Purple moor grass induces a rapid decrease of photosynthesis in young oak after forest clear-cutting

The purple moor grass (Molinia caerulea (L.) Moench) is a well-knownresource competitor to the detriment of tree regeneration in many boreal or temperate forests of the Northern hemisphere. This study aimed at investigating to what extent soil nitrogen capture in interaction with light availability drives the early establishment of competition between oak (Quercus petraea (Matt.) Liebl.) and Molinia seedlings. Two-year-old oak was grown in 20 l pots, alone or in combination with Molinia, for two levels of light availability (11 and 55% of incident photosynthetically active radiations) in a greenhouse. Leaf photosynthesis measurements and soil 15N-labelling were used to monitor changes in carbon assimilation and soil nitrogen uptake between and within species under well-watered conditions. Presence of Molinia hadno significant effect on short-term oak seedling growthregardless of the light availability. However, increase in incident light resulted in the rise of both nitrogen uptake and photosynthesis capacityin Molinia. Meanwhile, N taken up by oak grown with Molinia under unshaded environment hardly changed but photosynthesis capacity strongly decreased when compared to shady conditions. In conclusion, these results pointed out a short-term response of photosynthesis and N capture in mixture of Molinia – oak seedlings in unshaded conditions (i.e. after forest canopy opening), which might partly result from a larger nitrogen capture by Molinia