Drought is becoming more common in the southwestern US, resulting in higher tree mortality. Surviving trees may experience legacy effects of drought: decreases in growth rates for multiple years. These trees may not fully recover before the next drought arrives, increasing the risk that more will die. One potential mechanism driving these legacy effects is related to how trees store and use sugars. Drought may change trees' ability to use stored sugars, because they are moved by water in tree tissues. This Doctoral Dissertation Improvement Grant (DDIG) will provide funds to compare trees under different levels of drought stress to understand how drought changes the availability of sugars in trees, and how these changes are related to tree growth rates and legacy effects, using tree rings and the age of sugars in different rings. This research is important because it will improve understanding of how trees respond physiologically to drought, which could improve predictions of how environmental change will affect forests. Accurate predictions about drought effects on US forests is critical for management of associated local (fire, flooding, etc.) and global (altered carbon cycling) negative impacts.
This study will address: (1) what are the effects of drought stress on storage dynamics of non-structural carbohydrates (NSCs) in foundation tree species, and, (2) how are these dynamics related to legacy effects of drought in southwestern US forests? This project focuses on two foundational Populus species, because angiosperms exhibit complex drought legacy effects and are more reliant on NSC storage. Tree cores from 14 genotypes of Populus fremontii experiencing varying levels of apparent drought stress at the Palo Verde Common Garden were collected in 2015. Tree cores and respired CO2 will also be collected from Populus tremuloides individuals at four sites differing in drought stress across the Four Corners region. This study will analyze tree-ring wood and gas samples for 14C content to assess carbon fixation date, age, and availability of NSCs in tree rings of different ages, and for delta13C to assess drought stress. 14C and delta13C data will be related to ring widths and water stress/use indices using hierarchical models to understand the role of NSC dynamics in drought legacy effects. This study will develop an inexpensive, novel incubation method for obtaining CO2 for 14C analysis, bypassing existing difficulties with NSC extraction procedures. These isotopic methods will help elucidate links between drought stress and NSC storage, with implications for how foundation tree species will respond to climate change.
