Fecundity and terrain‐climate interactions that shape tree niche differences across North America and Europe

Abstract 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. 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. 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 (~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. 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.