Water, carbon, and light use efficiencies in an old hemiboreal coniferous forest: nine-year patterns under hydroclimatic variability

Abstract. Hemiboreal forests bridge boreal and temperate biomes by combining functional and compositional features of both and playing a key role in regional carbon and water cycling. Water (WUE), carbon (CUE), and light (LUE) use efficiencies provide integrative indicators of how effectively ecosystems convert available resources into carbon uptake, yet their long-term dynamics and controlling factors remain poorly explained in hemiboreal forests. We analysed nine consecutive growing seasons (2016–2024) of eddy covariance measurements from an old upland hemiboreal coniferous forest in southern Estonia to quantify WUE, CUE, and LUE and to identify their controls at daily and growing-season scales and along a standardized precipitation-evapotranspiration index (SPEI) defined hydroclimatic gradient. Growing-season air temperature and vapour pressure deficit (VPD) increased over the study period. Despite this trend, WUE remained generally stable across years, with only one deviating growing season (2022) linked to intensified carbon uptake over a shorter season length. In contrast, CUE exhibited pronounced variability among growing seasons, driven primarily by changes in net ecosystem production and respiration dynamics. LUE was remarkably stable and showed no indication of age-related decline. At the daily scale, VPD controlled WUE and LUE, whereas photosynthetically active radiation exerted dominant control over CUE. Along the hydrometeorological gradient, all three resource use efficiencies responded non-linearly, but WUE and LUE remained generally stable, while CUE was the most variable metric and reflected shifts in the balance between carbon uptake and respiratory losses. Together, these results reveal differentiated sensitivities of ecosystem efficiencies to atmospheric drying and identify CUE as the most responsive indicator of hydroclimatic variability. Our findings provide new insight into the functional stability and potential thresholds of hemiboreal coniferous forests undergoing climate change.