The neighborhood effects of ancient trees on stand growth: evidence, mechanisms and retention implications from individual trees to ancient-tree groups

Large old and ancient trees are persistent spatial drivers of forest neighborhood conditions, but their effects on neighboring plants cannot be reduced to either competition or facilitation. This review synthesizes evidence for how ancient trees influence nearby plant growth, regeneration and stand development across spatial and temporal scales. We first examine spatial patterns and growth responses that reveal where ancient-tree effects occur, then distinguish aboveground constraints, including canopy-mediated light limitation and litter filtering, from belowground processes involving roots, mycorrhizal connectivity, hydraulic redistribution and rhizosphere communities. We show that the net effect of an ancient tree is context dependent, varying with distance, ontogenetic stage, environmental stress, neighbor traits and neighborhood composition. We further distinguish nonlinear distance-dependent responses from genuinely non-additive effects among multiple neighbors. Following tree death, canopy gaps, coarse woody debris and soil biological legacies can extend neighborhood effects into subsequent regeneration. Finally, we examine how overlapping crowns, roots and microbial networks may generate emergent effects when individual ancient trees occur in groups. This framework identifies ancient-tree neighborhoods as context-dependent ecological units and provides a mechanistic basis for adaptive retention rather than universal removal or retention prescriptions.