Macroclimate drives vegetation distributions, but fine‐scale topographic variation can generate microclimate refugia for plant persistence in unsuitable areas. However, we lack quantitative descriptions of topography‐driven microclimatic variation and how it shapes forest structure, diversity, and composition. We hypothesized that topographic variation and the presence of the forest overstory cause spatiotemporal microclimate variation affecting tree performance, causing forest structure, diversity, and composition to vary with topography and microclimate, and topography and the overstory to buffer microclimate. In a 20.2‐ha inventory plot in the North American Great Plains, we censused woody stems ≥1 cm in diameter and collected detailed topographic and microclimatic data. Across 59‐m of elevation, microclimate covaried with topography to create a sharp desiccation gradient, and topography and the overstory buffered understory microclimate. The magnitude of microclimatic variation mirrored that of regional‐scale variation: with increasing elevation, there was a decrease in soil moisture corresponding to the difference across ~2.1° of longitude along the east‐to‐west aridity gradient and an increase in air temperature corresponding to the difference across ~2.7° of latitude along the north‐to‐south gradient. More complex forest structure and higher diversity occurred in moister, less‐exposed habitats, and species occupied distinct topographic niches. Our study demonstrates how topographic and microclimatic gradients structure forests in putative climate‐change refugia, by revealing ecological processes enabling populations to be maintained during periods of unfavorable macroclimate.

Climate change is causing rapid shifts in the abiotic and biotic environmental conditions experienced by plant populations, but we lack generalizable frameworks for predicting the consequences for species. These changes may cause individuals to become poorly matched to their environments, potentially inducing shifts in the distributions of populations and altering species’ habitat and geographic ranges. We present a trade-off-based framework for understanding and predicting whether plant species may undergo range shifts, based on ecological strategies defined by functional trait variation. We define a species’ capacity for undergoing range shifts as the product of its colonization ability and the ability to express a phenotype well-suited to the environment across life stages (phenotype–environment matching), which are both strongly influenced by a species’ ecological strategy and unavoidable trade-offs in function. While numerous strategies may be successful in an environment, severe phenotype–environment mismatches result in habitat filtering: propagules reach a site but cannot establish there. Operating within individuals and populations, these processes will affect species’ habitat ranges at small scales, and aggregated across populations, will determine whether species track climatic changes and undergo geographic range shifts. This trade-off-based framework can provide a conceptual basis for species distribution models that are generalizable across plant species, aiding in the prediction of shifts in plant species’ ranges in response to climate change.

The southern pine beetle, Dendroctonus frontalis Zimmermann is an important mortality agent of Pinus in the eastern United States of America where it commonly shares hosts with the black turpentine beetle, Dendroctonus terebrans (Olivier), which infrequently kills trees. Unlike D. frontalis, which must kill its hosts to become established in the bark and reproduce, D. terebrans can occupy living hosts as a parasite. Olfactory mechanisms whereby D. frontalis initially locates hosts have not been demonstrated, whereas D. terebrans responds strongly to host odors. Because D. terebrans produces frontalin, the primary aggregation pheromone component for D. frontalis, and commonly arrives on hosts prior to D. frontalis, it has been hypothesized that D. terebrans pheromone components can mediate D. frontalis location of suitable, living trees. We assessed this possibility with studies of the semiochemical interactions between D. frontalis and D. terebrans. Coupled gas chromatography-electroantennographic detection analyses indicated that D. terebrans produces nine different olfactory stimulants for D. frontalis, nearly all of them known semiochemicals for D. frontalis. A trapping experiment designed to address the potentially confounding influence of lure contamination confirmed that the D. terebrans pheromone component exo-brevicomin enhances attraction of D. frontalis and thus could be an attractive kairomone. In ambulatory bioassays, male D. frontalis were strongly attracted to odors of frass of solitary female and paired D. terebrans, indicating their attraction to the naturally occurring semiochemicals of D. terebrans. Cues from D. terebrans may influence host and mate-finding success of D. frontalis and, thereby, the latter’s virulence.

Globally, forests provide important ecosystem services, but anthropogenic change may shift the boundaries of forested biomes, because small-scale environmental changes govern biome transitions. This is especially true in semi-arid forests, where minor topographic and microclimatic changes influence forest functioning and transitions to open biomes such as grasslands. However, we lack quantitative descriptions of topographically driven microclimate variation and how it shapes forest structure, diversity, and composition in these transition zones. Leveraging a 20.2-ha forest inventory plot (Niobrara plot) at a semi-arid forest-grassland transition zone in the Niobrara River valley (Nebraska, USA), we paired data on abundances and distributions of seedlings, saplings, and adults of woody species with topographic and microclimate data to test the hypothesis that if topographic variation causes variation in microclimate that affects forest function, then forest structure, diversity, and composition should vary significantly with topography and microclimate. Microclimatic variation within the Niobrara plot strongly corresponded with topography, creating a sharp water availability and exposure gradient from the river floodplain to the forest-grassland transition zone. The magnitude of microclimate variation corresponded to that of regional macroclimate variation. Mean soil moisture was 10.2% lower along the higher-elevation transition zone than in the canyon bottoms, corresponding to variation across approximately 2.5 degrees of longitude. Mean air temperature increased by 2.2 °C from the canyon bottoms to upper canyon, corresponding to variation across approximately 3 degrees of latitude. Forest structure, diversity, and composition correlated strongly with topographic and microclimatic gradients. More complex forest structure and higher species richness of adults and saplings occurred in moister, less exposed habitats with steeper slopes and lower elevations, whereas seedling stem density and richness were higher in higher-light, moister habitats at lower elevations. Species occupied well-defined topographic niches, promoting high beta diversity along topographic and microclimatic gradients and high species turnover from the floodplain to the transition zone. Synthesis : Microclimatic and topographic variation drive patterns of structure, diversity, and composition in the forests at this forest-grassland transition zone. As the macroclimate becomes warmer and drier, topographically mediated microclimatic refuges supporting diverse, structurally complex forested ecosystems may shrink in semi-arid regions.

Plants can express considerable plasticity in leaf functional traits, but that plasticity may be constrained by coordination and trade‐offs among multiple functions. The leaf economics spectrum (LES) describes a well‐supported trade‐off between durability versus productivity but does not capture all dimensions of leaf functional responses to environmental variation. We hypothesized that traits related to photoprotection, desiccation tolerance, and water‐use efficiency (WUE) would occupy axes distinct from, and potentially orthogonal to, the LES, and that species would differ in how they resolve multifunctional trade‐offs along environmental gradients, influencing the strength of within‐ versus between‐species trait relationships. We tested these hypotheses by analysing 11 leaf traits on 219 individuals of 16 woody species in a 20.2‐ha temperate woodland plot spanning a topography‐driven desiccation gradient in the North American Great Plains. Using multivariate ordinations, environmental vector fitting, and complementary bivariate correlation and standardized major axis analyses, we quantified axes of trait coordination and compared relationships within and between species. More productive leaves were less desiccation tolerant, durable, and water‐use efficient, invested less in carotenoid‐related photoprotection, and occurred in more mesic topographic positions. Ordinations revealed novel functional relationships and trade‐offs: WUE was coordinated with carotenoid‐related photoprotection, with more water‐use efficient leaves investing more in photoprotection and less in chlorophylls relative to carotenoids, while desiccation tolerance aligned with durability and traded off with productivity traits. Within‐species analyses confirmed strong, consistent relationships among LES traits and desiccation tolerance, whereas relationships involving WUE and photoprotection—reflecting more dynamic physiological processes—varied among species, producing weak between‐species correlations. Together, these patterns indicate that WUE and photoprotection are coordinated along an axis approximately orthogonal to the LES, capturing covariation among dynamic physiological rather than structural functions. Supporting the idea that how individuals resolve functional trade‐offs along environmental gradients shapes between‐species trait relationships, our study not only expands the leaf economics framework but also reconceptualizes it in relation to critical trade‐offs involving carotenoid‐related photoprotection, desiccation tolerance, and WUE. By identifying new dimensions of trait relationships, our study advances functional ecology by highlighting leaf phenotypic variation along gradients as an integrated expression of multiple interacting functions. Read the free Plain Language Summary for this article on the Journal blog.

Several species of aggressive bark beetle in the genus Dendroctonus (Coleoptera: Curculionidae) undergo large fluctuations in population density with distinct outbreak and non‐outbreak phases. We investigated attributes we hypothesized as subject to density‐dependent variation (in particular, those likely to express phenotypic plasticity) in the southern pine beetle (SPB), Dendroctonus frontalis, as possible indicators of population fluctuations. These traits were morphology (body size and hindwing shape) and the sex ratio of trap‐captured, dispersing SPB populations. We compared attributes of beetles from locations that ranged from having zero to high numbers (>1500) of SPB infestations (spots) at the county level for two summers. Southern pine beetle were obtained from six states in the southeastern USA and had been collected during a springtime, region‐wide trapping survey used for forecasting outbreaks annually. Although we detected an expected but weak sexual dimorphism in both size and shape‐related traits, no morphological differences were found between SPB collected from traps in counties with low or high densities of spots (≤10 or >10 per county, respectively). We found no relationship between numbers of SPB spots per county and trapped sex ratios in 2016, but we observed a strong trend in 2017, with about three times higher proportions of females trapped in counties with low compared with high numbers of spots. This implies that one or more known or possible factors influencing trapped sex ratios (e.g., disparities between the sexes in their responses to semiochemicals or in their propensity to disperse) can be density‐dependent. Including trap‐captured sex ratios in prediction models may improve current forecasting of SPB outbreaks in the southeastern USA, informing more timely and effective management of one of the most economically and ecologically important beetle species of this region.