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首页> 外文期刊>Global change biology >Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses? [Review]
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Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses? [Review]

机译：地中海三个常绿站点上的严重干旱对生态系统CO2和H2O通量的影响：当前假设的修订？ [评论]

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Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top-down approaches in conjunction with a mechanistic ecosystem gas-exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO2/H2O exchange. The three sites include two nearly monospecific Quercus ilex L. forests - one on karstic limestone (Puechabon), the other on fluvial sand with access to ground water (Castelporziano) - and a typical mixed macchia on limestone (Arca di Noe). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO2 exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO2 fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman-Monteith equation. In a top-down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity (Q(10)) in response to drought. Soil temperature and soil water content explained 70-80% of the seasonal variability of ecosystem respiration. During the drought, light-saturated ecosystem gross carbon uptake and day-time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water-use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water-use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO2, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.
机译：通过自上而下的方法，结合机械化的生态系统气体交换模型，分析了来自三个地中海生态系统的涡流协方差和树汁流量数据，以检验当前有关干旱对生态系统呼吸和冠层CO2 / H2O交换的影响的假设。这三个地点包括两个近乎单种的栎属森林-一个位于喀斯特石灰岩（Puechabon）上，另一个位于可接触地下水的河滩上（Castelporziano）-以及典型的石灰岩混合玛奇（Arca di Noe）。生态系统呼吸的估计值来自净生态系统二氧化碳交换的光响应曲线。随后，根据涡度协方差CO2通量和生态系统呼吸随土壤温度和湿度的函数估算出生态系统总碳吸收量。通过Penman-Monteith方程的反演计算大树冠电导。从上至下的分析表明，就干旱的总体响应而言，所有三个地点都表现出相似的行为。与通常的假设相反，在所有地点，生态系统呼吸都显示出响应干旱而降低的温度敏感性（Q（10））。土壤温度和含水量解释了生态系统呼吸的季节性变化的70-80％。在干旱期间，光饱和的生态系统的总碳吸收量和日平均冠层电导率下降了90％。这些变化与土壤含水量密切相关。无论是否使用涡流协方差的蒸散量或树液流量的蒸腾量，干旱期间的生态系统水分利用总碳吸收效率都会降低。我们证明，这与目前的树冠功能模型形成鲜明对比，后者预测干旱期间生态系统的水分利用效率（WUE）会增加。确定了对这些结果的四个可能的解释（气孔闭合，光合作用的生理能力变化，对CO2的叶肉电导降低和光抑制），这些将在即将发表的论文中进行测试。建议进一步测试后将新发现纳入当前的生物地球化学模型中，因为这将改善对（半）干旱生态系统碳平衡的气候变化影响的估计。

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