Colonization of tree species along an interior-exterior gradient across the forest edge in a tropical montane forest, northwest Thailand

We investigated the environmental factors and tree species characteristics that are important for colonization of an interior-exterior gradient across the forest edge, for application to the restoration of abandoned shifting-cultivation areas in tropical montane forests in the Umphang Wildlife Sanctuary, northwest Thailand. The relative importance of physical environment and recruit limitation was evaluated in relation to the regeneration traits of tree species. Three belt transect plots (150 m x 20 m) were established at the transition from secondary forest (edge interior) to open areas (edge exterior) of different ages (1, 3, and 5 years) after abandonment of shifting cultivation. We also set three belts (20 m x 50 m each) in a primary forest remnant. The species composition of canopy trees, regenerated seedlings, and saplings was studied, together with aspects of the physical environment. We found that it was difficult for primary forest species to effectively colonize the forest edge exterior, mostly due to recruitment limitations rather than the physical environment. Many of secondary forest species and generalists were also affected by recruitment limitation (significant negative correlation with the distance from forest edge), though they were also affected by factors related to the physical environment and forest structure and more abundant in open area. Only a few species, like Choerospondias axillaris (primary forest species), Wendlandia tinctoria (secondary forest species), Colona elobata, and Ficus hispida (generalist species) did not suggest recruitment limitation. These results suggested that natural regeneration of secondary forest and generalist species could be utilized as a first step in restoration expecting their facilitation effects for primary forest species.