NUTRIENT-USE EFFICIENCY - A LITTERFALL INDEX, A MODEL, AND A TEST ALONG A NUTRIENT-AVAILABILITY GRADIENT IN NORTH-CAROLINA PEATLANDS

The efficiency of using nutrients to produce new biomass may be an important adaptation of plants to infertile habitats. We distinguish resource-use efficiency (production per unit resource uptake) from resource-response efficiency (production per unit resource availability) and employ Monod functions to investigate changes in these two efficiencies along resource gradients. The model predicts increasing nutrient efficiency with decreasing nutrient availability or uptake to some optimum resource level, but further nutrient deficiency causes a decrease in nutrient efficiency. We tested this model along a natural fertility gradient in three North Carolina peatland communities, using the ratio of litterfall production to litterfall nutrient content as an index of nutrient-use efficiency and the ratio of litterfall production to various measures of soil nutrient pool size as an index of nutrient-response efficiency. The model did an excellent job of describing both nutrient efficiencies, and, as predicted, efficiency decreased at suboptimal nutrient levels in infertile pocosin peatlands. Additionally, we tested the generality of our model in northern and tropical forests. Our model described the results in both data sets well, and nutrient-use efficiency declined at suboptimal concentrations for the primary limiting nutrient. Thus, the model appears to work on numerous different ecosystems and to be relatively scale invariant, working from local resource gradients to regional and global scales.