Experiments on Disturbance in Old‐Field Plant Communities: Impact on Species Richness and Abundance

Abstract

Changes in species richness and abundance following experimental disturbance were studied in a 7th—yr old field (the “older” field) dominated by Solidago canadensis, and 2nd—yr old field (the “younger” field) dominated by Ambrosia artemisiifolia. The two fields were disturbed in late spring 1981 by clipping and removing aboveground plant biomass in experimental plots, which were compared with undisturbed control plots. Treatments represented three intensities of disturbance (low, intermediate, and high), as defined by amount of biomass removed and extent of canopy disruption. Different patterns of change in richness were observed in each field during the 1981 growing season. In the older field the number of species decreased in control plots during the summer. In disturbed plots, richness increased with intensity of disturbance, resulting in maximum richness at the highest disturbance intensity during mid— and late summer. Several species increased significantly in cover and/or frequency in plots representing intermediate and high intensity of disturbance. In the young field, species richness fluctuated in control plots, with peaks in early and late summer. In disturbed plots, richness decreased with intensity of disturbance in early summer and increased in midsummer. By late summer no difference in richness or species composition was detected between control and clipped plots. These patterns of change in richness can be explained by the different effect of treatments on the vegetation structure and by the role of the dominant species in each field. In the bi—layered older field, disturbances greatly disrupted the closed canopy of S. canadensis, thus providing resource patches (i.e., increased light, moisture) for suppressed and invading species. In the younger field, the impact of disturbance was only detectable in early and midsummer, and changes in richness were mainly related to the seasonal cycle of the dominant annual A. artemisiifolia. Results from these experiments provide insights into the mechanisms of species coexistence under natural disturbance regimes comparable to our disturbance treatments. They also indicate that the impact of disturbance on species richness depends largely on the nature of the dominants and rates of successional or seasonal change of the community.