Unexpected impact of N availability on the interaction between Quercus and Deschampsia

Ecological theories on species occurrence and interactions between them deeply rely on resources availability. Grasses are known for their ability to preempt belowground resources, especially nitrogen to the detriment of tree seedlings. Objective of the study was to determine how combination in contrasted soil inorganic nitrogen x light availability influence plants in terms of growth and resource acquisition in mixed stand of oak seedling (Quercus petraea) and Deschampsia cespitosa during early phase of mixture growth. We designed a greenhouse experiment testing the Deschampsia effect on oak, grown in pots, alone or as a mixture under two levels of crossed resources, nitrogen (fertilized, N+, or unfertilized pots, N-), and light (shaded, L-, or well-lit pots, L+). Our results showed oak endured competition from grass in all resource combinations except in N-/L-. Light environment increased intensity and importance of competition on oak. Unexpectedly Deschampsia was facilitated by oak in N+ pot. A higher root biomass, length, SRL, and lower root diameter confer to Deschampsia better prospection and capitation capacities, allowing it to take 96.6% of a source of 15N labeled inorganic nitrogen up. No root plasticity concordant with the foraging theory was observed. Why was Deschampsia facilitated by oak when nitrogen was not limiting is an intriguing question. One answer, still needing research, is that a better oak growth, and so production of more exudates, fosters Deschampsia growth. As a result, Deschampsia ability to harvest resources is reinforced. Oak seedling responds through driving the few resources it can collect to storage organs, following a conservative strategy