Spatio-temporal variation in the demography of a bunch grass in a patchy semiarid environment.

摘要

Plants face different environmental pressures in different patches of vegetation mosaics, so their demography cannot be completely understood if it is not studied in each patch-type. Banded patterns of vegetation surrounded by bare areas occur in semiarid landscapes. At one level, two phases of the mosaic are the banded vegetation-patches (vegetation arcs) and the bare areas, but at another level two phases can be distinguished inside the vegetation arc. One phase (frontal zone) is always in the upslope boundary of the arc, has only herbs and it has been suggested that it functions as a colonization area, while the other one (central zone) is at the middle of the arcs and has both shrubs and herbs. The demography of a tussock grass (Hilaria mutica) growing in the two phases of the vegetation arcs was studied under the hypothesis that it will show the demographic parameters of a ruderal species in the frontal zone and those of a more competitive species in the central zone. Temporal variability was assessed through annual, average, periodic and stochastic matrices. lambda -values are higher in the frontal than in the central zone, and lower in dry years than in years with moderate precipitation. The influence of the demographic processes on lambda -values shows spatial and temporal variation. In dry years, lambda -values are more sensible to stasis (permanence in the same size class) and retrogression (transition to a smaller size class) in both zones, whereas in years of moderate precipitation the influence of fecundity and growth increases in the frontal zone and the influence of stasis and retrogression continue to be the most important in the central zone. Variations in the demographic parameters observed in the frontal zone are evidences of a life history plasticity finely tuned with environmental variation, and these results support the hypothesis that frontal zones function as colonization areas.