Vegetation structure and composition along an interior-edge-exterior gradient in a Costa Rican montane cloud forest.

摘要

A study was carried out in the Costa Rican Cordillera de Talamanca in order to assess changes in vegetation structure and floristic composition along a gradient from the interior of tropical upper montane cloud forest into adjacent pasture land. A 150X10 m belt transect was established perpendicular to the forest/pasture edge at 4 sites with differing recent and present disturbance, which could be arranged along a gradient of increasing land-use pressure (forest from no logging to selective logging, with understorey ranging from zero to high disturbance; and pasture land subjected to low to high grazing pressure, and either no or some cultivation). Vegetation transect censusing included recording of seed plant species presence and abundance (aerial crown/shoot cover), as well as assessment of seedling, sapling and tree stem diameter and height. A total of 290 seed plant species were found in 159 genera and 85 families. The Asteraceae with 47 species was the most speciose family. Shannon-Wiener'sdiversity index showed no significant difference between forest and pasture land, with the exception of a pasture land site which had experienced intensive land use. Sorensen's similarity index showed significant differences in species composition between forest and pasture land. Vegetation at the forest edge consisted mainly of secondary shrub and tree species. From the forest interior towards the edge, numbers of seedlings, saplings and secondary trees increased, while numbers of mature forest trees declined. Edge effects are expressed in a reduction of canopy height and an increase in subcanopy stature from the forest interior towards the edge, causing the merging of forest canopy and subcanopy layers at the edge. Intensively used pasture lands onlyharboured a low herbaceous vegetation, while their abandoned equivalents showed the presence of patches of secondary shrub vegetation and recovering forest in a grassland matrix. Expected spatio-temporal changes at the forest edge include the encroachment of secondary vegetation and the levelling of the forest/pasture border as time advances and land use becomes less intense. Transitional second growth edge vegetation may buffer the remaining mature forest core and simultaneously induce forest recoveryprocesses in adjacent abandoned pasture lands. It is recommended to include forest edge tree species in programmes focused at the ecological restoration of tropical montane cloud forests.