Response of water use efficiency to summer drought in boreal Scots pine forests in Finland

Abstract. The influence of drought on plant functioning has received considerable attention in recent years, although our understanding of the response of carbon and water coupling in terrestrial ecosystems remains unclear. In this study, we investigated the response of water use efficiency to summer drought in boreal forests at daily time scales mainly using eddy covariance flux data. In addition, simulation results from the JSBACH land surface model were evaluated against the observed results. Two Scots pine (Pinus sylvestris) sites at Hyytiälä (southern Finland) and Sodankylä (northern Finland) were used in the study. Based on observed data, the ecosystem level water use efficiency (EWUE) showed a decrease only during a severe soil moisture drought at Hyytiälä, whereas the inherent water use efficiency (IWUE) increased when there was a severe soil moisture drought at Hyytiälä and a moderate soil moisture drought at Sodankylä. This indicates a decrease in surface conductance at the ecosystem level, but the decrease in evapotranspiration (ET) was alleviated because of the increased vapor pressure deficit (VPD) during drought. Moreover, the changes in IWUE implied that Scots pine has weaker response to drought in the southern site than in the northern site. Thus, IWUE is a more appropriate metric than EWUE for capturing the impact of soil moisture drought on plant functioning at daily time scales. In general, the results from transpiration based ecosystem level water use efficiency (EWUEt) and IWUE, and the transpiration based inherent water use efficiency (IWUEt) from JSBACH simulations were similar to the observed results. The deviated groups of gross primary production (GPP) and evapotranspiration (ET) under severe soil moisture drought in observed data at Hyytiälä were also successfully captured in the simulated results. However, deficiencies in the model were clearly seen by the limitation effect of air humidity on stomatal conductance in observed data. Our study provides a deeper understanding of carbon and water dynamics in the major boreal ecosystem. These findings highlight the importance of choosing a suitable plant functioning indicator when investigating the effects of drought, and suggest possible improvements to land surface models, which play an important role in the prediction of biosphere-atmosphere feedbacks in the climate system.