In typical plant size hierarchies, a small number of large individuals are present while most other individuals remain small. Such asymmetric size distributions are expected to become increasingly uneven over time. Observations of populations of Pinus rigida (pitch pine), the dominant species in northeastern Pine Barrens communities, have instead suggested that this model of asymmetric competition structuring size distributions of trees may not be uniformly supported in these populations. As part of a long-term study of P. rigida population dynamics, we followed the development of size hierarchies in natural populations of seedlings that germinated after fires in 1995 at three different sites in the Long Island Pine Barrens. Almost 7,000 seedlings were initially tagged; mortality led to differential losses at sites over the next eleven years. We examined the shapes of distributions of seedling height and stem diameter and quantified inequality among individuals using Gini coefficients.
Results/Conclusions
The shapes of the distributions of plant sizes differed greatly among the populations of P. rigida at the different sites, and these size hierarchies changed in different ways among the sites over time. The dwarf pine plains, where mature trees were the smallest in stature (1-3 m), were structured in a typical size hierarchy with a few larger trees and many smaller individuals. The dwarf sites retained this form throughout all censuses, even as tree heights and the range of sizes increased. The intermediate-stature site (with mature trees of 6-10 m) maintained a similar size hierarchy over time, although less well-defined than that of the dwarf sites. However, at the site with the largest stature mature trees (10-15 m tall), the size distributions became increasingly even through time, as the frequencies of the largest and smallest trees increased and the frequency of the intermediate-sized trees decreased. The development of this type of size structure is contrary to most previous findings and to the predictions of theory, suggesting that factors such as asymmetric competition are not likely to be good explanations for the type of size hierarchy that develops over time in at least one of these populations. This evidence also suggests that resources and other environmental factors and/or competition are not uniform across pitch pine populations in the Long Island Pine Barrens, and one or more of these differences cause stands of identical age and of the same species to develop into size hierarchies with very different structure.
