Global environmental changes have increased the frequency and distribution of tree mortality. As trees die, they release stored carbon dioxide to the atmosphere, creating a positive feedback on global change and accelerating warming. To predict the future state of forest health, a predictive understanding of how and when trees die is needed, but there are still major uncertainties regarding the physiology of tree mortality from drought. Investigators in this project, will measure the age of stored carbon (sugars and starches) in tree tissues as they die from a severe drought. These data will reveal novel information about how the connections between different tree organs fail under severe drought. The hypothesis to be tested is that trees which experienced drier conditions ahead of the drought will have altered stores of carbon, and die more quickly, revealing a way to predict future mortality risk in forest trees. Results from this study will be shared with local communities on the Colorado Plateau, including students from the Navajo Nation, to help provide scientific knowledge about the changes that will likely take place in regional forests under future drought.
A decade of tree physiological research has focused on the dichotomy among two posited modes of tree mortality from drought, hydraulic failure and carbon starvation, with limited exploration of the interaction between these two modes. This project will leverage an experimental drought treatment being imposed upon pinyon pine trees at the Sevilleta LTER, to measure the age of non-structural carbon (NSC; sugars and starch) stored in and respired from multiple tree organs under lethal drought. By measuring the age of NSC used in trees under drought, the impacts of hydraulic stress upon the use and mobility of different-aged carbon pools will be revealed. Specifically, the project will address the role of stored (deep sapwood and roots vs. shallow sapwood and branches) and recent NSC under lethal drought. This experiment also includes a 'legacy' treatment, which will upgrade an existing 75% precipitation exclusion treatment to 90% exclusion, allowing evaluation of how previous drought stress influences the physiological responses of trees to subsequent drought. NSC being respired from branches and the trunk, and NSC stored in branches, different sapwood depths, and coarse roots, will be sampled in order to measure the age of the NSC with radiocarbon techniques. The expectation is that drought trees use older NSC as the drought progresses, and that trees experiencing transport or phloem limitation will show divergence of NSC age in use among different tree organs as the connections between deep storage and metabolically active tissues (leaves) are disrupted.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
