'Latent Reserves': A Hidden Treasure In National Forest Inventories

In a Europe shaped by centuries of forest management, the task of today's scientists in characterising, understanding and modelling natural forests is highly challenging. Although numerous forest reserves exist, most remain hardly comparable case studies. Contrarily, National Forest Inventories (NFIs) consist of systematically distributed sample plots with varying time since last intervention and provide representative data. These characteristics make NFIs a unique opportunity to investigate hidden natural forests. Here we propose using NFI plots free of human influence for >40 to >70 years ('latent reserves') to conduct large-scale studies on near-natural forests. We tested this original concept in Swiss forests. We characterised compositional and structural attributes of 'latent reserves' and compared them with those of managed forests to assess whether the former demonstrated more signs of naturalness than the latter. As an example of an application, we analysed the tree- and stand-level factors affecting natural tree mortality in 'latent reserves'. Up to 15.3% of Swiss NFI plots fulfilled the criteria of 'latent reserves', and most of these plots were distributed at mid- to high elevations where accessibility and management opportunities are limited. 'Latent reserves' showed more signs of naturalness than managed forests-a higher proportion of broadleaves, higher mortality rates, higher stand density and more deadwood. However, their size structure and basal area did not differ from those of managed forests, most likely because of a lower site productivity. Although 'latent reserves' were transitioning towards a natural state, more time without management might be required for these forests to become fully detached from the effects of past management, especially at high elevations. Mortality analyses in 'latent reserves' showed that species-specific tree mortality had a U-shaped response to tree size, was negatively related to tree growth and was higher when competition was stronger. Synthesis. Our findings demonstrate the potential of 'latent reserves' to study near-natural forests at the country level, and point to further opportunities for larger-scale collaborations. Investigating 'latent reserves' represents a first step towards a deeper understanding of such forests using existing long-term data and shows promise for further research in Europe.