasured microsite differences could help account for variation observed at the hottest and driest site. Results from less climatically favourable sites suggest that overstorey trees neither directly limit nor facilitate juvenile growth, though further study over longer timeframes is needed to resolve the pace and magnitude of potential recovery or decline. Overall, juvenile vigour may be promoted following overstorey mortality only in a narrow spatial (site) and temporal (weather) environmental context, suggesting additional vulnerabilities for pi & ntilde;on populations under more arid conditions.

1. If niche differences contribute to biodiversity, then landscapes must vary and species must respond differently to that variation. Terrain, through its effects on solar radiation and moisture, is an important contributor to habitat variation, but its effects on demography are largely unknown. Understanding fecundity responses to this landscape heterogeneity could provide insights on niche differences and on how terrain buffers climate change effects. Here, we use a hierarchical Bayesian state-space model to quantify how topographic features buffer tree fecundity from intensifying climate change. We estimate the effects of slope, aspect and drainage on tree fecundity across North America and Europe while accounting for individual condition and climate. 2. We analysed 2,874,955 tree-years from 292 species across forest inventory plots spanning mountainous regions in North America and Europe. We fitted species-specific fecundity responses to terrain variables and climate interactions, then constructed predictive distributions of terrain effects at landscape scales. We quantified community-level hypervolume to assess how diversity in fecundity responses across species relates to topographic heterogeneity. 3. Topography influences tree fecundity, with terrain as an important contributor for most species. In dry portions of their ranges, tree species in southeastern North America and south-central Europe increase fecundity towards northeast aspects, while highest fecundity of species in southwestern North America shifts towards south-facing aspects. Terrain-sensitive species (terrain explains >20% of fecundity variance) are most abundant in southwestern North America (similar to 42% of species), but southeastern North America shows the strongest terrain effects on fecundity and highest per-species fecundity variance, despite having the gentlest slopes. Community hypervolume is exceptionally high in southeastern United States. Whether this variation maintains diversity depends on how fecundity combines with other demographic rates. 4. Synthesis: Fine-scale terrain variation creates reproductive differences through species-specific fecundity responses. The effectiveness of topographic heterogeneity as a climate refuge depends on regional landscape structure and species sensitivity to terrain. In topographically diverse regions, species may buffer climate change through fine-scale redistribution to favourable microsites. Quantifying these differences helps identify factors limiting reproduction and habitat features with greatest potential as local refuges.

As climate warms, trees are expected to track their ideal climate, referred to as ‘range shifts’; however, lags in tree range shifts are currently common. Disturbance events that kill trees may help catalyse tree migrations by removing biotic competition, but can also limit regeneration by eliminating seed sources, and it is unknown whether disturbance will facilitate or inhibit tree migrations in the face of climate change. Here we use national forest inventory data to show that seedlings of 15 dominant tree species in the interior western United States occupy historically cooler areas than mature trees, as expected with climate warming. However, the climatic differences between adults and seedlings are the result of widespread regeneration failures in the hottest portions of species’ ranges. Disturbances did not uniformly catalyse climatic range shifts; differences were species- and disturbance-specific. Assisted migration programmes may thus be needed to help trees adapt their ranges to climate change. Climate change and disturbances are changing forest tree composition, but it is not clear if disturbances assist trees in tracking their climate ranges. This study shows that the impact of disturbance on range shifts is dependent on the tree species and type of disturbance.

Predicting seed production is challenging because many plants produce highly variable crops among years (i.e. masting), but doing so can inform forest management, conservation, and our understanding of ecosystem trajectories in a changing climate. We evaluated the ability of an existing model to forecast masting in an ecologically and culturally important tree species in the southwestern United States, Pinus edulis. Annual seed cone production was predicted using cross-validation techniques on two unique out-of-sample datasets, representing different collection methods and spatial scales (cone scars and cone counts). We then hindcasted this model into the historical past to evaluate whether seed production has declined with the onset of extreme drought conditions in western North America. The evaluated model had fair skill, with root-mean-squared error of 6%. The model had better skill predicting the interannual variability within a site than among sites (i.e. within years). Hindcast analyses indicated recent (2000-2024) mean annual cone production was 30.6% lower than in the past century (1900-1999). Mast forecasts are within reach, but much room remains for improvement. Forecasts may be a powerful tool to anticipate the effects of climate change on forests and woodlands.

Global changes in temperature and aridity are increasing the frequency of extreme drought events. Such changes can have pronounced impacts on dryland ecosystems, which exist at the margins of plant physiological tolerances. Pinyon-juniper (PJ) woodlands-a dryland vegetation type spanning 40 million ha in western North America-are a model system for the impacts of drought, where recurrent short-interval drought events may trigger feedback mechanisms that influence future drought resistance. Leveraging a long-term monitoring network in PJ woodlands of the United States (US) Southwest, we sought to understand how interactions between recurrent drought events influence tree mortality risk. We developed generalized linear mixed models to predict patterns of recent (i.e., 2014-2023) tree mortality based on biophysical variables, tree size, and prior drought-driven changes (ca. 1998-2014) in forest conditions. We then used these models to quantify how mortality risk has shifted over time. Tree density and stand basal area declined substantially throughout our 1998-2023 monitoring period. Since 2014, tree mortality was more common and spatially extensive than new tree recruitment, and nearly half of the surviving trees experienced crown dieback. Tree size influenced biotic interactions and responses to environmental conditions, and soil organic matter and mycorrhizal fungi communities buffered individuals against drought. Shifts in woodland demographics (e.g., reduced stand densities, crown dieback) led to a 28.2% increase in mortality risk between 2014 and 2023 for trees that survived this period, a pattern that was consistent across species. Recent drought events have triggered widespread tree mortality and dieback in PJ woodlands of the US Southwest. These events also increase future tree mortality risk, overcoming system inertia created by local edaphic conditions and compensatory responses.

Structurally heterogeneous forest conditions provide diverse microclimates which can support juvenile tree regeneration, survival, and growth. However, the extent to which juvenile trees depend on moderated microclimates depends on their physical maturity and species-specific tolerances for environmental stresses related to variables like soil moisture, heat, and shade. Linking juvenile responses to microclimates for different life-stages and species facilitates more nuanced understanding of regeneration niches and is especially important in dry conifer forests where structurally heterogeneous conditions are a frequent objective of forest restoration treatments. In this study, we sampled microclimate conditions and planted seeds and greenhouse-grown seedlings across a range of overstory structure and microclimate conditions in a heterogeneous forest restoration treatment in Colorado, USA. We assessed how fine-scale (similar to 1-20 m) spatial variation in overstory structure and topography related to variability in microclimate conditions and, collectively, how these biophysical conditions influenced survival and growth of ponderosa pine (Pinus ponderosa var. scopulorum) and Douglas-fir (Pseudotsuga menziesii var. glauca) over 3 years. We found evidence of strong canopy buffering of mean daily vapor pressure deficit (VPD), but buffering of daily maximum VPD was moderate, and buffering of soil moisture was weak except in the hottest and driest month of our 3-year study. Although canopy cover provided refuge from hot and dry conditions, survival and growth of juvenile trees across life-stages and species were greatest in above-average canopy openness, with warm and dry microclimate conditions in May. The spatial patterns of seedling establishment relative to canopy cover, driven by early-season microclimate conditions, did not match canopy-mediated, moderate microclimates later in growing seasons when weather conditions were most limiting. Importantly, our results reflect a specific regeneration trajectory initiated by favorable early-growing season conditions in the first year of study, which promoted survival and growth resilience through subsequent limiting conditions, most notably a hot and dry second year of study. The patterns observed in this study are consistent with drivers of landscape-scale regeneration previously identified for dry conifer forests and show that overstory structure and microclimate variation can mediate these patterns at very fine spatial scales. The narrow spatial and temporal ranges of conditions which supported survival and growth in this study highlight opportunities for restoration treatment planning in similar forests, but also underscore the susceptibility of these dominant dry forest species to potential future changes in the timing and intensity of abiotic conditions which limit regeneration success.

The fundamental trade-off between current and future reproduction has long been considered to result in a tendency for species that can grow large to begin reproduction at a larger size. Due to the prolonged time required to reach maturity, estimates of tree maturation size remain very rare and we lack a global view on the generality and the shape of this trade-off. Using seed production from five continents, we estimate tree maturation sizes for 486 tree species spanning tropical to boreal climates. Results show that a species' maturation size increases with maximum size, but in a non-proportional way: the largest species begin reproduction at smaller sizes than would be expected if maturation were simply proportional to maximum size. Furthermore, the decrease in relative maturation size is steepest in cold climates. These findings on maturation size drivers are key to accurately represent forests' responses to disturbance and climate change.

Plants display a range of temporal patterns of inter-annual reproduction, from relatively constant seed production to "mast seeding," the synchronized and highly variable interannual seed production of plants within a population. Previous efforts have compiled global records of seed production in long-lived plants to gain insight into seed production, forest and animal population dynamics, and the effects of global change on masting. Existing datasets focus on seed production dynamics at the population scale but are limited in their ability to examine community-level mast seeding dynamics across different plant species at the continental scale. We harmonized decades of plant reproduction data for 141 woody plant species across nine Long-Term Ecological Research (LTER) or long-term ecological monitoring sites from a wide range of habitats across the United States. Plant reproduction data are reported annually between 1957 and 2021 and based on either seed traps or seed and/or cone counts on individual trees. A wide range of woody plant species including trees, shrubs, and lianas are represented within sites allowing for direct community-level comparisons among species. We share code for filtering of data that enables the comparison of plot and individual tree data across sites. For each species, we compiled relevant life history attributes (e.g., seed mass, dispersal syndrome, seed longevity, sexual system) that may serve as important predictors of mast seeding in future analyses. To aid in phylogenetically informed analyses, we also share a phylogeny and phylogenetic distance matrix for all species in the dataset. These data can be used to investigate continent-scale ecological properties of seed production, including individual and population variability, synchrony within and across species, and how these properties of seed production vary in relation to plant species traits and environmental conditions. In addition, these data can be used to assess how annual variability in seed production is associated with climate conditions and how that varies across populations, species, and regions. The dataset is released under a CC0 1.0 Universal public domain license.

Mast seeding, the synchronous and highly variable production of seed crops by perennial plants, is a population-level phenomenon and has cascading effects in ecosystems. Mast seeding studies are typically conducted at the population/species level. Much less is known about synchrony in mast seeding between species because the necessary long-term data are rarely available. To investigate synchrony between species within communities, we used long-term data from seven forest communities in the U.S. Long-Term Ecological Research (LTER) network, ranging from tropical rainforest to boreal forest. We focus on cross-species synchrony and (i) quantify synchrony in reproduction overall and within LTER sites, (ii) test for relationships between synchrony with trait and phylogenetic similarity and (iii) investigate how climate conditions at sites are related to levels of synchrony. Overall, reproductive synchrony between woody plant species was greater than expected by chance, but spanned a wide range of values between species. Based on 11 functional and reproductive traits for 103 species (plus phylogenetic relatedness), cross-species synchrony in reproduction was driven primarily by trait similarity with phylogeny being largely unimportant, and synchrony was higher in sites with greater climatic water deficit. Community-level synchrony in masting has consequences for understanding forest regeneration dynamics and consumer-resource interactions.

In contrast to many other arid region rivers, streamflow in the South Platte River is heavily augmented by trans-basin water imports and irrigation return flows. Hydrological changes began in the 1880s, resulting in channel narrowing and the development of a continuous Populus-Salix forest by the mid-twentieth century. We assessed the composition, structure and regeneration status of the riparian forest and identified environmental variables affecting annual Populus deltoides tree growth. We sampled forest structure at four sites in 2015, and conducted dendroecological analysis at seven additional sites in 2019. The riparian forest was dominated by P. deltoides, which occurred at all sites, comprising 79% of total tree basal area and 62% of total tree density. Age structure data indicated ongoing though episodic recruitment of P. deltoides, at least over the past similar to 130 years. We tested 14 linear mixed effects models to describe the effect of climate and streamflow on individual tree growth (modeled as the log of BAI, n = 237 trees). The most parsimonious model selected with AICc explained 28.6% of BAI variability, and included hydrology and climate factors during the growing season (i.e., June-August streamflow, June-July PDSI), some aspects of off-season (i.e., previous November and March) streamflow, along with tree age and study site effects. The riparian forest developed in response to, and has been maintained by, current climate conditions and water management regimes. It may be negatively affected by future climate change and increased urban water demand in the basin.

Tree-removal treatments have been broadly applied across pinon-juniper ecosystems of the western United States to reduce tree cover, stimulate understory plant production, and promote habitat for shrub- and grassland-obligate wildlife species. Mastication treatments have become an increasingly common approach, yet the efficacy of these treatments can vary on the basis of a variety of factors, including soil characteristics, woodland structure, and grazing pressures. Here, we assessed vegetation responses to mastication treatments across three dominant soil types in two-needle pinon (Pinus edulis Engelm. [Pinaceae])- one-seed juniper (Juniperus monosperma [Engelm.] Sarg.) woodlands in southeast Colorado, United States, a region characterized by monsoonal precipitation, limited presence of introduced plant species, and relatively high grazing intensity by cattle and wildlife. We found that mastication treatments were effective at increasing herbaceous plant cover and species diversity (by 1.2 x and 1.5 x) and at reducing the amount of exposed soil (60% reduction) 3 yr following treatment. This was mainly due to increases in native perennial grasses. Further, there were limited (and insignificant) increases in cover of annual plants and low abundance of introduced species in treated plots. Understory plant responses to treatment were similar across soils with a range of available water capacities. The increase in understory plant cover and richness paired with the low abundance of introduced species suggests that mastication treatments increase forage production for cattle and wild ungulates. In addition, the lack of soil type differences in treatment response suggests that mastication treatment placement does not need to prioritize soil type and can instead focus on other key areas of importance, such as wildlife habitat connectivity, historic woodland structure, and treatment feasibility. (c) 2023 The Society for Range Management. Published by Elsevier Inc. All rights reserved.

Masting describes the spatiotemporal variability in seed production by a population of plants. Both abiotic and biotic factors drive masting, but the importance of these factors can vary among individuals and populations. To better understand how a changing climate, altered disturbance regimes, or novel management strategies might affect future seed production, we quantified the joint influence of multiple factors on annual cone production in a widespread conifer species, Rocky Mountain ponderosa pine (Pinus ponderosa var. scopulorum). We reconstructed individual-level annual cone production across climatic gradients using the cone abscission scar method, and explored the causes and drivers of masting in this species. Site-level responses between weather and masting were highly variable, notably differing in the strength of their response to either summer or spring weather. In addition, the relationship to summer precipitation during cone initiation, a primary driver of annual seed production in this species, was strongest at hotter and drier sites. Additionally, we found that masting was strongly influenced by tree- and stand-level factors such as diameter, age, and local neighborhood density, all of which were associated with the mean, interannual variability, and between-tree synchrony of cone production at the individual-level. Larger and older trees produced more cones, more frequently, and with less synchrony than smaller and younger trees. Open grown trees experiencing lower levels of neighborhood competition also produced more cones with less interannual variability, but with higher between-tree synchrony. Because tree- and stand-level traits appear to regulate seed production more strongly than climate or weather in this species, management interventions targeting these factors could be powerful tools to manage future tree recruitment. Thus, current efforts to reduce stand density and conserve large trees in some ponderosa pine forests may enhance tree-level seed production and reduce variability in seed crops among years.

Trees must allocate resources to core functions like growth, defense, and reproduction. These allocation patterns have profound effects on forest health, yet little is known about how core functions trade off over time, and even less is known about how a changing climate will impact tradeoffs. We conducted a 21-year survey of growth, defense, and reproduction in 80 ponderosa pine individuals spanning eight populations across environmental gradients along the Colorado Front Range, USA. We used linear mixed models to describe tradeoffs among these functions and to characterize variability among and within individuals over time. Growth and defense were lower in years of high cone production, and local drought conditions amplified year-to-year tradeoffs between reproduction and growth, where trees located at sites with hotter and drier climates showed stronger tradeoffs between reproduction and growth. Our results support the environmental stress hypothesis of masting, which predicts that greater interannual variation in tree functions will be associated with more marginal environments, such as those that are prone to drought. With warming temperatures and increased exposure to drought stress, trees will be faced with stronger interannual tradeoffs, which could lead to further decreases in growth and defensive efforts, ultimately increasing risks of mortality.

The benefits of masting (volatile, quasi-synchronous seed production at lagged intervals) include satiation of seed predators, but these benefits come with a cost to mutualist pollen and seed dispersers. If the evolution of masting represents a balance between these benefits and costs, we expect mast avoidance in species that are heavily reliant on mutualist dispersers. These effects play out in the context of variable climate and site fertility among species that vary widely in nutrient demand. Meta-analyses of published data have focused on variation at the population scale, thus omitting periodicity within trees and synchronicity between trees. From raw data on 12 million tree-years worldwide, we quantified three components of masting that have not previously been analysed together: (i) volatility, defined as the frequency-weighted year-to-year variation; (ii) periodicity, representing the lag between high-seed years; and (iii) synchronicity, indicating the tree-to-tree correlation. Results show that mast avoidance (low volatility and low synchronicity) by species dependent on mutualist dispersers explains more variation than any other effect. Nutrient-demanding species have low volatility, and species that are most common on nutrient-rich and warm/wet sites exhibit short periods. The prevalence of masting in cold/dry sites coincides with climatic conditions where dependence on vertebrate dispersers is less common than in the wet tropics. Mutualist dispersers neutralize the benefits of masting for predator satiation, further balancing the effects of climate, site fertility and nutrient demands. A new method to quantify three masting components from individual tree-years has revealed that globally, masting is uncommon in tree species that depend on mutualist dispersers, with its distribution further mediated by climate and nutrient availability.