Plantation expansion reshapes water and carbon patterns in Northeast China's forests: A high carbon sink at the cost of inefficient water use

Climate warming is reshaping forest ecosystems by altering both carbon and water cycles, yet it remains poorly understood whether rapid plantation expansion can match the ecological efficiency of natural forests. This knowledge gap is critical, given global afforestation efforts amidst escalating climate stress. Here, we challenge the simplistic ‘high carbon gain–high water demand’ paradigm by positing that plantations and natural forests operate under fundamentally distinct biogeochemical regimes. Across Northeast China (2005–2020), we integrated multi-source remote sensing and meteorological data to quantify gross primary productivity (GPP), evapotranspiration (ET), and water-use efficiency (WUE). Both forest types increased in GPP, ET, and WUE, yet plantations consistently showed higher GPP and ET but lower WUE. In long-term stable stands, mean WUE was 1.84 g C m −2 mm −1 in natural forests, higher than 1.80 g C m −2 mm −1 in plantations. Mean GPP was 1067.72 g C m −2 in plantations, exceeding 965.42 g C m −2 in natural forests, and its annual increase was markedly greater in plantations (6.04 vs. 5.63 g C m −2 yr −1 ). ET increased significantly in plantations (1.89 mm yr −1 ) but not in natural forests. Structural equation and random forest models revealed contrasting association patterns: stand structural and compositional factors made the largest contribution to WUE in both natural and planted forests, accounting for 60.9% and 52.7% of the total normalized importance, respectively, while soil factors also played an important role in planted forests, contributing 30.1%. These results indicate that plantation expansion in temperate humid forests can enhance carbon uptake, but often was associated with greater water consumption and lower WUE.