Effects of Seasonal Drought on Old-field Plant Communities

Abstract

-We investigated the effects of the 1988 drought on old-field plant communities treated with contrasting types of nutrient subsidies. The study site was divided into eight 0.1-ha plots: three plots received monthly applications of sludge (Milorganite?; 6-20, N-P-K); three plots received an equivalent subsidy of fertilizer, and two plots were left as untreated controls. Control plots were dominated by perennials (especially Solidago canadensis, Poa compressa and P. pratense), whereas the nutrient-enriched plots were dominated by summer annuals (especially Setaria faberii, Ambrosia artemisiifolia and A. trifida) and resembled old fields in the initial stages of secondary succession. Control plots had greater species richness and diversity but lower productivity than nutrient-enriched plots for several years preceding 1988. Daily aboveground net primary productivity (daily NPP) for July 1988 samples, when drought stress was most severe, was 45% of the 1985-1987 mean for control plots but only 17% and 11% for sludge and fertilizer plots, respectively. This pattern was reversed in August 1988 following alleviation of the drought due to above-normal precipitation. Daily NPP in August 1988 in sludge and fertilizer plots were 224% and 140%, respectively, of the 1985-1987 average, compared to 100% for control plots. Thus, primary productivity in perennial-dominated plant communities (i.e., the control plots) was not as depressed during the peak of the drought as in the annual-dominated, nutrientenriched plots. However, the nutrient-enriched communities exhibited more rapid responses to the alleviation of drought stress than the perennial-dominated communities. Species diversity (H') was lower during the drought stress in control plots when compared to the 1985-1987 average whereas H' in sludge and fertilizer plots was not affected. However, after the drought, H' in sludge plots was over twice the 1985-1987 mean. In contrast, species richness was not significantly affected by drought stress in any of the treatments.