Partition evapotranspiration into transpiration and evaporation in a natural boreal larch forest in China: A framework from concurrent eddy covariance systems

Abstract

Partitioning forest ecosystem evapotranspiration (ET) into transpiration (T) and evaporation (E) has been widely discussed, as it is crucial for understanding hydrological processes and functions. However, quantifying the hydrological importance of the understory ecosystem is limited, particularly in China’s boreal forests. In this study, we first employed a framework with concurrent eddy covariance (EC) systems and the underlying water use efficiency (uWUE) method to partition ET in a natural boreal larch forest (Larix gmelinii) of China during the growing seasons of 2020 to 2022. The results showed that understory transpiration (Tu) and overstory transpiration (Tos) contributed 21.8 and 78.2% to T, respectively, while the contributions of understory evaporation (Eu) and overstory evaporation (Eos) to E were 41.2 and 58.8%, respectively. Understory evapotranspiration (ETu) contributed 32.6% to the total ET, suggesting the considerable importance of understory vegetation to water flux in the boreal larch forest. Interestingly, Tos and Eos had a similar contribution to ET. While the controlling mechanisms in Tu/T, Eu/E, and ETu/ET were more complex than those in Tu, Eu, and ETu, the leaf area index (LAI) of the overstory and solar radiation (SR) played greater roles in regulating each component and its respective ratios. Those results highlight the non-negligible role of understory vegetation in hydrological processes in boreal forests, which must be considered when managing or modelling forest-water relationships. Our study also demonstrates the effective utility of concurrent EC systems in partitioning ET in natural boreal forest ecosystems.