Conspecific Adult Dominance Interacts With Microsite Factors to Influence Forest Regeneration Patterns Across Elevation

ABSTRACT Aim As climate warms, contemporary tree regeneration patterns will affect future forest composition. Yet the factors influencing forest regeneration across environmental gradients remain difficult to disentangle. We studied how conspecific canopy tree dominance (as a proxy for seed availability) interacts with microsite factors along elevational climatic gradients to affect regeneration patterns and recruitment limitations across temperate deciduous and montane boreal forests. Location Adirondack and Appalachian Mountains (NY, VT, NH, and ME), USA. Taxa Abies balsamea , Acer saccharum , Betula alleghaniensis , B. papyrifera var. cordifolia , Fagus grandifolia , and Picea rubens . Methods We measured overstory tree dominance (via species‐specific basal area), understory seedling abundance, and microsite factors (e.g., understory vegetation cover, canopy openness) across elevation on ten mountains. Generalised linear regression models were fit to quantify the importance of conspecific tree basal area (representing seed availability), microsite factors, and their interaction on recruitment for each species along elevational climatic gradients. Results Seedling abundance increased with greater conspecific adult basal area (i.e., overstory‐understory relationships were positive) for all species except the shade‐intolerant and wind‐dispersed B. papyrifera var. cordifolia . Sparse understory vegetation amplified positive overstory‐understory relationships for A. saccharum and B. alleghaniensis , while closed canopies dampened them for the shade‐intolerant B. alleghaniensis and B. papyrifera var. cordifolia . The positive overstory‐understory relationship for F. grandifolia was weaker in warmer microclimates, while that for the shade‐tolerant A. balsamea was greater under closed canopies, likely owing to seedling banks. Main Conclusions Nearly ubiquitous positive overstory‐understory relationships indicated that local conspecific adult tree dominance drives recruitment (likely through seed dispersal) as expected. However, microsite conditions interacted with overstory basal area and modified these overstory‐understory relationships differently for each species to either inhibit or facilitate tree recruitment. This framework integrates broad‐scale density‐dependent processes (i.e., overstory‐understory conspecific relationships) with fine‐scale drivers to better understand tree species recruitment across biome boundaries.