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 across daily and interannual scales and along a standardized precipitation-evapotranspiration index (SPEI) defined hydroclimatic gradient.

Growing-season temperature and vapour pressure deficit (VPD) increased over the study period. Despite this trend, WUE remained comparatively 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 interannual variability 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. Across the hydroclimatic gradient, CUE displayed the strongest non-linear response, indicating rapid shifts in ecosystem balance under moisture anomalies, whereas WUE remained comparatively resistant. Changes in LUE suggested a moisture optimum but high efficiency was maintained under dry extremes, likely reflecting structural and physiological adjustments within the canopy.

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.