This project was carried out at three different sites along an elevation gradient within the Tropical Elevation Gradient Experiment in Rwanda (Rwanda-TREE project,
https://www.rwandatree.com(s’ouvre dans une nouvelle fenêtre)). We collected data on different physiological and morphological traits related to tree carbon gain and water use in 16 tropical montane rainforest tree species.
The key scientific findings from this project were that i) The thermal optimum of net photosynthesis (the process used by plants to fix atmospheric carbon dioxide for growth) did not shift with warming, suggesting that these tropical trees will operate at supra-optimal temperatures with reduced ability to fix carbon in a warmer climate. ii) The leaf respiration rates (the process that generates energy for cellular maintenance and growth, but also releases carbon dioxide in the process) adjusted to warming by remaining constant or decreasing with warming when measured at growth temperatures. These results imply that leaf respiration will have little constraint on the carbon available for tree growth in a warming world. iii) We also found that the biochemical processes underlying photosynthesis (maximum carboxylation capacity of the Ribulose-1,5-bisphosphate carboxylase/oxygenase enzyme – Vcmax and maximum electron transport rate – Jmax) responded differently to warming between potted seedlings (with the same soil across the three sites) and freely-rooted saplings in soils at different sites. In seedlings, both Vcmax and Jmax declined with warming. In freely rooted field-grown trees, only Jmax declined with warming, while Vcmax did not change. Overall, our results indicate that the responses of photosynthesis to warming will be critical in determining the ability of tropical trees to continue fix atmospheric carbon in a warmer climate.
The results from this project were presented to different stakeholders in the forestry sector in Rwanda, and were also published in two international scientific journals.
Some of the data generated from this project are currently being used to develop a simple model to predict the effects of warming on carbon uptake of tropical montane rainforests. Moreover, the data generated from this project will also contribute to a pan-tropical effort that aims to improve our understanding on how warming will affect carbon uptake and sequestration in tropical tree species, and its implications to the feedback to the climate system.
