Plant-plant interactions show differential responses to different combinations of available resources that has been underexplored. The short-term functional response of Quercus petraea seedlings and Deschampsia cespitosa tufts grown alone or in mixture was monitored in contrasting combinations of soil inorganic nitrogen x light availabilities in a greenhouse experiment. Growth, biomass allocation, functional traits and resource acquisition were quantified. Intensity and importance of interactions were calculated by organ biomass-based indices. Competition exerted by D. cespitosa on oak was primarily driven by light availability and secondly, for each light level, by nitrogen supply, leading to a strong hierarchy of resource combinations for each considered plant organ. Under high light, oak preferentially allocated biomass to the roots, underlining the indirect role of light on the belowground compartment. Unexpectedly, Deschampsia cespitosa grew better in the presence of oak seedlings under high nitrogen supply whatever the light availability. Oak short-term nitrogen storage instead of investment in growth might be a long-term strategy to survive D. cespitosa competition. Why Deschampsia had a higher biomass in the presence of oak under nitrogen fertilization is an intriguing question. The role of root exudates or change in balance between intra- nu s interspecific interactions may hold the answer. There may be an active mechanism of competition rather than only competitive resource exploitation. Forest managers sometimes practice adding nitrogen fertilizer to improve oak seedling growth in plantations or natural regeneration. Here, the higher biomass in mixture to the benefit of the competitor clearly questions this practice: oak may provide extra nitrogen to competitors during the early period of plant-plant interaction or it may influence the balance between intra- vs interspecific interactions. The identification and quantification of active competition may result in new practices for a broad diversity of plant-plant interactions such as tree regeneration, intercrop management and weed control in agriculture. (C) 2018 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.