SEED BANKS AND SEEDLING ESTABLISHMENT ON AN EXPERIMENTAL PRODUCTIVITYGRADIENT

A major cause of the lower species richness of nitrogen enriched grass lands with high productivity is the lower colonization rate of such ar eas by new species. To determine what role the soil seed bank might pl ay, we performed greenhouse germination tests of soil sampled in exper imental plots that had received various rates of annual nitrogen addit ion for II yr. Species composition of the soil seed bank depended on t he rate of N-addition, even though the species richness and the overal l density of viable seeds in soil were independent of N-addition. The proportion of the species shared between the seed bank and the above-g round vegetation decreased with N-addition. The density of forb seedli ngs emerging in field plots was negatively correlated with the nitroge n input, productivity, and litter depth, but the probability of seedli ng survival from May through August was not different among treatments . Thus, suppression of seed germination, rather than limitation of the availability of seeds in soil or seedling survivorship, was the prima ry mechanism of lower colonization rates in the high-N plots. Burning in spring decreased litter depth and promoted germination of forbs, es pecially in no-N control plots. However, germination was still very lo w in the high-N plots even after fire decreased litter depth to a leve l similar to that of controls.