Regeneration of Tree Species and Restoration Under Constrasted Mediterranean Habitats: Field and Glasshouse Experiments

Abstract

Understanding the processes of tree population recruitment and their limitations, is the scientific basis to assure the forest natural regeneration, and to improve techniques of restoration and afforestation. We present here preliminary results of a collaborative project (HETEROMED). The environmental heterogeneity of Mediterranean forests was related to natural patterns of seedling establishment. Factorial design experiments of seed addition were carried out and the resulting emergence and survival were related to the conditions of light and soil moisture. The fieldwork was carried out at two natural areas in South Spain: Sierra Nevada National Park (higher mountains on the Southeast) and Los Alcornocales Natural Park (lower mountains in Sierra del Aljibe range, on the southern tip of the Iberian Peninsula). The experiments were focussed in seven target species - Quercus suber, Quercus canariensis, Quercus ilex, Quercus pyrenaica, Pinus sylvestris, Acer granatense and Sorbus aria. In addition, a glasshouse factorial experiment investigated the differential response of the four Quercus species to the combined effects of shade and drought. In both field sites (Sierra Nevada and Sierra del Aljibe) seedling emergence differed among species, but was not significantly affected by habitat (light availability). In Sierra Nevada, seedling survival after dry summer was statistically different between tree species, habitat type and irrigation treatment. Summer drought was the main cause of seedling mortality. Survival was higher in deep shade habitats than in open stands. Irrigation boosted survival in open areas (full light) and under shrublands (medium shade), but had scant effect inside woodland stands (deep shade). In Sierra del Aljibe, there were significant differences in seedling survival of Quercus suber between habitat types. Most surviving seedlings after the dry summer were in habitats with medium shade (under tree canopy), rather than in full light (open areas) or deep shade (closed forest) habitats. In the glasshouse study, the seedling mass (after six months of growth) was statistically different between species, and between light and water treatments. In the deep shade treatment seedling mass was not affected by stopping irrigation (simulating seasonal drought), unlike in the full light. A comparison of field and glasshouse results is carried out. A conflict between components of regeneration (seedling emergence, survival, and growth) is demonstrated as common in Mediterranean forests. The implications for ecological restoration are discussed and future perspectives of HETEROMED project are advanced.