High vulnerability of highland Andean forests to warming

Tropical forests are expected to be highly vulnerable to climate change. Observations from the tropical montane Andean forests report a change in composition towards a greater relative abundance of warm affiliated species, i.e thermophilic species. These shifts are hypothesised to result from differential responses to warming of cold- and warm-affiliated species, with the former experiencing mortality and the latter migrating upslope. However, the drivers of these changes are poorly understood. Along a 2000m altitudinal gradient/thermosequence in the Colombian Andes, we planted 2-yr old individuals of cold- and warm-affiliated species under common soil and water conditions, exposing them to the hot and cold extremes of their thermal niches, respectively. We show that cold-affiliated species growing outside and on the hotter portion of their thermal ranges decreased their growth. Warm-affiliated species can survive but reduce their growth under the colder portion of their thermal distribution. We demonstrate that growth responses are related to species’ thermal distributions; survival probability increased as species’ distribution optima were warmer than the experimental site and decreased as species’ distribution optima were colder than the study sites. These results can be explained by the negative effects of heat stress on simulated photosynthesis.  Our findings highlight the potential effects of rapid warming on the composition of highland forest species in this biodiversity hotspot.