Short-Term Warming Effects on Leaf Photosynthesis and Transpiration of Picea koraiensis Saplings

Abstract

Rising temperatures across the cold-temperate forests of Northeast China are reshaping the carbon–water exchange of the young conifer plantations that now dominate the region, yet the leaf-level responses of Picea koraiensis Nakai, the principal afforestation species of the Greater Khingan Mountains, have not been characterised under field warming. We exposed 10-year-old saplings to four open-top chamber treatments (T0, ambient; T1, +0.45 °C; T2, +1.59 °C; T3, +2.15 °C) in 12 plots during the 2024 growing season and tracked needle gas exchange alongside the physiological and environmental factors that govern it. Net assimilation (A) and transpiration (E) both followed a single-peaked diurnal course, rising with irradiance and needle temperature to a mid-afternoon maximum near 14:00 and following the seasonal march of light and warmth from June to a peak in early August. Warming raised A at every level applied, with T1, T2 and T3 all exceeding T0 (2.23, 2.29 and 2.44 versus 1.65 µmol m−2 s−1), whereas E separated sharply only at the strongest treatment (2.63 versus 1.61 mmol m−2 s−1 in T0). Plot-level regressions identified stomatal conductance to water vapour as the strongest statistical association with A (adjusted R2 = 0.31–0.49, p < 0.001), with needle temperature and air temperature entering as secondary terms, while stomatal conductance to water vapour and leaf vapour pressure deficit led the models for E (adjusted R2 = 0.34–0.53, p < 0.001), pointing to a response organised around atmospheric demand. Warming of +1.59 °C raised the carbon return per unit of transpired water, whereas +2.15 °C lowered it below the control value, so the carbon–water trade-off in this species depends on the magnitude of the temperature increase. These results quantify how growing-season warming of 0.45–2.15 °C shifts the photosynthetic and transpirational performance of P. koraiensis saplings and supply leaf-level parameters for evaluating the water cost of carbon gain in the expanding plantations of the Greater Khingan Mountains.