Topographic controls on the distribution of tree islands in the high Andes of south‐western Ecuador

Abstract

AbstractAim To evaluate the hypothesis that geomorphometric parameters of upper montane Andean environments have an important influence on the regional fire ecology and consequently play a role in the spatial distribution of ‘remnant’ tree islands dominated by Polylepis.Location A glacial landscape located between 3600 and 4400 m elevation in Cajas National Park, south‐western Ecuador.Methods The eigenvalue ratio method was used to evaluate the regional geomorphometric parameters of a 30‐m digital elevation model for Cajas National Park. The landscape character was evaluated by quantifying the topographic roughness, organization, and gradient. This information was used to determine the spatial correlations between terrain characteristics and the distribution of tree islands in the region.Results We demonstrate a strong spatial correlation between areas of high topographic roughness and gradient, and the locations of the major tree islands. We find that there is a distinctive relationship between the topographic roughness and organization in the vicinity of the tree islands (e.g. increased upslope roughness and decreased topographic grain strength) that substantiates the notion that the tree islands are located in relatively inaccessible topography.Main conclusions In the northern and central Andes, the location of Polylepis‐dominated ‘forest islands’ has been shown to be a function of climate, terrain characteristics, and anthropogenic disturbances. Although the relative importance of various ecological factors has been debated, it remains clear that fires have exerted a strong influence on these ecosystems. Other authors have noted that tree islands are more likely to occur at the base of cliffs, above moist areas, and in other areas where fires do not burn frequently. Our results corroborate these observations, and demonstrate that the occurrence of Polylepis patches is strongly correlated with specific combinations of terrain features. Although we do not discount the importance of other factors in determining the spatial position and areal extent of these forests, we demonstrate strong support for fire‐related hypotheses.