Riparian woodlands occupy a small area of global drylands but are hotspots for carbon and water cycling because groundwater supplements a small moisture supply from precipitation (P). Despite their regional importance, it is unclear if and how climate variability alters water vapor and carbon dioxide (CO) fluxes in these ecosystems, and how ecosystem drivers vary across annual and seasonal scales. Here we use 21 years of eddy covariance measurements to understand land-atmosphere controls on water vapor and CO fluxes of a semiarid riparian mesquite () woodland with year-round access to deep groundwater and highly variable summer and winter rainfall. Access to groundwater supplemented evapotranspiration (ET) that exceeded precipitation (ET:P range 1.9–5.4), making this riparian forest a substantial CO sink (367 ± 83 g C myr). Contrary to general expectations of regional climatic drying, there was a shift to more favorable water conditions as P and soil moisture increased over time. Annual gross ecosystem production (GEP) and respiration (R) increased at the same rate (∼9 g C myr) due to GEP and R increases during the wettest periods of the year. Growth year separation based on GEP phenology and regression models show that water availability and antecedent phenophase fluxes control seasonal ET and GEP, with CO fertilization detected only during winter dormancy, the least-active phenophase. The major R driver during the spring and summer was GEP, and this coupling intensified following the onset of summer rainfall. Groundwater subsidies support ET during the dry growing season and decouple water vapor and CO fluxes. These results highlight the dynamic nature of water vapor and CO cycling in semiarid riparian woodlands, the value of groundwater subsidies to buffering ecosystem responses from the interannual effects of climate variability, and the importance of two water sources in driving seasonal ecosystem responses.

中文翻译：

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二十年来河岸林地水蒸气和二氧化碳通量对环境变化的响应


河岸林地仅占全球旱地的一小部分，但却是碳和水循环的热点，因为地下水补充了降水（P）的少量水分供应。尽管它们具有区域重要性，但目前尚不清楚气候变化是否以及如何改变这些生态系统中的水蒸气和二氧化碳（CO）通量，以及生态系统驱动因素在年度和季节尺度上如何变化。在这里，我们使用 21 年的涡度协方差测量来了解陆地-大气对半干旱河岸豆科林地水蒸气和二氧化碳通量的控制，该林地全年都有深层地下水，夏季和冬季降雨量变化很大。地下水的利用补充了超过降水量的蒸散量 (ET)（ET:P 范围 1.9–5.4），使这片河岸森林成为大量的二氧化碳汇（367 ± 83 g C myr）。与区域气候干燥的普遍预期相反，随着磷和土壤湿度随着时间的推移而增加，水条件变得更加有利。由于一年中最潮湿时期 GEP 和 R 的增加，生态系统年总产量 (GEP) 和呼吸 (R) 以相同的速度增加 (∼9 g C myr)。基于 GEP 物候和回归模型的生长年分离表明，可用水量和前期物候通量控制季节性 ET 和 GEP，仅在冬季休眠（最不活跃的物候期）期间检测到 CO 施肥。春季和夏季的主要 R 驱动因素是 GEP，这种耦合在夏季降雨开始后加剧。地下水补贴支持干旱生长季节的蒸散，并将水蒸气和二氧化碳通量分离。 这些结果强调了半干旱河岸林地水蒸气和二氧化碳循环的动态性质、地下水补贴对于缓冲气候变化年际影响的生态系统响应的价值，以及两种水源在驱动季节性生态系统响应中的重要性。