Tree plantations replacing natural grasslands in high biodiversity areas: how do they affect the mammal assemblage?

Abstract

Monoculture plantations of fast-growing trees are increasing worldwide and, in many cases, these replace highly diverse natural environments. The Mesopotamian Savannas and the Iberá marshes ecoregions of northeastern Argentina are a mosaic of habitat types in which extensive areas of natural grasslands have been replaced by the planting of non-native pines (Pinus spp.) Their impact on the natural communities is unknown. We evaluated the effect of these plantations, the landscape configuration and the forestry management practices on the assemblages of medium to large-size terrestrial mammals of this region at landscape and stand levels. Camera-trap stations were deployed in three environments: 89 in natural grasslands, 54 in native forest patches, and 91 in tree plantations. At a landscape level, we evaluated the effect of the type of environment and of the extent of different land covers on mammal richness per station using GLM, and on species composition using one-way PERMANOVA and redundancy analysis. At a stand level, we assessed the effect of understory vegetation, and of the age and density of tree plantation stands, on the similarity in mammal species composition between tree plantations and native environments, using non-metric multidimensional scaling analyses and GLM. Species richness and species composition differed between native forests and other environments, but not between grasslands and plantations. The variations in composition between the environments differed among the two ecoregions: the species composition in the plantations was different from the grassland assemblage only in the Iberá marshes, which suggests that the impact of tree plantations depends on the local pool of species. Mammal assemblages were also affected by the proportion of forests and wetlands and the environmental heterogeneity in the landscape. The similarity between the mammal assemblages of pine plantations and those of native forests increased with the age of the stand and at intermediate tree densities. In this grassland-dominated but heterogeneous landscape, it seems that most medium and large grassland mammals do not perceive tree plantations as a barrier, regardless of their management. Actions at a stand level, such as planting at intermediate tree densities and promoting longer rotation times, are desirable to encourage the forest mammal assemblage to use the plantations. Forests and wetlands are acting as keystone habitats for many species that are clearly associated with the presence of these environments in the landscape. Keeping large areas of natural grasslands and wetlands sprinkled with forests is essential for the conservation of the mammal assemblages of this region.