The response of tropical montane forest trees to warming

Globally, the tropics have experienced the largest recent increases in heat extremes relative to historical variation; and mean temperatures are predicted to continuously exceed bounds of historical variability in concert with more intense and frequent heatwaves. Recent warming has been associated with widespread reduction of growth and biomass carbon stocks in the warmest locations in lowland tropical forests in all continents. In the Andes, large increases in mortality of montane adult and juvenile tree species in the warm extreme of their thermal ranges have been observed, triggering a compositional change towards an increased representation of lowland species across all elevations. The specific mechanisms behind these changes remain unknown. 

We investigated tropical forest responses to warming using a field transplant experiment on an elevation gradient in the Colombian Andes in which juvenile trees from 15 dominant Andean species were planted across elevations and associated ambient temperatures. Montane species were exposed to a range of temperatures within and outside the warm edges of their thermal ranges and lowland species were exposed to temperatures on their cold edges, replicating the observed upward migration of these species in the Andes. We measured the impact of warming on tree survival, growth, and physiology. Overall, we found differential responses of montane and lowland species to warming. Highlights include higher thermal plasticity of montane species than in lowland species, which might have implications for lowland tropical forest under warming. Despite the large thermal plasticity of photosynthetic capacity and respiration in montane species, only 55% of the studied species were able to sustain growth when growing at the warm extreme of their thermal ranges. These findings suggest that the negative effects of warming are likely to promote increased homogeneity in community composition of these forests with a possible loss of diversity in this globally recognised biodiversity hotspot region.