Manipulating mammal herbivory in South Patagonia forests: effects on plant species assemblage, survival and short-term growth response of Nothofagus antarctica seedlings

Livestock browsing and large wild herbivores in forested areas have been perceived as conflicting with silviculture and forest conservation for a long time. However, silvopastoral systems in native forests entail livestock manipulation in space and time, to reduce negative impacts on forest structure and dynamics. Competition from other vegetation and browsing by mammal herbivores are two factors that usually limit tree regeneration. In 2014, we established eight 15×15m fences (and the respective control plots with browsing) in four Nothofagus antarctica (ñire) silvopastoral sites in Tierra del Fuego Island, to protect tree seedlings and vegetation from cattle and guanaco browsing. In 2019, after five growing seasons, the exclusion treatment had favored regeneration of ñire by reducing browsing pressure while increasing seedling growth rate, though the effect on seedling survival was weak. Ñire regeneration was absent only in one of the fields (inside and outside of fences), probably due to the high coverage of the exotic fodder grass Holcus lanatus. Soil moisture increased while soil compaction decreased over years in all fields, though the fences effect was weak. Moreover, other plants, and especially palatable herbs (e.g., Taraxacum sp., Osmorhiza sp.) and grasses (e.g., Agrostis sp., Bromus unioloides) increased much more in abundance and height than ñire seedlings within fences. Thus, the use of exclusion treatments to manipulate browsing impact enhances the short-term growth of ñire regeneration in south Patagonia forests used for cattle production. However, a negative effect by competing for understory vegetation in fences probably also occur in the long term.