RESPONSES OF LEGUMES TO HERBIVORES AND NUTRIENTS DURING SUCCESSION ON A NITROGEN-POOR SOIL

We measured legume abundance following 13- and 5-yr experiments testing for the effects of mammalian herbivores, nutrients, and climate on plant communities in an old field and a savanna in east central Minnesota. Total legume abundance was significantly greater in plots with herbivores excluded. Within herbivore exclosures, legumes were more abundant in plots to which P, K, S, Mg, Mn, Ca, Na, and trace minerals had been added. Legumes were significantly more abundant in the savanna than in the old field. Lathyrus venosus, a rapidly growing early-maturing species, was largely responsible for these results. Two other common legumes at our study site, Amorpha canescens and Lespedeza capitata, were not significantly affected by herbivore exclosures. However, within herbivore exclosures, addition of nutrients significantly reduced Lespedeza. Late summer total legume biomass within herbivore exclosures increased strongly following exclosure establishment in 1982, declined dramatically following a severe drought in 1988, and then increased again following the drought. This trend suggested that herbivore effects we measured in 1994 resulted from long-term accumulation of legumes following the establishment of exclosures. Our results suggest that herbivores and nutrients other than nitrogen can dramatically limit the abundance of some legume species that might otherwise dominate grassland plant communities on nitrogen-poor soils. Limitation of legumes by colonization and drought may also be important. Thus, herbivores and nutrients other than nitrogen may be critical in structuring grassland plant communities and influencing succession, even on nitrogen-poor soils.