Recovery of tree species functional composition in eucalypt plantations with natural regeneration differs among canopy strata

Tree monocultures have been promoted globally to supply timber; yet, a high diversity of native trees can establish in less intensively managed plantations, allowing to both harvest timber and transition towards a more natural forest. However, little is known about the functional recovery of native trees under plantations, which is critical for biodiversity conservation and ecosystem services. Here, we evaluate how functional composition of tree strata differs between two restoration methods (eucalypt plantations with natural regeneration and naturally regenerating forests) and how this is affected by stand age, climatic water deficit, and soil characteristics. We established 129 plots in two restoration methods and mature forest as reference, in São Paulo state, Brazil. We divided tree stratum in each plot into different canopy strata using perfect plasticity approximation. We measured five key traits that are important for fire resistance (bark thickness), drought tolerance (wood density), productivity (specific leaf area and leaf thickness), and nutrient cycling (nitrogen-fixing ability) for 393 species and calculated for each stratum community-weighted mean trait values. Community traits were mostly affected by canopy strata, the interaction between canopy strata and restoration method, and water availability. Eucalypt trees dominated the upper strata of plantations, presenting higher wood density, tougher leaves, and thicker bark, reflecting the drought and fire adaptation of eucalypts. The lower strata of eucalypt plantations and naturally regenerating forests had similar functional composition. Our results suggest that eucalypt plantations can be used as a tool to facilitate natural regeneration and restore ecosystem functioning in degraded areas.