Abstract
The composition of Mediterranean forests is expected to vary with ongoing changes in climate and land use. To gain a clearer understanding of the response to global change of growth and survival during regeneration it is necessary to take a closer look at the ecophysiological traits underlying seedling performance. Gas exchange, leaf water potential, chlorophyll fluorescence, soil moisture, temperature and global site factor were measured over 1 year in naturally regenerated young trees of three coexisting species (Pinus pinea, Quercus ilex and Juniperus oxycedrus) in two stands of different density. We modelled the photosynthetic response of plants to micro-climatic conditions via the parameterization of the non-rectangular hyperbolic model of photosynthesis, which relates gross photosynthesis to incident light through three biochemical parameters, and the subsequent expansion of these parameters as a function of environmental variables (light environment, soil moisture and temperature). We investigated the relationship between different photosynthetic performance and the species-specific strategies to cope with stress (stress tolerant or avoiders). The optimal light environment, defined through the global site factor (GSF), and the regeneration niche to maximize carbon assimilation differed between the three species. P. pinea showed high sensitivity to water availability in agreement with a drought avoidance strategy, attaining the maximum photosynthetic capacity of the three species following the spring rainfall. Q. ilex was the most thermophilic and light-demanding of the species. Under high light conditions, J. oxycedrus was more drought tolerant and displayed higher net CO2 assimilation than P. pinea over the course of a growing period. Optimal locations for P. pinea regeneration are below-crown environments, while for J. oxycedrus regeneration the optimal locations are open gaps. Q. ilex regeneration occupy open gaps where the other two species are unable to establish themselves because of excessive light, temperature or very low water availability. Competition between species will occur under a canopy gap fraction of 0.5. Higher GSF values will exclusively favour the regeneration of Q. ilex.
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