Tree regeneration in the threatened forest of Robinson Crusoe Island, Chile: The role of small-scale disturbances on microsite conditions and invasive species

Biological invasions are a major driver of biodiversity loss on islands. After disturbances, invasive plant species can establish affecting forest regeneration microsites. On Robinson Crusoe Island (33°S, Juan Fernández Archipelago, Chile) small-scale treefall canopy gap microsites are most frequently used by “endemic montane forest” species for regeneration. Regeneration can be hampered when invasive species establish and alter gap conditions. We evaluated the role of small-scale disturbances on regeneration and identified tree regeneration microsites in gaps, gap borders, and closed forest. We collected information on the effects caused by invasive species by sampling 30 gaps with a range of invasive species cover, including gaps where invasive species were removed. We analyzed the impact of native ferns and invasive species, regeneration substrates and light availability on native tree species regeneration and juvenile tree performance traits. Our aim was to analyze small-scale disturbances and identify threshold values for the variables related with tree regeneration presence-absence, density and performance, particularly considering invasive species competition. We used classification and regression trees to identify variables and their threshold values influencing native tree species regeneration. Gap borders and small gaps (<200 m2) were preferred microsites for regeneration. Native tree species seemed able to compete as long as invasive species cover did not exceed 10%. Fern cover >10% facilitated tree regeneration and performance. Competition from invasive species for space, water and nutrients was likely more important than for light. Restoration should attempt to recreate intermediate disturbance conditions considering the threshold values identified. Thresholds for variables important for restoration processes can help in the control of invasive species.