Impact of beaver foraging on structure of lowland boreal forests of Algonquin Provincial Park, Ontario

We investigated the effects of beaver browsing on woody plant community structure of lowland boreal forests by measuring spatial variation in plant diversity, foraging rates and sapling recruitment at 15 ponds occupied by beavers and one other pond abandoned by them in southern Algonquin Park, Ontario. Beavers fed preferentially on a small number of deciduous species and the number of cut stems declined sharply with increasing distance from ponds. The maximum stem diameter cut was 45.3 cm, and the average stem diameter cut was 15.1 cm. Conifers increased in relative dominance to deciduous species in the presence of beavers. Plant species richness, and stem and basal area diversity peaked at intermediate distances (ca. 25 m) from ponds. Sapling recruitment by non-preferred species was positively related to foraging pressure. Total stem abundance and basal area and sapling recruitment by four preferred species (trembling aspen, red maple, sugar maple and beaked hazel) were negatively related to foraging pressure. However, by including alder and willow (also preferred by beavers) these patterns changed, becoming positively related to foraging pressure. Diversity patterns changed three years after abandonment. However, sapling recruitment patterns in preferred and non-preferred species around the abandoned pond were similar to the occupied ponds. Hence, selective foraging by beavers does not completely shift the community structure towards nonpreferred species. These observations suggest that beavers do not act as keystone species, i.e., they increase the importance of the dominant plant species (conifers). However, the gaps they create facilitate the regeneration of stems of both their preferred food species and non-preferred species, with future ramifications to forest structure. (C) 1999 Elsevier Science B.V. All rights reserved.