Aggregate retention in two Tierra del Fuego Nothofagus forests: Short-term effects on bird abundance

Green-tree retention may mitigate detrimental effects of clear-cutting on forest birds. These beneficial effects would depend mainly on the way that retention changes habitat and landscape structure. In this study we examined the effect of aggregated retention on the abundance and richness of birds inhabiting two structurally different Nothofagus forests on Tierra del Fuego Island, Patagonian Chile. In the first year stands were undisturbed and in the second year parts of these stands were partial cut. After cutting abundance of 5 of 10 species was smaller in disturbed stands and only 2 species had increased abundance with aggregate retention. Retention effect was similar in both forest types for all species, but one. Nevertheless, we did not detect a significant aggregate retention effect when we compared pre- and post-harvesting scenarios, although eight species decreased in abundance during the second year in control stands in both forest types. The reduction in abundance of one species was greater in aggregated retention than in control stands and for three other species this reduction was greater in the structurally more simple forest. In retention stands a greater number of bird species were significantly affected by habitat and landscape variables than in control stands. Habitat structures that are changed drastically by retention, including canopy cover, tree density and ground branch cover, affected abundance of birds in addition to landscape structure of treated stands. Thus, the retention effect on bird abundance depends mainly on forest and landscape characteristics.