Disturbance Regimes of Midsuccessional Old Fields

We monitored the dynamics of natural gaps in vegetation in permanent plots in three old—field communities for 3 yr. The vegetation in all three fields was dominated by long—lived clonal perennials. Total area in gaps with exposed soil was very low, with <0.2% of the area of each field in gaps ≥10 cm in diameter (our minimum size). Most gaps were small; only 5 to 21% of all gaps found were °20 cm in diameter. Most (95%) of the new gaps were created by animal activity, with moles being the most common disturbance agent. There was considerable variation in disturbance and closure rates over space and time that resulted in differences in the availability of gaps within fields, among fields, among seasons, and among years. Within each field, disturbance rates were higher but closure rates tended to be lower in more xeric areas of the field, resulting in higher availability of gaps in the more xeric areas at each census. Within each year, both disturbance and closure rates were higher during the growing season than during the winter. These two trends seemed to balance each other, resulting in little change in the availability of gaps over the year. Disturbance rates did not differ among years, but closure rates were higher in a year with higher precipitation, resulting in a lower availability of gaps in the wetter year. Overall these gaps appear to be too rare and too small to play an important role in the persistence of the dominant perennials in the old—field communities we studied, many of which require large gaps to establish from seed. However, they may be important for the persistence of annuals with seed banks in these communities. Such gaps may also influence the rate of succession in these fields by influencing the pattern of colonization by woody plants.