Conspecific negative density dependence in American beech

Abstract

Background: One of the most important drivers of forest biodiversity is conspecific negative density dependence (CNDD), a reduction in performance when conspecific densities are high. While the majority of CNDD research has focused on tropical forests, evidence is mounting that CNDD may also play an important role in temperate forests. To further explore the potential reach of this phenomenon, we investigated CNDD in American beech (Fagus grandifolia) in a mature mid-Atlantic forest. Methods: We used bivariate point pattern analyses to examine spatial relationships between large beech trees and conspecific saplings, and we also contrasted these patterns with comparable patterns for heterospecifics. In addition, to address the possibility of dispersal limitation and the associated effects on spatial patterns, we analyzed seedling density as a function of adult conspecific abundance. Results: We found that beech saplings were more repelled from large conspecifics than large heterospecifics, despite the fact that beech seedling density was positively correlated with beech basal area. However, saplings of other canopy tree species were also repelled from adult beech trees, suggesting a general suppressive effect. Nonetheless, the discrepancy between beech seedling and sapling densities beneath adult conspecifics suggests that beech seedling survival rates were reduced in vicinity of conspecific adults. Conclusions: Regardless of the extent to which beech inhibits heterospecific trees, a negative effect on conspecific recruits may be critical for biodiversity maintenance. Without this conspecific suppression, a dense layer of shade-tolerant beech saplings could form beneath adult beech trees. If this were to occur, beech would have a substantial head-start following canopy disturbance, and this late-successional species could potentially dominate a stand in perpetuity, through repeated disturbance cycles.