Edge effects in a forest mosaic: implications for oak regeneration in the Highlands of Chiapas, Mexico.
Forest edges created by scattered-patch clear-cutting have become a common landscape feature in tropical montane forests, including pine-oak and evergreen cloud forests. Forest-edge-pasture gradients were characterised with respect to changes in vegetation structure and ecological function in the Highlands of Chiapas, Mexico. In this study two edge types were recognised by assessing vegetation structure and floristic composition along a gradient from adjacent clearings into the forest interior. The influence of edge type (either “hard” or “soft”) was investigated with respect to acorn production, predation, dispersal and germination, seedling establishment and abundance of small mammals during two consecutive years. There was no significant difference between the forest interior and the edge in plant density and basal area. Floristic richness and composition did not vary with depth-of edge influence. Edge type influenced the relationship between probability of acorn removal by small mammals and distance from the edge in both mast fruiting and non-mast fruiting years. Movements of metal-tagged acorns supported the hypothesis that soft edges are permeable to acorn predators and/or dispersers, and that rodents move acorns up to 15 m into the grasslands in soft edges. In hard edges, acorn dispersal and removal was greater within the forest. Peromyscus spp. were the main acorn predators and/or dispersers of acorns. The intensity and rate of acorn removal varied with the year, demonstrating that in mast-fruiting years some seeds may escape predation by animals, according to the predator satiation hypothesis. Grasslands presented fewer insect damaged acorns and higher seedling emergence than the edge and the forest, emphasising the importance of acorn dispersal. Seedling survival and growth were higher for 12 m into grassland from the soft edges, suggesting that the probability of oak expansion into the grassland increases when the grasslands have been abandoned and have a scattered cover of shrubs. The results indicate that edge structure alters edge function (edge permeability), which determines the probabilities of tree invasion into old-fields. These findings have implications for the conservation of montane forest in Mexico where scattered patch clear-cutting is widespread. Recommendations are made for different restoration practices to accelerate tree colonisation into the grasslands across hard and soft edges.