资源环境科技发展态势分析平台

Changing climate and increasing atmospheric CO2 significantly regulate forest water use efficiency (WUE). However, magnitudes of the WUE trends and underlying processes driving these patterns in two major forest types, deciduous broadleaf forests (DBFs) and evergreen needleleaf forests (ENFs), across the Northern Hemisphere remain poorly understood. We investigated the WUE trends over the past two decades using eddy covariance observations from 26 forest sites from the FLUXNET2015 data set. Our analyses revealed a greater increase in WUE in DBFs than that in ENFs. The decreased stomatal conductance (G(s)) mostly contributed to the increase in WUE in the DBFs, whereas the increased gross ecosystem productivity acted as the main trigger for the increase in WUE in the ENFs. The vapor pressure deficit substantially increased in the DBFs, triggering the decrease in G(s). In contrast, the slight CO2 fertilization and the limited stomatal constraint contributed to the increased gross ecosystem productivity in the ENFs.

Plain Language Summary Forests absorb CO2 from the atmosphere for photosynthesis and grow at the cost of water losses due to transpiration from stomata on the leaves. Atmospheric CO2 levels have increased in recent decades, and forests are expected to benefit from this elevated CO2 (eCO(2)), which enhances the photosynthetic rates and stomatal closures and thus limits water losses. These effects cause forests to more efficiently use water for CO2 absorption. Our study indicated that the WUE in deciduous broadleaf forests increased more than that in evergreen needleleaf forests. Our study found that this increased WUE in the deciduous broadleaf forests could be attributed to a reduction in stomatal conductance that was triggered by an increased vapor pressure deficit despite the eCO(2). The increase in water stress would partially inhibit the enhanced photosynthesis and water conservation resulting from eCO(2). However, increases in the photosynthetic rates in the evergreen needleleaf forests contributed more to the increased WUE, and no marked stomatal constraint on photosynthesis was observed. Our study implies that the effects of climate and eCO(2) on forest WUE are dependent on the forest type. Although eCO(2) could benefit forest growth and the WUE, climate changes in the future, especially increased water stress, might offset these benefits.