New adaptation and mitigation strategies are needed to manage temperate forests in the context of climate change with more frequent droughts. Thinning intensification has been proposed as an adaptation strategy to improve forest resistance to drought. Partial cutting of forest stands by thinning operations mimics the natural disturbance by treefall events and can affects tree leaf litter traits by altering interactions among forest plants. The importance of plant litter traits and decomposability for C and nutrient cycling processes has been largely emphasized. However, the role of biotic interactions as drivers of intraspecific variability in litter traits remains surprisingly little studied. In this study, we used a large-scale, multi-site network of long-term tree removal experiments manipulating the abundance of a foundation tree species, i.e. Quercus petraea, to assess how plant interactions control intraspecific variation in oak leaf litter traits and decomposability. We studied 19 plots across eight experimental sites covering a large gradient of oak abundance, stand age and local abiotic context. Oak leaf litter quality strongly declined with tree removal in early forest successional stage. Litter became poorer in nutrients such as N and Mg and richer in secondary metabolites such as lignin and condensed tannins. Variance partitioning indicated that oak abundance explained as much variation in oak leaf litter traits as oak age and twice as much as soil inherent fertility. Confirmatory path analysis revealed that the decline of oak leaf litter quality induced by tree removal was most likely driven by a shift in understory plant species composition. This response pattern could reflect the plasticity of oak leaf litter traits to the shortage of nutrient supply related to the development of understory plants competitors with higher nutrient capture and retention ability. Our data also give consistent but weaker support that the decrease in oak leaf litter quality with tree removal could be driven by alleviated competition for light among canopy trees and subsequent enhanced crown exposure to light. This plasticity in oak leaf litter traits had important consequences for ecosystem functioning. The decline of oak leaf litter quality in turn slowed its decomposition and induced a decline of the forest floor turnover. This slower forest floor dynamics largely mitigated forest floor C loss associated to reduced litterfall caused by tree removal, resulting in a weak net forest floor C loss. Interestingly, the decline of oak leaf litter quality also induced a shift of litter N loss from N release to N immobilization. Overall, our study provides evidence that biotic factors such as plant interactions are major drivers of plasticity in leaf litter traits and decomposability. This finding contributes to the emerging view that phenotypic plasticity is fundamentally related to biotic interactions for sessile organisms, especially for long-lived and large plant species such as trees. Further, we demonstrated that this plasticity shaped by plant interactions can be a major driver of ecosystem processes. Taking this source of functional diversity into account could help us to better understand how changing biodiversity affects ecosystem functioning in the context of global change.