期刊
HYDROLOGY AND EARTH SYSTEM SCIENCES
卷 23, 期 2, 页码 1199-1210出版社
COPERNICUS GESELLSCHAFT MBH
DOI: 10.5194/hess-23-1199-2019
关键词
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资金
- Swiss Federal Office of the Environment
- European Commission's Seventh Framework Programme (FP7/2007-2013) [290605]
- forest department of the canton AG
- forest department of the canton BE
- forest department of the canton BL
- forest department of the canton BS
- forest department of the canton GR
- forest department of the canton SO
- forest department of the canton TG
- forest department of the canton ZH
- forest department of the canton ZG
- environmental offices of Central Switzerland
Rain recharges soil water storages and either percolates downward into aquifers and streams or is returned to the atmosphere through evapotranspiration. Although it is commonly assumed that summer rainfall recharges plant-available water during the growing season, the seasonal origins of water used by plants have not been systematically explored. We characterize the seasonal origins of waters in soils and trees by comparing their midsummer isotopic signatures (delta H-2) to seasonal isotopic cycles in precipitation, using a new seasonal origin index. Across 182 Swiss forest sites, xylem water isotopic signatures show that summer rain was not the predominant water source for midsummer transpiration in any of the three sampled tree species. Beech and oak mostly used winter precipitation, whereas spruce used water of more diverse seasonal origins. Even in the same plots, beech consistently used more winter precipitation than spruce, demonstrating consistent niche partitioning in the rhizosphere. All three species' xylem water isotopes indicate that trees used more winter precipitation in drier regions, potentially mitigating their vulnerability to summer droughts. The widespread occurrence of winter isotopic signatures in midsummer xylem implies that growing-season rainfall may have minimally recharged the soil water storages that supply tree growth, even across diverse humid climates (690-2068 mm annual precipitation). These results challenge common assumptions concerning how water flows through soils and is accessed by trees. Beyond these ecological and hydrological implications, our findings also imply that stable isotopes of delta O-18 and delta H-2 in plant tissues, which are often used in climate reconstructions, may not reflect water from growing-season climates.
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