Responses of Andean Montane Forests to Climate Change

Abstract

Andean montane forests are ecosystems of global significance. They have exceptionally high levels of biodiversity and endemism, provide drinking water and livelihoods for millions of people, and are an important carbon store and sink. The tropics, however, are already being affected by heat extremes beyond the bounds of recent climate variability, and warming is fastest at high elevation due to factors such as decreasing cloud cover and increasing humidity. Additionally, the stable conditions of the Holocene have left many montane species adjusted to narrow climate bounds, so they are disproportionately vulnerable to rapid warming. Rising temperatures are already leading to compositional changes in Andean montane forests, with cold-affiliated highland species being displaced by warm-affiliated species from lower elevations. This is hypothesised to be driven by increasing mortality of cold-affiliated species and upward migration of warm-affiliated species, but the exact mechanisms underlying these changes remain uncertain. We investigated the potential for both cold- and warm-affiliated Andean species to adjust their physiology and morphology to changing temperatures by using three common garden sites at high (14°C), mid (22°C) and low (26°C) elevation in the Colombian Andes, with the higher temperatures of the lower sites simulating climate warming. Both species groups were best suited to temperatures closest to the thermal means of their natural distributions. Cold-affiliated species were able to acclimate photosynthetic capacity, leaf dark respiration and foliar nutrient content to warming, but were unable to shift leaf structural traits or water-use efficiency, whereas warm-affiliated species were able to acclimate all traits to cooling. Climatic origin was a key driver of acclimation potential, but within-species variability was also identified as an important factor contributing to species’ ability to shift traits in response to changing temperature. The ability of cold-affiliated species to acclimate certain key leaf traits to warming could slow the upslope advance of lowland species; however, the greater thermal plasticity of warm-affiliated species, coupled with their optimal performance at high temperatures, will lead to inevitable changes in the community composition of Andean montane forests as temperatures rise.