Altered herb assemblages in fragments of the Brazilian Atlantic forest

Understanding the response of tropical plant communities to human disturbance is critical for conserving biodiversity in a changing world. Here we examine the shifts experienced by understory herb assemblages while inhabiting small forest fragments in a fragmented Atlantic forest landscape to infer about community-level shifts imposed by either habitat loss or fragmentation. We established 100 25-m2 plots, placed randomly in 10 forest fragments and 10 forest interior patches, in which all herb species were recorded, litter accumulation, soil temperature and moisture were estimated. We recorded a total of 6027 herbs belonging to 134 species, with a predominance of ferns, grasses, aroids, sedges and calatheas. Forest fragments and forest interior exhibited similar densities of herbs: 64.4 ± 57.8 herbs/25 m2 vs. 56.1 ± 44.1, respectively. Species richness was reduced by a half in forest fragments at plot and habitat spatial scales. Fragments were particularly impoverished in terms of ferns, aroids and calatheas, but supported a subset of proliferating native herbs and indicator/exclusive species; i.e. a taxonomically and ecologically distinct herb flora. Fragments also supported less humid soils covered by a thicker litter layer and these attributes correlated to species distributions in both forest habitats. Our results suggest that habitat loss and fragmentation, particularly the establishment of illuminated and desiccated forest edges, result in the extirpation of particular ecological groups with a few species/ecological groups experiencing proliferation, such as light-demanding species. Collectively, these processes result in impoverished/altered assemblages at multiple spatial scales, potentially limiting the conservation services provided by human-modified landscapes.