Recent analyses on the length of drought recovery in forests have shown multi-year legacies, particularly in semi-arid, coniferous ecosystems. Such legacies are usually attributed to ecophysiological memory, although drought frequency itself, and its effect on overlapping recovery times, could also contribute. Here, we describe a multi-decadal study of drought legacies using tree-ring carbon-isotope ratios (δ13C) and ring-width index (RWI) in Pinus ponderosa at 13 montane sites traversing a winter-summer precipitation gradient in the Southwestern U.S. Sites and trees were selected to avoid collection biases that exist in archived tree-ring databanks. The spatial hydroclimate gradient and winter-summer seasonal patterns were well predicted by seasonal and inter-annual correlations between δ13C and atmospheric vapor pressure deficit (VPD). Using VPD, we found that the probability of extreme drought has increased up to 70% in this region during the past two decades. When the recent increase in drought frequency was not considered, multi-year legacies in both δ13C and RWI were observed at most sites. When the increase in drought frequency was detrended from tree-ring chronologies, some sites exhibited short legacies (1-2 years) in both δ13C and RWI, and there was a sight trend for longer legacies in RWI. However, when considered broadly across the region and multiple decades, no significant legacies were observed, which contrasts with past studies. Our results reveal that a contribution to observed multi-year legacies is related to shifts in the climate system itself, an exogenous factor, that must be considered along with physiological memory.

中文翻译：

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最近干旱频率的增加导致在半干旱森林的树木年轮中观察到多年干旱的遗留物。

最近对森林干旱恢复时间的分析显示了多年的遗产，特别是在半干旱，针叶生态系统中。这些遗产通常归因于生态生理记忆，尽管干旱频率本身及其对重叠恢复时间的影响也可能有所贡献。在这里，我们使用美国西南部站点的13个山地站点的冬青降水梯度，使用树轮碳同位素比（δ13C）和环宽指数（RWI）在美国黄松中进行了数十年的干旱遗留研究选择树和树是为了避免在已归档的树环数据库中存在收集偏差。通过δ13C和大气蒸气压亏缺（VPD）之间的季节和年际相关性，可以很好地预测空间水气候梯度和冬季和夏季的季节性模式。使用VPD，我们发现，在过去的二十年中，该地区发生极端干旱的可能性增加了70％。当不考虑近期干旱频率的增加时，在大多数地点都观察到了δ13C和RWI的多年遗留物。当干旱频率的增加与树木年轮的时间变化趋势相抵触时，一些站点的δ13C和RWI都表现出较短的遗留期（1-2年），并且RWI的遗留期呈明显趋势。但是，当在整个地区和数十年中广泛考虑时，没有观察到重大遗产，这与以往的研究形成了鲜明对比。我们的研究结果表明，对观察到的多年遗产的贡献与气候系统本身的变化有关，气候系统本身是一个外在因素，必须与生理记忆一起考虑。