DISSERTATION RESEARCH: Role of non-structural carbohydrate dynamics in legacy effects of drought in Southwestern forests

干旱在美国西南部变得越来越普遍，导致树木死亡率更高。存活下来的树木可能会经历干旱的遗留影响：多年来生长速度下降。在下一次干旱到来之前，这些树木可能无法完全恢复，这增加了更多树木死亡的风险。驱动这些遗留效应的一个潜在机制与树木储存和利用糖的方式有关。干旱可能会改变树木利用储存的糖的能力，因为它们是通过树木组织中的水来移动的。这项博士论文改进基金（DDIG）将提供资金，通过树木年轮和不同年轮中糖的年龄，比较不同干旱胁迫水平下的树木，了解干旱如何改变树木中糖的可用性，以及这些变化与树木生长速度和遗留效应的关系。这项研究很重要，因为它将提高对树木对干旱的生理反应的理解，这可以改善对环境变化如何影响森林的预测。准确预测干旱对美国森林的影响对于管理相关的局部（火灾、洪水等）和全球（改变的碳循环）负面影响至关重要。本研究将探讨：(1)干旱胁迫对基础树种非结构性碳水化合物（NSCs）储存动态的影响；(2)这些动态与美国西南部森林干旱的遗留效应有何关系？由于被子植物表现出复杂的干旱遗留效应，并且更依赖于NSC储存，因此本项目主要关注两种基础杨树物种。2015年，在帕罗佛得角公共花园收集了14个基因型的白杨（Populus fremontii）的树核，这些树核经历了不同程度的表观干旱胁迫。在四角地区干旱胁迫程度不同的四个地点，收集了金针杨个体的树芯和呼吸CO2。本研究将分析树木年轮木材和气体样品的14C含量，以评估不同年龄树木年轮中NSCs的碳固定日期、年龄和可用性，并分析delta13C以评估干旱胁迫。14C和delta13C数据将与环宽度和水分胁迫/利用指数相关，使用分层模型来了解NSC动态在干旱遗留效应中的作用。本研究将开发一种廉价、新颖的培养方法，以获得用于14C分析的CO2，绕过现有NSC提取程序的困难。这些同位素方法将有助于阐明干旱胁迫和NSC储存之间的联系，并对基础树种如何应对气候变化产生影响。

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.