Data from: Water use by Swedish boreal forests in a changing climate

The rising levels of atmospheric carbon dioxide concentration ([CO2]) and temperature have the potential to substantially affect the terrestrial water and energy balance by altering the stomatal conductance and transpiration of trees. Many models assume decreases in stomatal conductance and plant water use under rising [CO2], which has been used as a plausible explanation for the positive global trend in river run-off over the past century. Plant water use is, however, also affected by changes in temperature, precipitation and land use, and there is yet no consensus about the contribution of different drivers to temporal trends of evapotranspiration (ET) and river run-off. In this study, we assessed water-use responses to climate change by using both long-term monitoring and experimental data in Swedish boreal forests. Historical trends and patterns in ET of large-scale boreal landscapes were determined using climate and run-off data from the past 50 years, while explicit tree water-use responses to elevated [CO2] and/or air temperature were examined in a whole-tree chamber experiment using mature Norway spruce (Picea abies (L.) Karst.) trees.
The results demonstrated that ET estimated from water budgets at the catchment scale increased by 18% over the past 50 years while run-off did not significantly change. The increase in ET was related to increasing precipitation and a steady increase in forest standing biomass over time. The whole-tree chamber experiment showed that Norway spruce trees did not save water under elevated [CO2] and that experimentally elevated air temperature did not increase transpiration as decreased stomatal conductance cancelled the effect of higher vapour pressure deficit in warmed air. Our findings have important implications for projections of future water use of European boreal coniferous forests, indicating that changes in precipitation and standing biomass are more important than the effects of elevated [CO2] or temperature on transpiration rates.

大气二氧化碳浓度([CO₂])与气温的持续上升，可通过改变树木的气孔导度与蒸腾作用，显著影响陆地水热平衡。诸多模型假设，在二氧化碳浓度升高的情境下，气孔导度与植物水分利用会出现下降，这一假设被用作解释过去一个世纪全球河流径流量正向增长趋势的合理依据。然而，植物水分利用同时也受气温、降水与土地利用变化的影响，目前学界对于不同驱动因子对蒸散发(ET)与河流径流量时间趋势的贡献尚未达成共识。本研究依托瑞典北方森林的长期监测与实验数据，评估了水分利用对气候变化的响应：借助过去50年的气候与径流数据，明确了大型北方景观蒸散发的历史趋势与格局；同时通过整树箱实验，针对成熟挪威云杉（Picea abies (L.) Karst.）开展研究，探究其在二氧化碳浓度升高和/或气温升高情境下的树木水分利用响应。研究结果显示，基于集水区尺度水量平衡估算的蒸散发，在过去50年间提升了18%，而径流量未出现显著变化；蒸散发的增加与降水量上升以及林分生物量随时间的稳步增长密切相关。整树箱实验结果表明，挪威云杉在二氧化碳浓度升高的环境下并未实现水分节约，而实验性升温并未提升蒸腾作用——这是因为气孔导度的下降抵消了升温空气中更高水汽压亏缺带来的影响。本研究发现对欧洲北方针叶林未来水分利用的预测具有重要启示意义，表明降水与林分生物量的变化，相较于二氧化碳浓度升高或气温对蒸腾速率的影响，更为关键。