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 × 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 intravs 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 intravs 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.

Gross primary production (GPP) is a key component of the forest carbon cycle. However, our knowledge of GPP at the stand scale remains uncertain, because estimates derived from eddy covariance (EC) rely on semi-empirical modelling and the assumptions of the EC technique are sometimes not fully met. We propose using the sap flux/isotope method as an alternative way to estimate canopy GPP, termed GPP iso/SF , at the stand scale and at daily resolution. It is based on canopy conductance inferred from sap flux and intrinsic water-use efficiency estimated from the stable carbon isotope composition of phloem contents. The GPP iso/SF estimate was further corrected for seasonal variations in photosynthetic capacity and mesophyll conductance. We compared our estimate of GPP iso/SF to the GPP derived from PRELES, a model parameterized with EC data. The comparisons were performed in a highly instrumented, boreal Scots pine forest in northern Sweden, including a nitrogen fertilized and a reference plot. The resulting annual and daily GPP iso/SF estimates agreed well with PRELES, in the fertilized plot and the reference plot. We discuss the GPP iso/ SF method as an alternative which can be widely applied without terrain restrictions, where the assumptions of EC are not met. K E Y W O R D S boreal forest, intrinsic water-use efficiency, mesophyll conductance, nitrogen fertilization, phloem δ 13 C, PRELES, sap flux, stand transpiration 1 | INTRODUCTION Gross primary production (GPP) represents a key flux in the carbon (C) budget of a forest ecosystem. GPP has been commonly estimated using many approaches, such as eddy covariance (EC), empirical models and upscaling ecophysiological measurements at stand scale

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