CONVERGENCE AND DIVERGENCE OF OLD-FIELD VEGETATION AFTER 11 YR OF NITROGEN ADDITION

Eleven years of data from experimental nitrogen gradients in three old fields and in native savanna are used to test the hypothesis that sim ilarity of resource availability should influence similarity of plant species composition, Vegetation became less similar on plots that rece ived different amounts of nitrogen, The difference in vegetation betwe en plots was a function of both the difference in added nitrogen and t he absolute levels of added nitrogen, Changes in similarity on plots t hat received the same amount of nitrogen were influenced by initial sp ecies composition, the amount of added nitrogen, and the rate of colon ization by species not initially present on plots, Replicates of many treatments initially became less similar, only to increase in similari ty after 3-6 yr. Data are generally consistent with predictions made i n 1988, and clearly illustrate the importance of long-term responses t o experimental manipulations.