Herbivores regulate native-alien plants dynamics in sub-Antarctic beech (Nothofagus antarctica) forests of Tierra del Fuego, Argentina

Questions: What is the role of native and domestic herbivores in native-alien vegetation changes in sub-Antarctic forests? Does herbivory suppression increase native while reducing alien species diversity?Location: Central-eastern Tierra del Fuego Island, Argentina.Methods: In four sites, we set up three different treatments: excluded grazing by livestock (cattle), by livestock and native (guanaco) herbivores, and grazed control plots in Nothofagus antarctica forests. We then monitored the dynamics of understorey native and alien plants (species richness, abundance) of dicots, monocots, and ferns over 6 years. Generalized Linear Mixed Models were used to determine the effects of herbivore exclusion and time on species richness and abundance. In addition, we calculated the abundance rate of change to quantify temporal dynamics.Results: Overall species richness did not change in the exclusion plots, but native and alien abundances were affected by herbivory and time since exclusion. The abundance of native monocots changed after herbivore exclusion, increasing its dominance significantly inside exclusion plots. However, the cessation of herbivory did not reduce alien species in general. Only alien dicots significantly declined in the livestock exclusion treatment. Further, we found that the two dominant alien grass species (Holcus lanatus and Poa pratensis) respond differently to grazing exclusion, and their response was modulated by forest basal area.Conclusions: Our results indicate that cattle and guanaco grazing had variable effects on native and alien plants, and that herbivore exclusion has a substantially positive effect on native grasses. However, the magnitude of temporal changes was more significant for vegetation richness and abundance than the effect of exclusion treatments. Herbivore exclusions can initiate a rapid recovery of compositional attributes of native vegetation in N. antarctica forests grazed by livestock.