The Renaissance of Mixed Forests? New Insights Into Shifts in Tree Dominance and Composition Following Centuries of Human-induced Simplification of Iberian Forests

Anthropic activities have modelled and simplified southern European forest landscapes for centuries. Over recent decades, new drivers related to human-mediated global change have induced the redistribution of tree species and an increase in more complex forests. However, the current large-scale patterns and drivers of these changes are yet to be fully described for the Mediterranean Basin. In this frame, this work identifies and examines changes in dominance and composition from pure to mixed forests across bioclimatic gradients and forest types in Iberian forests over recent decades based on data from the Spanish National Forest Inventory from 1960 to 2020. Then, considering different environmental, anthropic, and disturbance variables we also identify some of the most important drivers associated with the shifts observed from 1986 to 2020. Our results confirm an ongoing increase in mixed forests involving the replacement of conifers by broadleaved species. These shifts are greater in the Atlantic biogeoregion and in pure broadleaved deciduous forests. Climate warming-associated disturbances such as drought severity together with land use legacies and forest types showed the strongest relationships with the observed changes in the studied forests. Our results support the premise put forward by palaeoecologists which states that the increase in tree mixtures is a natural process reversing the historical human-induced simplification of Iberian forests. The increasing importance of mixed forest in southern Europe makes decisive the revision of forest classifications as well as forest management and conservation plans in order to include these increasingly abundant novel stands in forest policies.