Abstract
<title>Abstract</title> <p>Leaf and woody biomass in boreal forests are often expected to increase together under wetter conditions, yet whether their responses remain coupled under hydroclimatic change remains unclear. Here, we combined multi-source remote sensing products to quantify changes in leaf biomass and woody biomass across Eurasian boreal deciduous forests. We found woody biomass increased across large regions, whereas leaf biomass declined. This divergence was driven by a shift in carbon allocation from leaves to woody tissues, indicating that recent vegetation changes reflect a reorganization of aboveground carbon partitioning rather than uniform biomass accumulation. Increasing precipitation was the dominant climatic factor associated with this allocation shift. Under wetting conditions, evapotranspiration and carbon gain per unit leaf biomass increased, whereas ecosystem-scale water-use efficiency declined, showing that woody biomass accumulation was supported by greater water use and higher leaf-specific carbon gain rather than improved water-use efficiency. However, this wetting-associated carbon storage benefit was accompanied by greater drought-induced carbon loss. Regions experiencing stronger long-term wetting showed larger carbon losses following drought events. These findings reveal an allocation-mediated trade-off in boreal forest responses to hydroclimatic change, highlighting that enhance woody carbon storage, but may also increase vulnerability to abrupt carbon loss under future climate extremes.</p>