Tree regeneration patterns in cork oak landscapes of Southern Portugal: The importance of land cover type, stand characteristics and site conditions

Abstract

Agricultural intensification and abandonment of traditional uses have led to the degradation of Mediterranean oak landscapes, most apparent in a severe lack of tree regeneration. In the Alentejo region of Southwestern Portugal the landscape mosaic is composed of three prevalent land cover types: cork oak forests, open cork oak woodlands (montados) and shrublands dominated by Cistus ladanifer. The objective of this study is to understand how natural tree regeneration differs among these three land cover types and how stand characteristics and site conditions affect regeneration at each land cover type. We collected data on stand characteristics (adult tree layer), site conditions (topography, soil conditions, understory vegetation and light conditions) and tree regeneration (species composition, density, size structure), and analysed differences among the three land cover types. An information-theoretic approach and model averaging were applied to a set of generalized linear mixed models (GLMMs) to identify stand characteristics and site conditions that best explain regeneration occurrence of cork oak (Quercus suber) and strawberry tree (Arbutus unedo) at each land cover type. Our results show that regeneration failure was widespread at all land cover types, being strongest in shrublands. Regeneration density was higher in forests than in montados, and marginal in shrublands. Differences in regeneration patterns among land cover types seem to be related to differential processes of seed availability, seed dispersal and the availability of safe sites for recruit establishment, which are affected by vegetation cover and (past) land use practices. Cork oak regeneration was more likely to occur at steeper slopes in both forests and montados, with lower soil fertility and higher available water storage capacity in forests, and with higher proportion of ground cover in montados. In shrublands, cork oak regeneration was more likely to occur with higher soil fertility and rock content, and with less solar radiation. Regeneration of strawberry tree in forests was more likely to occur with lower available water storage capacity, and with the presence of adult strawberry trees, while in shrublands it was more likely to occur at flatter slopes. Overall, our results indicate that differences in cork oak and strawberry tree regeneration within land cover types seem to be a result of indirect effects of stand and site factors on competition and facilitation between tree recruits and coexisting plants. This study provides recommendations that can support sustainable management and restoration efforts in Mediterranean oak landscapes.