Invasive plants contribute to arrested succession in highly disturbed tropical dry forests: A field experiment in the Brazilian Caatinga

Abstract

Dry forests have been historically modified by multiple human disturbances resulting in a mosaic of forest areas with different successional stages embedded within anthropogenic landscapes. Anthropogenic drivers, such as the spread of invasive species, can favour the establishment of alternative successional trajectories in regenerating dry forests, thereby shifting communities into a novel functional and structural composition. Here, we use a randomized complete block factorial field experiment to assess the role of invasive plant species and nutrient enrichment in the community-level attributes of native plants in a regenerating forest stand of Caatinga dry forest, northeastern Brazil. We sampled 48 3 × 3 m plots corresponding to eight treatments showing different levels of exotic grass invasion and nutrient input (NPK). Our results show that plant assemblages were impoverished and dominated by a few species of herbs, shrubs and subshrubs indicating a scenario of arrested succession. We demonstrate cross-treatment differences in community-level attributes of native plants with most exclusive plant species restricted to treatments with low presence of invasive species. The number of rare native species declined with increasing invader cover, while native plant species richness increased. However, this positive effect of invasive species on total species richness was associated with the proliferation of native grass, herbs and shrubs. Our findings show the important role of both high densities of an invasive grass species and land-use history on native plant assemblages, with drastic impoverishment in terms of tree species, which clearly indicates a scenario of arrested succession.