Barriers to tree seedling emergence on human-induced grasslands in Sri Lanka

1. Colonization by woody plants is often very slow or absent on grasslands occupying degraded land in the tropics. Seed dispersal limitation is widely reported, but the constraints to forest succession imposed by barriers to seedling establishment are poorly understood. We tested the hypotheses that seedling emergence of woody plants is limited by interactions of anthropogenic fire, vertebrate herbivory and competition with the dominant grass sward in human-induced montane grasslands in Sri Lanka.

2. Seedling emergence was determined fortnightly for 18 months in response to experimental manipulation of fire regimes, access by vertebrate herbivores and competition from the dominant grass canopy at the forest/grassland edge and at 10, 20 and 40 m from the edge into four blocks of grassland. Seedling emergence was also monitored in the absence of any experimental manipulation at 10, 20 and 40 m into adjacent blocks of lower montane rainforest.

3. Emergence of seedlings of woody plants was much lower in the grassland (mean < 0.1 seedlings m(-2) year(-1)) than in natural forest patches (mean 6.0 seedlings m(-2) year(-1)), but was maximal at the forest/grassland edge (mean 9.5 m(-2) year(-1)). In the grassland, fire reduced seedling emergence by 36% in fenced vertebrate exclosures but increased seedling emergence by 68% in unfenced plots. Exclusion of vertebrate herbivores had no impact on seedling emergence at the forest/grassland edge. Removal of the grass canopy by clipping or tilling increased seedling emergence at the edge, but had no effect in the grassland. Seedlings of the woody pioneer species Macaranga indica dominated the community of emergents in the grassland and at the edge.

4. Synthesis and applications. Although dispersal limitation represents the primary constraint to forest succession on degraded montane grasslands in Sri Lanka, management of fire regimes, vertebrate herbivory and competition from the dominant grasses would influence the abundance and composition of tree seedling emergents. Tilling in narrow strips, 2-5 m wide and within 10 m of the forest edge, would facilitate the emergence and establishment of early successional trees in the grassland, but the strips would require protection from fire for at least 2 years. Within the interior of grassland patches, a combination of controlled burning and short-term access to vertebrate dispersers would promote tree seedling emergence, but the burned patches would then require long-term protection from fire and grazing subsequently to allow the emergents to establish and catalyse forest succession.