Stages and Spatial Scales of Recruitment Limitation in Southern Appalachian Forests

Recruitment limitation of tree population dynamics is poorly understood, because fecundity and dispersal are difficult to characterize in closed stands. We present an approach that estimates seed production and dispersal under closed canopies and four limitations on recruitment: tree density and location, fecundity, seed dispersal, and establishment. Consistent estimates are obtained for 14 canopy species using 5 yr of census data from 100 seed traps and several thousand mapped trees and seedlings from five southern Appalachian forest stands that span gradients in elevation and moisture. Fecundity (seed production per square centimeter of basal area) ranged over four orders of magnitude, from 100 cm2 basal area/yr (Carya, Cornus, Nyssa, Quercus) to >103 cm2/yr (Betula). Mean dispersal distance ranged from 20 m (Acer, Betula, Liriodendron, Tsuga) and was positively correlated with fecundity. Species also differ in the degree of seed clumping at fine (1 m2) spatial scales. Dispersal patterns can be classed in two groups based on dispersal vector: wind-dispersed taxa with high fecundities, long-distance dispersal, and low clumping vs. animal-dispersal taxa with low fecundities, short-distance dispersal, and a high degree of clumping. “Colonization” limitations caused by sizes and locations of parent trees, fecundity, and dispersal were quantified as the fraction of sites receiving seed relative to that expected under null models that assume dispersal is nonlocal (i.e., long-distance) and not clumped (i.e., Poisson). Difference among species in colonization levels ranged from those capable of saturating the forest floor with seed in most stands (Acer, Betula, Liriodendron) to ones that leave much of the forest floor without seed, despite presence of adults (Carya, Cornus, Nyssa, Oxydendrum). Seedling establishment is one of the strongest filters on recruitment in our study area. Taken together, our results indicate (1) that fecundity and dispersal can be resolved, even under a closed canopy, and (2) that recruitment of many species is limited by the density and location of source, dispersal patterns, or both.