A gap dynamics simulation model of succession in a semiarid grassland

Abstract

We applied a gap dynamics conceptualization of succession to a semiarid grassland by focusing on gaps produced in the belowground resource space by death of an individual of the dominant species, the perennial grass blue grama ( Bouteloua gracilis (H.B.K.) Lag. ex Griffiths). We developed a gap dynamics simulation model to examine long-term successional dynamics on gaps, time required for B. gracilis to recolonize gaps, and spatial variability of gap dynamics at the scale of the landscape. The model is similar to gap models used in forest, but is based on belowground resource use and life history characteristics of plants in shortgrass communities, and incorporates effects of small-scale disturbances and stochastic environmental factors. Average time required for B. gracilis to dominate biomass on a simulated plot was 20 years, a more rapid recovery time than has been reported from experimental studies of large-scale disturbances. Our initial simulations assumed the only control on establishment of B. gracilis seedlings was the occurrence of a restrictive set of microenvironmental conditions. A second set of simulations incorporating effects of seed availability resulted in an average recovery time of 65 years. The scale dependence of successional processes important to recruitment of B. gracilis plants is indicated since this is still a faster recovery time than was observed experimentally. Aboveground biomass on the simulated plots was dominated by B. gracilis through time and space. Relative proportion of aboveground biomass and average biomass values for each species were comparable to the composition of shortgrass plant communities at the Central Plains Experimental Range study site in northcentral Colorado. This initial analysis suggests that gap processes are important to shortgrass communities, and that a gap dynamics conceptualization based on the importance of small, frequently occurring disturbances is a promising alternative to the previous conceptual model of shortgrass communities based on effects of large-scale disturbances.