Selective herbivory on a nitrogen fixing legume (Lathyrus venosus) influences productivity and ecosystem nitrogen pools in an oak savanna

Herbivory is known to change the structure of vegetation, but the possible effects of herbivory on ecosystem nitrogen pools are not well documented. Here we report that 13 years of deer exclusion significantly influenced ecosystem nitrogen pools and caused ecosystem productivity to more than double in a regularly burned Minnesota oak savanna. Herbivore exclusion greatly increased the abundance of Lathyrus venosus, a native nitrogen fixing legume. Primary productivity also increased through time, as did total soil nitrogen. This increase in productivity did not occur in unfenced plots, where there was a loss of total soil nitrogen, probably because fire-induced nitrogen losses exceeded gains. This study documents that herbivores, through 'top-down' effects on foodwebs, can strongly influence nitrogen pools in terrestrial ecosystems, and that legumes can play a critical role in replacing fire-induced nitrogen losses in Midwestern oak savannas.