Does Pinus halepensis facilitate the establishment of shrubs in Mediterranean semi-arid afforestations?

Aleppo pine (Pinus halepensis) dominates forest ecosystems in dry and semi-arid areas of the Mediterranean Basin. In addition to its natural distribution, this species has been used extensively in afforestation projects during this century. The improvement in soil fertility and microclimate provided by the P. halepensis canopy is known to facilitate the establishment of perennial grasses, but little is known about the interactions of this species with woody species in natural stands and afforestations under semi-arid climate. In this study, we analyze the effects of the P. halepensis canopy on the survival and ecophysiological features of experimentally planted seedlings of several Mediterranean shrub and tree species in two semi-arid afforestations of SE Spain. Soils under P. halepensis (pine microsite) showed small differences in nutrient content relative to those outside the canopy (open microsite). Radiation reaching pine microsites was four times lower than in open microsites. Soil moisture did not show significant differences between both microsites during most of the year. One year after planting, survival of the introduced seedlings was significantly higher in the open microsites than in the pine microsites. Predawn and midday water potentials and chlorophyll fluorescence of seedlings measured before the summer did not show any microsite effect. Our results suggest that the changes in understorey microclimate associated with R halepensis are not sufficient to facilitate the establishment of shrubs under semi-arid conditions. (C) 2002 Elsevier Science B.V. All rights reserved.