Evaluation of Andean alder as a catalyst for the recovery of tropical cloud forests in Colombia

Andean alder (Alnus acuminata) is a tree species often used in revegetation programs in the Colombian Andes, because it is native to the area and grows rapidly even in nitrogen-poor soils. Previous work implied that Andean alder plantings might not lead to as rich a diversity of native vegetation as would natural regeneration. At Ucumari Regional Park in the Central Andes of Colombia, 30-year-old alder plantations are juxtaposed to naturally regenerated forests of the same age. This study quantifies the impact of Andean alder on plant species composition and forest structure, compared to natural regeneration. Plantations and naturally regenerated forests differed in physiognomy, species composition and in some structural aspects. Alder plantations produced an even 20 m tall canopy. The foliage was vertically stratified into two distinct strata separated by a gap that exposed the boles. The naturally regenerated forest had an uneven 16-18 m tall canopy, vertically continuous foliage cover and significantly higher canopy cover. In both forest types, basal area and stem density were equivalent and distributed among size classes in the same way. Snags in naturally regenerated forests were thicker, perhaps offering a better resource base to organisms using this resource. Plantations had up to one-third fewer species per unit than naturally regenerated forests. Plantations showed a low alpha and beta diversity, with lower species turnover from plot to plot. Qualitatively, the vegetation in the plantations was distinct from that in naturally regenerated forests, as it was composed of a particular subset of species. Approximately one-fourth of species in plantations were exclusive to that forest type. The same patterns were found in the understorey vegetation (1.3-10 m high), where alder is not recruited. This suggests that these patterns are not an artifact of numerical abundance of alder, but rather that alder actually has an effect on species composition. Although alder may create a forest cover in a shorter time, natural regeneration might be a better strategy for the recovery and conservation of biodiversity as long as a propagule source is available. Presence of both forest types, however, may increase species richness and spatial heterogeneity at the landscape level. (C) 1997 Elsevier Science B.V.