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Tree community composition, structure and diversity along an elevational gradient in an Andean forest of Northern Ecuador

Abstract:

The Andean forests of northern Ecuador are known for their high levels of plant diversity relative to the area they occupy. Typically, these forests grow on steep slopes that lead to dramatic habitat gradients across short distances. These extreme habitat gradients make the Andean forest ecosystem an excellent natural laboratory for understanding the effect of elevation on forest community diversity, structure and composition. We established 31 plots (50 m × 5 m) which are divided between two elevational transects in the cloud forest of the Siempre Verde Reserve in the western foothills of the Andes Mountains of northern Ecuador. All trees and tree ferns with a diameter at breast height (dbh) ≥ 5 cm were measured and identified. We examined changes in community composition, structure, and diversity along and between the elevational transects and three elevational zones: low (2437–2700 m), middle (2756–3052 m), and high (3163–3334 m). We found four main trends associated with the elevational gradients at this site: (1) community composition differed between the two transects and among the three elevational zones according to N-MDS, ANOSIM, and percentage of shared species, with some species having limited distributions, (2) metrics of community structure showed opposite relationships with elevation, depending on the transect, with the only significant relationship (negative) found between basal area and elevation in the open trail transect, (3) alpha diversity, in general, peaked at mid-elevations, and (4) beta diversity consistently increased with distance between plots along elevation. The complexity of changes in community composition, structure, and alpha diversity along elevation may be related to the heterogeneity of the environment on a local scale, such as topography, soil composition, and even human impact, or to dispersal limitation and should be investigated further. These changes in community composition and the relatively high beta diversity found at this site exemplify the biological complexity of montane forest, reinforcing arguments from other studies on the importance of their conservation.