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，降水年际变化很大。在过去20年里，该地区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几倍。干旱年份会增加森林和林地树木的死亡率，而在干旱中幸存下来的树木需要几年的时间才能恢复正常的生长能力。虽然冬季降雨量大通常会促进更好的生长，但情况可能是这样的，一棵树利用好年份的能力取决于它的干旱历史。这是因为幸存下来的树木的解剖和生理变化似乎降低了树木的运输水分、支持高光合作用和新生长的能力。该项目将使用西南部四角地区的一组研究地点，这些地点覆盖了当前厄尔尼诺现象导致的冬季降水增加的广泛范围。由于之前在这些地点进行的研究记录了最近干旱对生长的影响，因此该项目现在可以测试遭受更大干旱影响的树木是否更难对厄尔尼诺提供的更多水分做出积极反应。研究物种包括白杨、皮尼松和刺柏，这些物种的抗旱性和利用浅层土壤水分和深层土壤水分的能力不同。该项目的结果将加强对降水变异性如何影响西南生态系统生产力的理解，预计降水变异性将随着气候的持续变暖而增加。这项研究将为包括来自代表性不足群体的个人在内的多名本科生提供研究经验。当前的厄尔尼诺现象在四角地区的冬季(2015年10月至2016年2月)产生了罕见的降水量梯度，跨度为长期平均值的25%至400%。四年前，2011-2012年的干旱对树木种群、土壤水分有效性和生态系统碳固定产生了显著的负面影响。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年的时间才能从过去的严重干旱中恢复过来。未知的是，过去干旱的这些长期影响是否会影响树木和生态系统对厄尔尼诺现象造成的降水极端增加的反应。因此，该项目试图确定过去的气候条件，特别是干旱，在多大程度上支配着树木在雨季的生理和生长反应。为了实现这一目标，该项目将对代表四角地区气候变异性的多个美国林业局森林调查(FIA)站点进行抽样，并提供补充的树木和地块级别的数据。这些地点支持三个重点的基础树种(白杨、皮尼翁、刺柏)，它们的耐旱性策略不同。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，使人们能够对气候遗产、它们对树木生理过程的影响以及在潜在有利时期对树木生长的后续影响有新的见解。条件。这项研究将在相对较短的时间内产生一个独特而广泛的数据集，可以用来检验高降雨量显著刺激树木生长的天真假设。预计这将证明树木对罕见的潮湿年份的生长反应受过去气候影响的控制。实地活动和实验室研究将涉及多名本科生、一名早期职业研究助理，并将寻求让代表性不足群体的个人参与。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.