Distinguishing colonisation modes from spatial structures in populations of the cushion plantAzorella madreporicain the high‐Andes of central Chile

Abstract

Abstract We developed multiplea priorihypotheses to link the observed spatial patterns with colonisation processes in the high alpine cushion plant,Azorella madreporica. We conducted this study in the Molina River basin (33°20′ S, 70°16′ W, 3600 m a.s.l.), in the Andes of central Chile, approximately 50 km east of Santiago. We mapped and measured size (as a surrogate for age) of individual cushions in two populations and used a standard spatial analytical tool (semivariograms) to test our alternativea priorihypotheses related to colonisation mode of the cushion species. In both populations, the size distribution ofA. madreporicareflected a negative exponential or inverse‐J pattern, typical of uneven‐aged populations, where most of the cushions belonged to relatively smaller size classes, in effect, a regular success in the establishment of seedlings, where all size classes of cushions were represented in the population. The results were site‐specific, where best‐fit semivariograms for spatial cushion's size distribution suggested a gradual colonisation in one population and an episodic colonisation in the other population. Microsite distribution proved to be homogeneous at both sites. Thus, the study of the spatial explicit size‐age population distribution of an alpine species provides valuable information about the frequency, magnitude and site variation of the reproductive pulses in these harsh environments.