Vegetation dynamics along a disturbance gradient: Spatial and temporal perspectives

In this study we investigated the spatio-temporal dynamics of three shrub communities along a disturbance gradient. We examined whether these dynamics may provide an indication for the relative importance of facilitation and competition processes among shrubs occurring along the gradient. During a 3-year period we used aerial photographs to detect changes in each of the shrub communities located in the Nizzana sand-dune ecosystem, Israel, each characterized by different disturbance intensities caused by sand movement. We measured different demographic parameters, namely recruitment and mortality rates, recruitment and mortality probabilities as a function of neighbouring vegetation cover, and spatial patterns of recruitment and mortality. The number of individuals monitored ranged from 210 in the intensively disturbed community to 480 in the least disturbed community. In the most intensively disturbed community, overall recruitment and mortality rates were highest varying from 20.7% to over 100%. In addition, regression analysis demonstrated that negative correlations between mortality probabilities and neighbouring vegetation cover exist at spatial scales of 3.5 m, and that the spatial patterns of recruitment and survival were highly clumped. These results suggest the importance of facilitation in this community. In the least disturbed community overall recruitment and mortality rates were lower, ranging from 12.8% to 17.9%. Regression analysis yielded positive correlations between mortality and neighbouring vegetation cover at spatial scales of up to 2 m, and the spatial patterns of recruitment and mortality were regular or random. These results suggest the dominance of competitive processes within less disturbed communities. We propose that along disturbance gradients associated with sand movement, shifts in the relative importance of facilitation and competition processes exist.