Succession in Old‐Field Plant Communities: Effects of Contrasting Types of Nutrient Enrichment

Abstract

We investigated the effects of monthly nutrient applications on succession in two old—field plant communities. Succession was monitored for 3 yr in 1—yr (younger) and 4—yr (older) experimental plots. Three 0.1—ha plots in each old field were treated with sludge, three with fertilizer, and two were left as untreated controls. In the younger community, herbaceous perennials and winter annuals replaced summer annuals by the 3rd yr of succession in control plots. Summer annuals, however, dominated enriched plots throughout the study. Species richness was significantly higher in enriched plots than in control plots during the 1st yr, but was significantly lower than control plots thereafter. Annual aboveground net primary productivity (ANPP) was significantly greater in enriched plots than in controls in the 1st yr, but the converse occurred in the 3rd yr. The type of nutrient enrichment affected ANPP; sludge plots had significantly lower ANPP than fertilizer plots in the 1st and 3rd yr. In the older community, two summer annuals and a biennial displaced dominant perennial grasses in enriched plots. In contrast to the younger community, ANPP was consistently greater in nutrient—enriched plots than in controls, and nutrient enrichment did not alter species richness. The type of nutrient enrichment affected the older community in an opposite manner from the younger community; sludge plots had significantly greater ANPP than fertilizer plots in the 3rd yr. We concluded that the age and physiognomy of the old—field community, the type of nutrients applied, and the duration of enrichment, each influenced the course of succession; responses observed in the 1st yr of enrichment were not indicative of later trends.