Topographically driven microclimatic gradients shape patterns of forest structure, diversity, and composition at a forest-grassland transition zone

O_LIGlobally, 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. C_LIO_LILeveraging 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. C_LIO_LIMicroclimatic 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 {degrees}C from the canyon bottoms to upper canyon, corresponding to variation across approximately 3 degrees of latitude. C_LIO_LIForest 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. C_LIO_LISynthesis: 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. C_LI