Role of Resources and Disturbance in the Organization of an Old‐Field Plant Community

The roles of disturbance and resource availability in influencing species richness and plant abundance were examined by conducting a factorial experiment for 2 yr in an old field in New Jersey, USA, dominated by goldenrods (Solidago spp.). Replicate plots were treated by (1) adding macronutrients, (2) adding water, (3) tying back tall herbs to increase light to ground layer species, and (4) creating gaps in the vegetation by disturbing the soil with hand trowel early in the growing season. Nutrient additions increased the cover of the herbaceous canopy and enhanced the dominance of Solidago spp. Water additions also increased canopy during the second growing season, a year with low seasonal rainfall. Light enrichment as a consequence of canopy manipulation resulted in the competitive release of subcanopy species, primarily Fragaria virginiana. Other subcanopy dominants were limited by different resources; Hieracium pratense responded most strongly to water and Rumex acetosella to nutrients. These differences in limiting resources may be responsible for the continued coexistence of these three subcanopy species. Additionally, we observed marked seasonal and yearly fluctuations in the severity of resource limitation which might serve to favor different species in this community at different times. Species richness was decreased by light enrichment alone, whereas light with irrigation interacted to increase species richness. Nutrient additions had little direct effect on species richness A single small disturbance at the growing season had little impact on species richness or abundance. Relative to the influence of resources, small—scale disturbance plays only a minor role in this community. Our results suggest that nonequilibrium organization of old—field communities can occur via a shifting resource base in the absence of natural disturbances.