Unexpected Large Photosynthetic Thermal Plasticity of Montane Andean Trees

Tropical forests play a significant role in global carbon sequestration. However, our understanding of how tropical tree species adjust to climate warming remains limited to studies on seedlings grown in pots and highly controlled growth conditions. To reduce this knowledge gap, we used a field experiment with 5-year-old juvenile trees of 12 naturally co-occurring dominant tropical Andean montane and lowland species growing in three common gardens established along a natural thermosequence in the tropical Andes. Based on a few previous studies, we hypothesized that montane species would exhibit a weaker photosynthetic thermal acclimation capacity compared to lowland counterparts. Our results showed that montane tree species can thermally acclimate net photosynthesis by shifting their thermal optimum (T_opt) by 0.6°C per 1°C of warming. This strong shift in T_opt was correlated to simultaneous strong shifts in T_opt of apparent photosynthetic capacity parameters (V_cmax and J_max), which increased by 0.7°C per 1°C of warming. This strong thermal acclimation resulted in similar rates of net CO₂ assimilation between montane and lowland species across different thermal environments. At last, rates of net photosynthesis at growth temperature explained 30% of the variation in the relative tree growth rates across the two species groups and thermal environments. Our results suggest that the strong physiological acclimation of photosynthesis to warming among montane Andean tree species should be considered when predicting future impacts of warming on Andean plant communities.