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

Abstract

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 pastureland. At four sites a 150 × 10 m belt transect was established perpendicular to the forest/pasture edge. 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. Asteraceae is with 47 species the most speciose family. Shannon–Wiener's diversity index showed no significant difference between forest and pastureland, with the exception of a pastureland site which had experienced intensive land use. Sørensen's similarity index showed significant differences in species composition between forest and pastureland. 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 pasturelands only harboured 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 recovery processes in adjacent abandoned pasturelands. It is recommended to include forest edge tree species in programs focused at the ecological restoration of tropical montane cloud forests.