Characterizing reference sites to inform restoration of the last remnants of the endangered Nothofagus alessandrii forest ecosystem

The ecological restoration of threatened ecosystems is a global priority, particularly for those with only a few remaining intact fragments. A critical first step in restoring such ecosystems is the development of restoration targets based on the compositional and structural conditions that would have existed in the absence of degradation, which requires a reference model. However, many restoration projects lack adequate reference information, especially regarding spatial structure and species interactions. In this study, we assessed tree species composition, size diversity, and spatial structure in three of the last remaining stands of Nothofagus alessandrii forests in central Chile, with the aim of informing reference models for ecological restoration. Despite relatively high species richness ( n = 14–19), the sites exhibited moderate species diversity (Shannon index: 0.62–0.68). All sites showed a negative exponential diameter distribution and moderate size inequality (Gini coefficient ≈ 0.55). Trees were spatially clustered in conspecific groups with radii ranging from 2 to 6 m, and species interaction varied across spatial scales (1–15 m), exhibiting both attraction and repulsion. Smaller trees displayed higher levels of interspecific mingling than larger individuals. Our findings provide critical data for designing restoration prescriptions, evaluating project outcomes, and guiding the spatial pattern of N. alessandrii plantations. More broadly, this study demonstrates the value of incorporating spatial structure and biotic interactions into reference models. Spatially explicit reference conditions, such as those summarized here, offer an ecologically realistic foundation for restoring forest ecosystems.