Stand composition modulates water use efficiency in adjacent mature boreal forests

Boreal forests play a key role in the global carbon and water cycles, with water use efficiency (WUE) linking their carbon gain and water loss. However, the variation in WUE among contrasting boreal forests and its response to environmental conditions remains poorly understood. We utilised six years (2015–2020) of eddy-covariance data to compare the ecosystem (EWUE), canopy (WUEc), and underlying (UWUE) WUE of two adjacent mature boreal forests: a monoculture pine stand and a mixed spruce-pine stand growing under the same climatic conditions in Northern Sweden. Our results suggest that the mixed forest exhibited lower EWUE, WUEc, and UWUE than the pine monoculture, driven by disproportionately higher evapotranspiration. This difference diminished as atmospheric dryness increased. Daily WUE variability was primarily controlled by solar radiation and vapour pressure deficit, with no functional difference between the two forest stands. Mixed forest EWUE was more sensitive to soil moisture variability, with no difference in canopy WUE, highlighting the role of soil evaporation and higher water availability in the mixed stand. The mixed forest exhibited stronger WUE responses to drought, with both increases and decreases depending on drought intensity, reflecting a more reactive adjustment of carbon gain relative to water loss. In contrast, the pine monoculture maintained comparatively stable WUE across wide drought conditions, indicating a more conservative strategy that sustains ecosystem functioning under atmospheric water limitation. Our findings demonstrate that forest composition and structure strongly influence WUE, with important implications for predicting boreal forest resilience under different management and climate change scenarios.