The inter-forest line could be the master key to track biocoenotic effects of climate change in a subtropical forest

Abstract

At least three strata of vegetational formations can be recognized in many mountainous ecosystems. For instance, the Andean Yungas exhibits: 1) upper fog grasslands, 2) intermediate montane forest, and 3) lower montane rain forest. The respective delimiters are the treeline and the inter-forest line. We develop an approach to perform an optimal tripartition of mountainous ecosystems using elevational cutoffs per watershed. This pair of cutoffs allows us to transform heights of a given digital elevation map onto a universal adimensional scale. Normalized elevation facilitates worldwide comparisons and entails synthetic information (e.g., >0 means above treeline and < −1 below inter-forest line). To test the reliability of delimiting elevation lines, we compare them against the hand-drawn treeline and we assess the agreement with another tripartition of sampling points, but based on a different set of organisms (i.e., mayflies, an ancient group of insects that mimic ecological patterns found in many other aquatic taxa). The resulting delimiters of strata pass the test successfully. The upper delimiter fits the detailed treeline well. The lower delimiter segregates different types of mayflies on either side. Only the intermediate layer holds a distinctive set of co-occurring mayfly species. We finally stress the relevance of the inter-forest line to track the imprints of global change on community composition.