From microsite selection to population spatial distribution: Pinus halepensis colonization in mediterranean-type ecosystems

This paper examines the effect of landscape heterogeneity on pine establishment and survival at both juvenile and mature stages. A field survey of seedlings (juvenile stage) was carried out in two sites (Shaharia and Nahal HaHamisha) close to planted forests of Pinushalepensis in the dry mediterranean region of Israel. Seedling establishment rate at the microsite level was determined by comparing to the proportions of actual land-cover categories—bare ground, rock, shrub, and tree, using a spectral classification of aerial photographs. The dynamics of tree colonization at one of the sites (Shaharia) during the last 30 years were examined using aerial photographs, GIS tools and generalized linear models, to assess the effects of distance from planted forest, aspect, slope, and land cover on the density of pine trees, was conducted. Shrub cover was significantly favorable for both juvenile and mature pines at both sites, while the opposite was true for bare ground. Trees had a negative effect at one site. Rock had a positive effect on juveniles in both sites and a negative effect on mature pine distribution, reflecting its facilitative effect in the early stage and constraints for the late stage. A combined land-cover category of shrub+rock in one site, undetectable from aerial photographs, had the strongest effect of both covers on pine establishment. Proximity to the planted forest significantly increased pine density. Facilitation mechanisms provide the necessary mitigation of stresses imposed by irradiance, moisture shortage, and competition from neighboring vegetation on seedling establishment and survival. These mechanisms, combined with the spatial aspect of seed dispersal from planted forests, are the key to understanding the multiple–scale dynamics of pine expansion.