RAPID: Leveraging the 2015-2016 El Nino to evaluate drought legacy effects on tree growth responses to rare wet events

美国西南部是一个干旱地区，降水量年际变化很大。该区域在过去二十年中经历了一系列严重干旱的年份，最近一次是在2012年。然而，在厄尔尼诺年，如2016年，冬季降水量可能比平均水平高出数倍。干旱年份增加了森林和林地树木的死亡率，在干旱中存活的树木需要几年的时间才能恢复正常的生长能力。虽然冬季降水量高通常会促进更好的生长，但一棵树利用好年份的能力可能取决于其干旱历史。这是因为存活下来的树木的解剖学和生理学变化似乎减少了一棵树？它具有运输水分和支持高速率光合作用和新生长的能力。 该项目将利用西南部四角地区的一组研究地点，这些地点涵盖目前厄尔尼诺现象造成的冬季降水量增加的广泛范围。 由于以前在这些地点进行的研究记录了最近干旱对生长的影响，该项目现在可以测试遭受较大干旱影响的树木是否不太能够对厄尔尼诺提供的水作出积极反应。 研究的物种包括白杨、皮农松和杜松，这些物种在耐旱性和利用浅层与深层土壤水分的能力上存在差异。该项目的结果将加强对降水变化的理解，预计降水变化将随着气候变暖而增加，影响西南部生态系统的生产力。 本研究将为多名本科生提供研究经验，包括来自代表性不足群体的个人。目前的厄尔尼诺现象在冬季（2015年10月至2016年2月）在四角地区产生了罕见的降水梯度，跨度为长期平均值的25%至400%以上。四年前，2011-2012年的干旱严重影响了树木数量，土壤水分供应和生态系统碳固存。最近的研究表明，树木生长、生态系统碳通量和净生态系统生产力需要3-5年才能从过去的严重干旱中恢复过来。 目前尚不清楚的是，过去干旱的长期影响是否会影响树木和生态系统对厄尔尼诺造成的降水极端增加的反应。因此，该项目旨在确定过去的气候条件，特别是干旱，在潮湿时期树木的生理和生长反应的程度。 为了实现这一目标，该项目将对代表四角地区气候变化的多个美国林务局森林清查（FIA）站点进行采样，并提供补充的树木和地块数据。这些网站支持三个焦点，基础树种（白杨，皮农，杜松），不同的耐旱策略。这项研究将采用重复的生理测量，树木年代学方法和统计建模的独特组合，这将使新的洞察气候遗产，其对树木生理过程的影响，以及随后对树木生长的影响，在潜在的？有利？条件这项研究将在相对较短的时间内生成一个独特而广泛的数据集，可用于测试高降水量显着刺激树木生长的天真假设。预计将证明树木生长的反应，罕见的潮湿年控制过去的气候影响。实地活动和实验室研究将涉及多个本科生，一个早期的职业研究助理，并将寻求涉及来自代表性不足的群体的个人。研究结果将通过科学文献和大众媒体传播;树木年轮数据将提供给可免费查阅的国际树木年轮数据库。

The U.S. Southwest is an arid region that experiences large interannual variation in precipitation. The region has experienced a series of severely dry years in the past two decades, most recently in 2012. However in an El Nino year, such as 2016, winter precipitation can be several times greater than average. Drought years increase mortality in forest and woodland trees, and the trees that survive drought require a period of several years before regaining normal growth capacity. Although high winter precipitation generally promotes better growth, it may be the case that a tree's ability to take advantage of a good year depends on its drought history. This is because the anatomical and physiological changes in trees that survive seem to reduce a tree?s ability to transport water and support high rates of photosynthesis and new growth. This project will use a set of study sites in the Four-Corners region of the southwest that span a wide range of increased winter precipitation from the current El Niño. Because previous studies at these same sites have documented the growth impacts of recent droughts, this project can now test whether trees that suffered larger drought impacts are less able to respond positively to the increased water provided by the El Niño. The study species include aspen, piñon pine, and juniper, species that differ in drought tolerance and ability to use shallow versus deeper soil moisture. The results from this project will strengthen understanding of how precipitation variability, which is expected to increase with continued climate warming, affects productivity of southwestern ecosystems. This study will provide research experience for multiple undergraduate students including individuals from underrepresented groups.The current El Niño has produced a rare gradient in winter (Oct 2015-Feb 2016) precipitation across the Four-Corners region, spanning 25% to over 400% of the long-term mean. Four years prior, the 2011-2012 drought significantly and negatively impacted tree populations, soil moisture availability, and ecosystem carbon sequestration. Recent work has demonstrated that it takes 3-5 years for tree growth, ecosystem carbon fluxes, and net ecosystem productivity to recover from past, severe droughts. What is unknown is whether these prolonged impacts of past droughts affect how trees and ecosystems respond to extreme increase in precipitation created by an El Niño. Thus, this project seeks to determine the extent to which past climatic conditions, especially droughts, govern tree physiological and growth responses during wet periods. To accomplish this, the project will sample multiple U.S. Forest Service forest inventory (FIA) sites representative of the climate variability in the Four-Corners region and for which complementary tree- and plot-level data are available. These sites support three focal, foundational tree species (aspen, piñon, juniper) that differ in their drought tolerance strategies. The study will employ a unique combination of repeated physiological measurements, dendrochronology approaches, and statistical modeling that will enable new insight into climate legacies, their impact on tree physiological processes, and subsequent implications for tree growth during potentially ?favorable? conditions. This study will generate a unique and extensive dataset over a relatively short time period, which can be used to test the naïve assumption that high precipitation significantly stimulates tree growth. It is expected to demonstrate that tree growth responses to rare wet years are controlled by past climatic influences. Field campaigns and laboratory studies will involve multiple undergraduates, an early career research associate, and will seek to involve individuals from under-represented groups. Findings from the study will be disseminated via the scientific literature and popular media; tree-ring data will be contributed to the freely accessible International Tree-Ring Data Bank.