Poster Session # 21: Biogeography, Biodiversity, Populations, and Genetics.

Thursday, August 7 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Resource ratio tradeoffs exist, but fail to explain effects of nitrogen on plant species richness.

Stevens, Martin Henry*,1, Carson, Walter*,2, 1 Ecology Graduate Program, Oxford, OH, USA2 Program in Ecology and Evolutionary Biology, Pittsburgh, PA, USA

ABSTRACT- Tradeoffs in plant traits associated with resource ratio specialization and response nonlinearity should explain variation in plant species richness along productivity gradients. Unfortunately, few experiments have documented the presence of such tradeoffs in general, nor have any experiments tested whether such tradeoffs can explain changes in relative abundance associated with the loss of diversity. We manipulated light and soil nitrogen in field experiments that tested for the presence of these tradeoffs, and tested whether these tradeoffs explain the loss of richness with increasing soil nitrogen. Common species appeared to occupy significantly different light and nitrogen niches that were consistent with tradeoffs assumed by resource ratio specialization and response nonlinearity. Tradeoffs existed between species success in low light versus species sensitivity to increasing light (r = -0.90), and between species sensitivity to increasing light versus sensitivity to increasing soil nitrogen (r = -0.66). These results are consistent with both resource ratio and response nonlinearity as general mechanisms of coexistence in plant communities that are especially relevant to resource gradients. These tradeoffs, however, failed to explain the change in species abundances along the resource gradient. Thus, while these tradeoffs appear to exist, they do not explain the loss of species richness along this soil resource gradient.

Key words: productivity, resource ratio, species diversity, tradeoff