Climate change sensitivity of photosynthesis and respiration in tropical trees

Thesis abstract: Tropical climate is getting warmer, with more pronounced dry periods in large areas. The productivity and climate feedbacks of future tropical forests depend on the ability of trees to acclimate their physiological processes, such as photosynthesis and leaf dark respiration, to these new conditions. However, knowledge on this in tropical tree species is currently limited due to data scarcity. In this thesis, I have studied temperature and seasonal drought responses of photosynthesis and leaf respiration (Rd) in 16 tree species belonging to late- (LS) or early- successional (ES) groups and grown under field conditions. My thesis covers three studies using two approaches: a study on mature trees of four species along an elevation gradient and a multispecies plantation elevation experiment named Rwanda Tropical Elevation Experiment (Rwanda TREE). In study I, I showed that stomatal conductance (gs), leaf transpiration (E) and light saturated net photosynthesis (An) decreased at warmer, lower-elevation sites during dry season, while patterns were absent or opposite (for E) in the wet season in mature trees. In study II, I showed that An declined at warmer low-elevation sites in LS species but not in ES species. This was a result of stomatal rather than biochemical limitations. In addition, the slope parameter of stomatal conductance vs. photosynthesis relationships did not vary with either site nor successional group. Moreover, drought strongly reduced An at the low and mid-elevation, warmer sites but not at the high-elevation, coolest site. In study III, I found that leaf Rd at 20 °C decreased at warmer sites, regardless if it was expressed per unit leaf area, mass, N or P. This resulted in complete or over-compensatory thermal acclimation at growth temperature. Moreover, drought reduced Rd, particularly in the ES species and at the coolest site. My results are important to increase the understanding of tropical forest responses to warming and drought, which is critical knowledge in the fields of global change ecology and ecosystem and climate modelling.