Ecotones, or transition zones between ecosystems, are spatially diverse and temporally dynamic ecological systems. Ecotones control the flow of energy and organisms between landscapes, are areas of steep gradients in plant structure and ecosystem properties, and their locations are often determined by species’ physiological limits. As a result, ecotones can serve as early warning systems of regional change. However, their spatial and temporal complexity makes studying ecotones inherently challenging. Using three case studies at differing spatial scales, we demonstrate how ecotones can serve as sentinels of climate change responses. First, at fine spatial scales, we use an experimental approach to quantify the functional trait responses of a caterpillar herbivore and its host plant to an anthropogenic ecotone within a tropical dry forest in northwestern Costa Rica. Second, at the landscape scale, we quantify species turnover and a key plant functional trait (specific leaf area) among woody and herbaceous species across elevation to examine upslope ecotone migration between tropical dry and cloud forests. Finally, at continental scales, we use biome and climatic distributions to quantify the location and climatic regime of one of the most important ecotones in the neotropics - that between tropical dry and wet forests.
Results/Conclusions
First, at the local scale, we demonstrate reduced growth and performance of a caterpillar herbivore and its host plant in response to an anthropogenic-induced ecotone among tropical dry forests. These changes were largely driven by temperature, which has important implications for tropical dry forests throughout the tropics. Next, at the landscape scale, we show that herbaceous species appear to track rising cloud-line across an ecotone between tropical dry and cloud forests. In contrast, woody species appear to lag behind, creating an ecotone ghost of climate past. Our work demonstrates how taxonomic and functional diversity can reveal shifting forests across mountains. Finally, we end with an example of how ecotones can reveal changes in the distribution of tropical forests at continental scales. We show that although ecotonal forests are intermediate between tropical dry and wet forests, they are not intermediate in climate, which suggests that the directionality and speed of ecotone shifts may not be linear. In short, ecotones are detection systems for climate change in tropical forests, and the mapping and monitoring of ecotones across spatiotemporal scales is an urgent priority. We end by outlining key methods that could be used to do so, including the use of satellite imagery.
