Site: Coweeta LTER

Simple models predict that biodiversity of forests should be much lower than that actually observed. Because only a few limiting resources are evident for trees, simple models would indicate that a forest would ultimately stabilize with only the few species that are the strongest competitors for the few available niches. However, observed diversity is much higher. High-dimensional regulation (for example, N limiting factors regulate N species) has long been recognized as a possible explanation, but the difficulty of identifying all the limiting factors and their interactions has proved prohibitory. To search for evidence of high dimensional biodiversity regulation, Coweeta researchers developed a hierarchical model allowing them to synthesize data from long-term experimental data sets with processes that control growth, maturation, fecundity, and survival. Their goal was to quantify interactions between demographic rates, within individuals, between individuals, and between species, as they respond to the environmental covariates of tree size and light availability

Data was taken from 26,000 trees across 268,000 tree years in forests spanning moisture and elevation gradients in the southern Appalachians, the Piedmont, and the transition between them. It was critical to examine so many trees because their hypothesis depends on individual level variation both between and within species, with the idea that individuals respond more like (and therefore compete more with) others of the same species, opening the door to greater coexistence between species. A long term approach was critical because such hypothesized high dimensional controls on diversity would vary within populations depending on the heterogeneity of conditions that different individuals experience in space and over time.

Their results are consistent with the hypothesis that species partition environmental variation, and that these tendencies to compete more with individuals of the same species can accumulate over time, providing a mechanism to promote and maintain diversity. Few long term data sets include observations of fecundity, growth, and survival from the same individuals, and none that the researchers are aware of analyze their interactions. The ability of the LTER to support such research over all three demographic schedules of individual trees was invaluable in building a successful model.

Graph for
The fraction of correlations between individuals of two species for which the correlation between individuals of the same species is greater than those for individuals of different species, as seen in forests characteristic of the region surrounding the Coweeta LTER
Clark et al 2010