Water use efficiency differs for mixed and monospecific boreal forests in Sweden

Ecosystem water use efficiency (WUE) is a key characteristic that describes the coupling of carbon and water exchange and can be used as an indicator of a forest's adaptability to varying climatic conditions. Mixed forests, characterized by the coexistence of two or more dominant tree species, may potentially exhibit higher productivity and greater resistance to extreme weather events due to possible niche differentiation among dominant species, leading to more efficient nutrient utilization. However, the increased productivity may also result in higher evapotranspiration demand, resulting in lower WUE compared to monospecific forests. 
In this study, we aim to assess the variation in WUE of mixed and monospecific boreal forests in response to different environmental factors using eddy-covariance measurements. The two study sites are represented by forest stands of similar age, growing under the same climatic conditions and located in close proximity (~10km distance) in Northern Sweden. The Rosinedalsheden site is a ~100-year-old monospecific pine (Pinus sylvestris) forest stand with sandy soils. The Svartberget site is a mixed ~110-year-old forest featuring pine (Pinus sylvestris, 61%), spruce (Picea abies, 34%), and birch (Betula sp., 5%) species, with soils dominated by till and sorted sediments. Our study spans a period of seven years (2014-2020) and covers a wide range of weather conditions, including the 2018 heatwave.