Climate Change sensitivity of Photosynthesis and Respiration in Tropical Trees

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 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 warming and seasonal drought responses of photosynthesis and leaf dark respiration (Rd) in early-successional (ES) and late-successional (LS) species originating from Afromontane and transitional rainforest vegetation zones. My research used an elevation gradient approach with different designs in different studies: existing mature trees of four species growing at five locations at different elevation (Paper I); multispecies plantations established at three sites at different elevation and vegetation zones in an elevation experiment named Rwanda TRopical Elevation Experiment (Rwanda-TREE), using either plants freely rooted in the soil (Paper II and III) or plants growing in pots with the same soil at all sites (Paper IV). The results demonstrated that in existing mature trees leaf stomatal conductance (gs), transpiration (E) and light saturated net photosynthesis (An) decreased at warmer, lower-elevation sites during dry season, while patterns were absent (for gs and An) or opposite (for E) in the wet season. In Rwanda-TREE, I found that An under non-drought conditions decreased in trees grown at the warmest, low-elevation site, in LS but not in ES species, while An was strongly and equally reduced in ES and LS species during the dry season at the two warmer sites, but not at the high-elevation site. Rates of leaf Rd measured at 20 ℃ were strongly reduced in trees grown at the warmer sites, leading to constancy or even declines in Rd at prevailing nighttime temperatures. Drought also reduced Rd. The pot study showed that the optimum temperature of An and its underlying biochemical processes did not significantly increase in warm-grown trees, indicating limited thermal acclimation capacity of photosynthesis. The findings of this thesis have several important implications for the projection of future tropical biosphere–atmosphere interactions. Firstly, the pronounced seasonality in altitudinal patterns suggest that tropical tree water use and CO2 uptake will be substantially reduced if dry seasons become more pronounced in a warmer climate. Secondly, the strong thermal acclimation of leaf Rd observed here should be accounted for to avoid model overestimation of the impact of global warming on leaf respiration in tropical forests. Thirdly, the contrasting responses of photosynthesis to warming in ES and LS species may imply potential functional shifts in tree community composition of tropical forests in a warmer climate. Fourthly, my results also indicate that acclimation capacity of the thermal optimum of photosynthesis may be considerably weaker in tropical montane tree species compared to temperate and boreal species. With these findings, my thesis contributes to reducing the knowledge gaps regarding tropical tree responses to climate change, which is key for improving projections of future climate change responses and feedbacks of tropical forests.