Periodic Reporting for period 1 - TTMCC (Tropical tree mortality in a changing climate)
Periodo di rendicontazione: 2019-09-01 al 2021-08-31
Tropical forests store more than half of the carbon sequestered by terrestrial vegetation. However, ongoing climate change is threatening this carbon sink by increasing the tree mortality in these tropical forests.While it is established that tropical tree mortality increases under hot conditions, key questions regarding why and how remain unresolved. Therefore, the overall goal of my project was to determine the sensitivity of tropical tree species to heat, and based on this knowledge develop a trait-based tree mortality model that can be used to provide recommendations on which trees are suitable to different climatic regions in Rwanda, central Africa.
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(si apre in una nuova finestra)). 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.
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.
The scientific findings from this project has certainly advanced our understanding of how tropical tree species will respond to warming. For example, our findings showed that tropical tree species exhibit a stronger acclimation of leaf respiration to warming compared to tree species from other biomes (temperate and boreal). This knowledge, once incorporated in global vegetation models, will improve predictions of feedback between terrestrial vegetation carbon cycling and the climate in the future. We also showed that, in contrast to temperate and boreal tree species, tropical counterparts show a limited capacity to shift their thermal optimum of net photosynthesis to warming. These findings show that tropical trees are at a much higher risk with climate change. This knowledge provides strong evidence to shape policies to combat climate change. This knowledge will also help vegetation modelers improve the representation of photosynthesis of tropical trees to improve the global vegetation models predictive capabilities.
The findings generated from this project will also inform foresters in Rwanda on which native tree species to use in ongoing reforestation programs.
The findings generated from this project will also inform foresters in Rwanda on which native tree species to use in ongoing reforestation programs.