Photosynthetic thermal acclimation capacity of tropical montane rainforest trees in Rwanda and in the Colombian Andes

Tropical montane forests are among the most productive ecosystems within the tropical region and store a significant amount of carbon in live biomass. With ongoing global climate warming, tropical climates are also getting warmer. The productivity and climate feedbacks of future tropical montane forests depend on the ability of trees to acclimate their photosynthetic metabolism to these new, warmer conditions. However, knowledge of acclimation ability of photosynthesis and its underlying biochemical processes to warming in trees grown under natural field conditions is currently limited due to data scarcity. To reduce this knowledge gap, we used two separate field experiments located in Colombia (with 15 species) and Rwanda (16 species), and for each experiment, tree species were grown at three different sites along an elevation gradient differing in ambient air temperature. At all three sites, we measured the responses of net CO₂ assimilation at different CO₂ concentration (50 to 2000 ppm) and at different leaf temperatures (15 to 40 °C) in ≈ 3 years old trees. We used these data to derive key photosynthetic biochemical parameters (maximum Rubisco carboxylation capacity - V_cmax and maximum electron transport rate - J_max) and their temperature sensitivity, as well as the thermal optimum of net photosynthesis (T_optA). We show that tropical montane tree species from the two continents are generally able to acclimate their T_optA by increasing in trees grown in warmer conditions, but the magnitude of change in T_optA differs among species from different successional groups (early- versus late succession) and climate of origin (lowland versus montane). Shifts in T_optA are largely driven by concomitant changes in thermal sensitivity parameters of underlying biochemical processes of photosynthesis (V_cmax and J_max) with warming. We also show that, at a standard temperature of 25 °C, V_cmax is largely constant, while J_max decreases with warming. Our findings indicate tropical montane tree species from Latin America and Africa can thermally acclimate their photosynthetic physiology, but that this thermal acclimation ability is related to species successional group and their climate of origin.