In Mediterranean drylands microclimate affects more tree seedlings than adult trees

Abstract Topography creates contrasting microclimates, especially between northern and southern slopes in drylands, which result in clear differences of tree cover patterns in the landscape. These local-scale differences in tree cover patterns may result from limitations occurring at different plant development stages, such as seed germination, seedling establishment, tree growth rate and survival, all crucial for the maintenance and expansion of plant populations through natural regeneration. Holm oak ( Quercus rotundifolia Lam.) is a key tree species in Mediterranean drylands. We aim at measuring the impact of microclimatic conditions on holm oak trees regeneration success using traits as indicators of adult tree performance in an average climatic year and tree’s distribution pattern at the landscape level as an indicator of tree establishment success. We hypothesize that under contrasting microclimatic conditions, seedlings performance is limiting the regeneration potential of holm oak trees rather than adult trees physiological performance. To quantify the microclimatic conditions originated by topography, two indicators were used: i) Potential Solar Radiation, as the amount of potential solar energy reaching the site above tree canopy; and ii) functional lichen diversity, as ecological indicators of microclimatic conditions under the tree canopy. The performance of holm oak was assessed on 40 adult trees growing in contrasting microclimatic conditions (high and low Potential Solar Radiation sites). Several indicators based on leaf, tree and stand response traits, frequently used as ecological indicators to evaluate tree performance and susceptibility to drought stress, were selected: specific leaf area, leaf reflectance indices, tree size, tree growth rate, tree age and tree density. We found that leaf physiological performance was similar under contrasting microclimatic conditions. However, in areas with higher Potential Solar Radiation tree age and density were significantly reduced. These results suggest that microclimatic differences located in southern slopes with high Potential Solar Radiation are limiting for germination and sapling establishment. Thus, forest regeneration, restoration and nature conservation practices aiming at increasing forests resilience in Mediterranean dryland climates should account for the importance of microclimate in defining the niche of seedlings and of adult trees.