Native and non-native herbaceous species dependence on tree cover in grazing systems from northern Chilean Patagonia.

摘要

Grazing systems that combine grasslands and woodlands represent changes in tree cover that influences herbaceous vegetation and may threaten the native forest flora. We studied these influences in Chilean Patagonia where recent colonization resulted in the fragmentation of the lenga (Nothofagus pumilio) forests leading to a mosaic-type landscape. The herbaceous vegetation, transmitted photosynthetically active radiation (PAR) and other forest structure characteristics were sampled in 15 (20 m x 20 m) plots differing in tree cover for two years, while considering the relative position (below or beyond) of tree crowns. Herbaceous species were classified as forest, non-forest, and forest indifferent species according to their preference, rejection or indifference to the forest environment. Species richness decreased with increasing PAR but this was dependent on the response of non-natives. Species composition varied gradually from open grasslands to dense forests, according to transmitted PAR values. Native species were associated to either forests or open areas, but the number of native species did not vary along the tree cover gradient (beyond tree crowns) or even decreased (below tree crowns). Non-native species dominated in the open grasslands and beyond tree crowns and its richness increased with transmitted PAR. The occurrence of grasslands and dense forests within this grazing system is shown to promote higher species richness. Management will consider that dense forest patches are necessary for maintaining a high diversity of native species, since isolated trees or low wooded areas do not guarantee the persistence of all native species associated to the original forests.