Groundwater can have a critical role in sustaining the functioning of natural ecosystems during droughts, especially in dry and seasonally dry climates. However, the response to droughts of ecosystems embedded in urban areas is not well known. This study investigates how different scenarios of groundwater availability control the water balance and vegetation productivity of two urban reserves hosting native vegetation in the Melbourne metropolitan area, Australia. Using a mechanistic ecohydrological model supported by field observations, long-term simulations were run to explore the impact of groundwater flow on water, carbon, and energy fluxes under present climatic conditions, including the Millennium Drought (2001–2009), and in response to perturbations in key environmental variables (air temperature, atmospheric CO2 concentrations, and rainfall). It was found that the presence of a water table and its capillary fringe within the root depths supports ecosystem transpiration and vegetation productivity. The effects of declining groundwater were found to be more severe in predominantly sandy soils because of the lower water holding capacity, identifying that the water status of vegetation differs significantly depending on soil type. Differences in rooting strategies and groundwater availability also had a pivotal role in helping plants soften the impacts of increased air temperature (Ta) and make use of higher atmospheric CO2 concentrations. Increased Ta strongly affected evapotranspiration, enhancing the competition for water between different vegetation types. These results provide quantitative insights of how vegetation responds to groundwater depletion and climate variability, highlighting the essential role of groundwater resources in urban ecosystems characterized by seasonally dry climates.

中文翻译：

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地下水缓冲了城市保护区的干旱影响和气候变化

在干旱期间，尤其是在干旱和季节性干旱气候中，地下水在维持自然生态系统的功能方面可以发挥关键作用。然而，城市地区的生态系统对干旱的反应并不为人所知。本研究调查了不同的地下水可用性情景如何控制澳大利亚墨尔本大都市区拥有原生植被的两个城市保护区的水平衡和植被生产力。使用由实地观测支持的机械生态水文模型，进行了长期模拟，以探索在当前气候条件下，包括千年干旱 (2001-2009)，以及对地下水流对水、碳和能量通量的影响，并响应关键环境变量（气温、大气 CO2 浓度、和降雨）。发现地下水位及其根部深度内的毛细管边缘的存在支持生态系统蒸腾作用和植被生产力。发现地下水下降的影响在主要是沙质土壤中更为严重，因为其持水能力较低，这表明植被的水分状况因土壤类型而异。生根策略和地下水可用性的差异在帮助植物减轻气温 (Ta) 升高的影响和利用更高的大气 CO2 浓度方面也起着关键作用。Ta 的增加强烈影响蒸发蒸腾，增强了不同植被类型之间的水分竞争。