On the inter‐ and intra‐annual variability of ecosystem evapotranspiration and water use efficiency of an oak savanna and annual grassland subjected to booms and busts in rainfall,Global Change Biology

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On the inter‐ and intra‐annual variability of ecosystem evapotranspiration and water use efficiency of an oak savanna and annual grassland subjected to booms and busts in rainfall
Global Change Biology ( IF 10.8 ) Pub Date : 2020-10-22 , DOI: 10.1111/gcb.15414
Dennis Baldocchi ₁ , Siyan Ma ₁ , Joe Verfaillie ₁

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Whether annual evapotranspiration of native ecosystems is increasing or decreasing with time as CO₂ concentrations are rising, the climate is warming and rainfall experiences booms and busts, remains an unanswered question in the field of global change biology. To answer this question, we measured evapotranspiration and carbon dioxide exchange over and under an oak savanna and over an annual grassland in the Mediterranean climate of California, USA, from 2001 through 2019 with the eddy covariance method; during this 19‐year period, CO₂ rose 40 ppm, air temperature increased by 1°C and annual rainfall ranged between 133 and 890 mm/year. No temporal trend in evapotranspiration or water use efficiency was observed over this time duration. Many competing positive and negative feedbacks among stomatal sensitivity to carbon dioxide concentrations, soil moisture, and vapor pressure deficit, the impact of temperature on saturation vapor pressure and access to groundwater muted the response of evapotranspiration to its changing world when integrated to the ecosystem scale and annual time steps. At the intra‐annual time scale, we found that plants transmit information on soil moisture status through their influence on the vapor pressure deficit of the atmospheric boundary layer. The inter‐annual variations in evaporative water use by the savanna and annual grassland were relatively decoupled from the booms and busts in rainfall. Instead, variations in length of growing season and access to groundwater explained much of this year‐to‐year variation in annual evapotranspiration. The access of groundwater by the oak savanna may make these ecosystems more robust in a warmer world, than was previously thought. This is a scale emergent property that needs better consideration in coupled climate‐ecosystem models.

中文翻译：

降雨过程中起伏不定的橡树大草原和一年生草地生态系统蒸散量和年水分利用效率的年际和年内变化

随着CO ₂浓度的增加，本地生态系统的年度蒸散量是随着时间增加还是减少，气候正在变暖，降雨经历了起伏，仍然是全球变化生物学领域尚未解决的问题。为了回答这个问题，我们使用涡度协方差法测量了美国加州地中海气候在2001年至2019年期间在橡树稀树草原上和下面以及每年草地上的蒸散量和二氧化碳交换。在这19年期间，CO ₂上升40 ppm，气温上升1°C，年降雨量介于133至890毫米/年之间。在这段时间内没有发现蒸散或水利用效率的时间趋势。当气孔对二氧化碳浓度，土壤湿度和蒸气压亏缺的敏感性，温度对饱和蒸气压的影响以及对地下水的获取之间存在许多相互竞争的正反馈和负反馈，这些误差在与生态系统规模和生态系统尺度结合后，使蒸散对其不断变化的世界的响应减弱了。年度时间步长。在年内时间尺度上，我们发现植物通过影响大气边界层的蒸气压亏缺而传递有关土壤水分状况的信息。稀树草原和一年生草地的蒸发水使用量年际变化与降雨的高峰和萧条相对不相关。取而代之的是，生长季节长度的变化和地下水的获取解释了每年蒸散量的逐年变化。橡树稀树草原获得的地下水可能使这些生态系统在一个温暖的世界中比以前想象的更健壮。这是一个规模新兴属性，在气候-生态系统耦合模型中需要更好地考虑。

更新日期：2020-12-15

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