Short-term avian responses to experimental variable-density thinning: Initial stand attributes outweigh effects of management in Austral temperate rainforests

Abstract

Anthropogenic environmental changes have led to secondary forests becoming dominant in many regions. Although ecological or restoration thinnings can help restore old-growth conditions, experimental evidence of their subsequent benefits to biodiversity remains sparse, particularly in Austral Temperate Rainforests in South America. Using mist-net sampling within 1-ha plots in mixed-evergreen and Nothofagus secondary forests, we examined short-term effects (4 years) of experimental Variable Density Thinning (VDT) on bird richness, abundance, and the individual body condition of two abundant breeding species: frugivore Elaenia albiceps and insectivore Aphrastura spinicauda. Based on 518 birds captured, richness and relative abundance were higher in Nothofagus than in mixed-evergreen forests, regardless of VDT treatment, except for E. albiceps which was more abundant in mixed-evergreen forests. Richness and abundance did not vary with VDT treatment, except for E. albiceps which increased in abundance following VDT. However, individual body condition of E. albiceps did not change due to VDT management, although it was better in mixed-evergreen than in Nothofagus forests, and increased over the course of the study. In contrast, body condition of A. spinicauda was not associated to VDT treatment, forest type, or season. Our results suggest that the short-term effects of VDT in secondary Austral Temperate Rainforests upon avian richness, abundance, and individual body condition were minimal, and that initial stand attributes (i.e. forest type) more strongly influenced these variables. This is likely due to the region’s high diversity of tree species, which drives substantial differences in tree species composition among forest types and affects the food resources available to birds. In this case, most trees in mixed-evergreen forests produce fleshy fruits that are important in the diet of E. albiceps and other bird species. Therefore, in geographical regions with a high alpha and beta tree diversity, restoration thinnings in secondary forests should explicitly target tree composition, which appears to be at least as important as vegetation structure in determining short-term avian responses.