Pine invasion impacts on plant diversity in Patagonia: invader size and invaded habitat matter

Conifers, which are widely planted as fast growing tree crops, are invading forested and treeless environments across the globe, causing important changes in biodiversity. However, how small-scale impacts on plant diversity differ according to pine size and habitat context remains unclear. We assessed the effects of different stages of pine invasion on plant communities in forest and steppe sites located in southern Chile. In each site, we sampled plant diversity under and outside the canopy of Pinus contorta individuals, using paired plots. We assessed the relative impact of pine invasion on plant species richness and cover. In both sites, richness and cover beneath pine canopy decreased with increasing pine size (i.e. height and canopy area). A significant negative impact of pines on species richness and plant cover was detected for pines over 4 m in height. The impact of pines on plant richness and cover depended on pine size (i.e. canopy area) and habitat type. Larger pines had more negative impacts than smaller pines in both sites, with a greater impact for a given pine size in the Patagonian steppe compared to the A. araucaria forest. Species composition changed between under and outside canopy plots when pines were 4 m or taller. Pine presence reduced cover of most species. The impacts of pine invasions are becoming evident in forested and treeless ecosystems of southern Chile. Our results suggest that the magnitude of pine invasion impacts could be related to how adapted the invaded community is to tree cover, with the treeless environment more impacted by the invasion.