Araucaria Forest conservation: mechanisms providing resistance to invasion by exotic timber trees

Abstract

Since only 12.6% of the Brazilian Araucaria Forest remains and timber tree monocultures are expanding, biological invasion is a potential threat to the conservation of natural forest remnants. Here, we test (1) the susceptibility of Araucaria Forest to invasion by Pinus taeda and Eucalyptus saligna, (2) the efficiency of different mechanisms controlling the early establishment of these two exotic timber tree species, and (3) the potential of the native timber tree Araucaria angustifolia to establish successfully in ecologically-managed monocultures of Araucaria, Pinus and Eucalyptus. In Araucaria Forest, more than a thousand Pinus seeds landed annually in a hectare; however, experimentally exposed seeds were 100% removed in only 6 days. Furthermore, all experimentally transplanted seedlings of Pinus taeda and Eucalyptus saligna died in less than a year in Araucaria Forest, but not in the monocultures. Correlative evidence suggests that this mortality was associated to plant community richness, plant abundance, and soil fertility. Araucaria angustifolia, in contrast, showed an establishment success in ecologically-managed tree monocultures as high as that exhibited in its natural habitat. The current resistance of Araucaria Forest to invasion by exotic timber trees is good news for the conservation of Araucaria Forest remnants and for its keystone species. The understanding of the mechanisms providing such resistance against invasion points towards management tools for minimizing future threats.