The North American Monsoon buffers forests against the ongoing megadrought in the Southwestern United States
The North American Monsoon buffers forests against the ongoing megadrought in the Southwestern United States
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DOI：
10.1111/gcb.16762
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发表时间：
2023-06
影响因子：
11.6
通讯作者：
B. Strange;R. Monson;P. Szejner;J. Ehleringer;Jia Hu
中科院分区：
文献类型：
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作者：
B. Strange;R. Monson;P. Szejner;J. Ehleringer;Jia Hu
美国西南部已经陷入了长达20年之久的大灭绝（MD），这是自公元800年以来最严重的一次，威胁到区域山地森林的长期活力和持久性。在这里，我们报告说，在面对创纪录的低冬季降水和大气干燥度增加，北美季风（NAM）气候系统的季节性活动带来了足够的降水在夏季的高度，以减轻极端的树木水分胁迫。我们研究了分布在NAM地理区域的17个黄松森林中57年时间序列（1960-2017）的季节性分解的树轮稳定碳同位素比率。我们的研究集中在晚材（LW），这是产生与NAM雨的同位素动力学。在MD期间，与NAM域外围生长的种群相比，NAM核心区域内生长的种群以较低的内在和较高的蒸发水利用效率（分别为WUEi和WUEE）运行，这表明那些能够获得NAM水分的种群的生理水分胁迫较小。周边地区人口用水效率的差异是由于较高的大气蒸汽压赤字（VPD）和夏季土壤水分的减少。然而，不结盟运动的缓冲优势正在减弱。我们观察到，自MD以来，WUEi和WUEE在核心NAM域的森林之间的关系正在转向干旱响应类似的森林周边的NAM。在对过去大气CO2浓度的增加进行校正后，我们能够将LW时间序列对气候的响应单独分离出来。这表明WUEi和WUEE之间关系的转变是由MD相关VPD的极端增加驱动的，大气CO2浓度的增加对气孔导度的有利影响很小。
The US Southwest has been entrenched in a two‐decade‐long megadrought (MD), the most severe since 800 CE, which threatens the long‐term vitality and persistence of regional montane forests. Here, we report that in the face of record low winter precipitation and increasing atmospheric aridity, seasonal activity of the North American Monsoon (NAM) climate system brings sufficient precipitation during the height of the summer to alleviate extreme tree water stress. We studied seasonally resolved, tree‐ring stable carbon isotope ratios across a 57‐year time series (1960–2017) in 17 Ponderosa pine forests distributed across the NAM geographic domain. Our study focused on the isotope dynamics of latewood (LW), which is produced in association with NAM rains. During the MD, populations growing within the core region of the NAM operated at lower intrinsic and higher evaporative water‐use efficiencies (WUEi and WUEE, respectively), compared to populations growing in the periphery of the NAM domain, indicating less physiological water stress in those populations with access to NAM moisture. The disparities in water‐use efficiencies in periphery populations are due to a higher atmospheric vapor pressure deficit (VPD) and reduced access to summer soil moisture. The buffering advantage of the NAM, however, is weakening. We observed that since the MD, the relationship between WUEi and WUEE in forests within the core NAM domain is shifting toward a drought response similar to forests on the periphery of the NAM. After correcting for past increases in the atmospheric CO2 concentration, we were able to isolate the LW time‐series responses to climate alone. This showed that the shift in the relation between WUEi and WUEE was driven by the extreme increases in MD‐associated VPD, with little advantageous influence on stomatal conductance from increases in atmospheric CO2 concentration.