Natural Advance Regeneration of Native Tree Species in Pinus radiata Plantations of South-Central Chile Suggests Potential for a Passive Restoration Approach

Restoration of natural forests previously replaced by plantations is a widespread challenge for forestry in Chile and elsewhere. However, there is little documented evidence for successful restoration, either through active or passive approaches. In this study, we aimed at (1) determining the potential for passive restoration in first-rotation Pinus radiata plantations through natural regeneration of native tree species and (2) identifying drivers of this advance regeneration. Across different regions in south-central Chile, we established nearly 260 plots to assess regeneration and environmental conditions along 26 transects running from plantations into adjacent natural forests. The regeneration was exclusively composed by native species, except for 7 individuals of P. radiata. Mean density and diversity of seedlings were significantly higher in natural forests than in plantations, but this was not the case for sapling density, and no differences in sapling diversity were supported. Additionally, significant differences in regeneration composition between plantations and natural forests were found only at two of the eight study sites. Compared to climatic and soil chemical variables, which varied mostly at regional scales, local environmental conditions showed little influence on regeneration, possibly due to the structural homogeneity of plantations. Yet, the significantly higher basal area, litter thickness and gap fraction of plantations compared to natural forests suggest that these factors may explain differences at the seedling stage. Our study indicates that the use of appropriate harvesting methods that maintain advance regeneration may facilitate the transition from plantations to native forests through passive restoration. The use this approach should be further investigated through analyzing regeneration's response to different forms of plantation harvesting.