Woody encroachment of <i>Pinus sylvestris</i> var. <i>mongholica</i> reduces ecosystem multifunctionality in Hulunbuir Steppe

Abstract Pinus sylvestris var. mongholica has been extensively planted in China’s Three-North Shelter Forest Program due to its ecological benefits in sand stabilization and carbon sequestration. However, its ongoing expansion into natural habitats—and the associated risks to grassland ecosystem functioning—has received insufficient attention. Here, focusing on a representative P. sylvestris var. mongholica–grassland ecotone in Hulunbuir Steppe, we systematically quantified how woody encroachment influences grassland ecosystem multifunctionality (EMF) along a habitat gradient spanning open grassland, inter-canopy grassland (IG), and under-canopy grassland (UG). We further explored the above- and belowground mechanisms driving these responses. Our findings revealed that woody encroachment significantly reduced grassland EMF, primarily due to declines in herbaceous diversity caused by canopy shading. Soil nutrient cycling, carbon storage, and soil multifunctionality consistently declined linearly with increasing encroachment intensity, underscoring the high sensitivity of belowground processes. n-dimensional hypervolume analyses and path modeling further indicated asynchronous responses between above- and belowground ecosystem functions, with soil functional spaces in UG and IG habitats exhibiting greater similarity than herbaceous functional spaces. Regional climatic conditions indirectly intensified negative impacts on above- and belowground ecosystem functioning by promoting tree growth and strengthening stand characteristics. Our study highlights the need for early-warning monitoring and adaptive management interventions to mitigate the impacts of P. sylvestris var. mongholica encroachment and maintain grassland multifunctionality.