Forest water-use efficiency: Effects of climate change and management on the coupling of carbon and water processes

Forests are essential in regulating global carbon and water cycles and are critical in mitigating climate change. Water-use efficiency, defined by the ratio of plant productivity per unit water use, is widely used to quantify the interactions between forest carbon and water cycles and could be potentially used to manage the carbon and water tradeoffs of forests under different environmental conditions. This paper reviews the literature on how biophysical variables and management practices affect forest water-use efficiency. We found that water-use efficiency varies greatly with forest type, species, age, environmental conditions, and forest management practices. Climatic stresses (e.g., drought and heatwave) often pose negative effects on forest instantaneous water-use efficiency (WUEins), particularly over a short term. Unexpectedly, plantations and natural forests have no statistical differences in WUEins. In addition, WUEins can be effectively improved by forest thinning. These results have important implications for managing the tradeoffs between carbon sequestration and water yield of forests. Finally, four important knowledge gaps, including species-specific water-use efficiency, long-term forest water-use efficiency dynamics, water-use efficiency responses to forest management, and the integrated effects of human and natural disturbances on plantation water-use efficiency are identified and discussed.