Are we missing the forests for all the trees? Spatial trends in diversity of overstory and understory temperate vascular plants
One of the central goals of ecology is understanding the mechanisms that enable coexistence among species. Negative density dependence (NDD), the process by which plant seedlings are unable to survive in the area surrounding adults of their same species, has received substantial empirical support and is thought to be a major contributor to plant species coexistence. NDD may stabilize the relative abundance of different species, thus preventing them from displacing each other and thereby maintaining species diversity. Recent high-profile studies demonstrate that NDD is particularly ubiquitous among forest canopy tree species in temperate and tropical forests. In temperate forests, however, canopy trees represent only 20% of the vascular plant species. For understory plants, which constitute the vast majority of temperate forest diversity, there is no evidence that NDD is a strong factor. We tested the extent to which woody plants demonstrate overdispersion (a spatial pattern indicative of NDD) using a fully spatial census of woody plants in the temperate forest at Powdermill Nature Reserve in southwestern Pennsylvania. We then used replicated point pattern analysis to calculate pooled estimates of Besag’s L function and bootstrapped this data 999 times to estimate 95% confidence intervals.
Our findings demonstrate unequivocally that understory and overstory plants differ significantly in the degree to which they demonstrate overdispersion. Overstory plants are indeed significantly more overdispersed, in accordance with NDD. Additionally, the degree to which these different plant groups demonstrate NDD changes in opposing ways with plant size.