Biotic effects of established vegetation on tree colonization in early successional shrublands on Chiloé Island, Chile

Background/Question/Methods The increasing extent of anthropogenic secondary shrublands that lack tree regeneration is a growing problem in southern South American temperate forests. Dense shrub layers may preclude establishment of forest species; however, in stressful environments, seedling establishment may be favored by such shading. The physical and biotic factors that limit tree regeneration under these conditions are poorly understood. Moreover, biotic processes such as competition and facilitation can shift in importance across environmental gradients and can vary with the identity and ontogeny of the colonizing species. We studied interactions between tree species and established vegetation in three early successional communities dominated by different shrubs species in the rural landscape of Chiloé Island, Chile. In separate experiments we sowed seeds and planted seedlings of five pioneer tree species under two treatments (aboveground vegetation removed vs. control) replicated in each vegetation type.  We then recorded seed germination and seedling survival. We used a randomized block design, with one replicate of each treatment in each of five blocks at three sites (vegetation types). We also quantified differences in soil fertility and light availability between treatments and among vegetation types that could alter the outcome of plant-plant interactions.
Results/Conclusions Established vegetation had positive or neutral effects on tree seedlings. Although responses differed among tree species and life history stages (germination and early survival), they were generally consistent among the communities studied. Soil fertility did not differ between vegetation removal and control treatments, but did vary among community types.  Significant reductions in light (and presumably temperature) by existing plant cover may explain the greater germination and survival of seedlings in the controls. Our results highlight the importance of positive interactions for the colonization of these early successional shrublands by trees. They also suggest that restoration of these stagnant successional communities can be accelerated by planting seeds or seedlings in association with shrubs, or beneath shading structures in more open areas.