Question: Does understorey richness, cover and biomass change during succession in abandoned Castor canadensis impoundments of riparian Nothofagus forests?

Location: Magellanic Nothofagus forests at Tierra del Fuego National Park (54°50′32.4″ S, 68°32′11.5″ W), Argentina.

Methods: Five meadows of different time since abandonment (1, 5, 6, 9 and 20 years ago) and two controls (pure N. pumilio and mixed N. pumilio - N. betuloides forests) were sampled. Understorey variables (species richness, cover and biomass) in beaver meadows were measured at eight plots, as well as sapling and seedlings age and height. In control treatments, ten plots on each forest type characterized forest structure.

Results: Beavers alter vegetation dynamics, modifying biomass and composition of the original forest communities. Richness, cover and biomass were significantly modified when compared to the original understorey. Ferns are the most affected group, while grasses became more abundant. Many species established in the impacted sectors, which did not grow in primary forests. Trees did not regenerate in impacted areas for long periods, and many understorey original species are missing. Nothofagus forests are not adapted to support a long-term beaver impact.

Conclusions: Beavers modify the original ecosystem from closed forest to a grass- and sedge-dominated meadow, due to the lack of adaptive regeneration strategies in the Nothofagus forests. The maintenance of the present level of the beaver population is not sustainable over time, due to utilized and impacted tree biomass which could not be replaced by the natural dynamics of the forest ecosystem.

Abbreviations: FA = Forest stand; GIS = Geographical information system; LF = Nothofagus pumilio forest; LGF = N. pumilio/N. betuloides mixed forest.

Nomenclature: Correa (1969–1998); Moore (1983).