Canopy structure and woody species distribution at the upper treeline in the Venezuelan Andes

Abstract

At the treeline in the Venezuelan Andes forest tongues and islands intermingle into the herbaceous páramo vegetation. The transition between both ecosystems is abrupt. Previous studies in the tropical Andes suggest radiation and grazing as key factors limiting the advance of woody species into the páramo. We analyzed juvenile and adult woody species composition along basal and apical transects from the páramo to the interior of three forest tongues. Canopy structure was determined along these transects. Our results show a higher species diversity in the basal transects. Most species seemed to be restricted to the interior and borders of forest tongues and associated with higher LAI (leaf area index), while Diplostephium venezuelense, previously suggested as a pioneer tree, was the only species able to colonize the low LAI environments of the open páramo. Our results also suggest that aspect plays an important role in the establishment patterns of treeline species, as a result of differential shading in the N and S facing borders of tongues and adjacent páramos. Hence, our results support the idea that radiation could be an important limiting factor in scenarios of forest expansion into the páramo matrix and that the abrupt nature of the treeline is independent of fire and grazing, since these factors were absent in our study site.