Influence of Pinus halepensis cover on seedling growth of two co-occurring Mediterranean oak species | Influence du couvert à pin d'Alep sur la croissance de deux chênes méditerranéens.

Change from pine forest towards mixed conifer-broadleaf woodlands is a scenario promoted by forest managers based on the rationale that mixed stands are more resilient. Oak seedlings naturally established in Aleppo pine in the course of succession but this process is long and uncertain. Moreover, the role played by P halepensis on broadleaves seedlings is still debated as some studies have emphasized the negative consequences of Allepo pine forests on spontaneous vegetation. In order to better assess the influence of Aleppo pine overstorey on the survival and development of oak seedlings in southern France we have designed an experiment in which acorns were sown under different types of pine cover. We installed twelve 25*25m plots in an area naturally afforested by pine in which we applied one of the three thinning treatments: heavy thinning removing 2/3 of the basal area, moderate thinning removing 1/3 of the basal area and the control (30 m2/ha). In each plot, 104 sowing points of three acorns-half of the points with Quercus ilex, half with Quercus pubescens- were introduced. Survival, growth of the emerged seedlings and development of the ground vegetation around the seedlings were monitored during 3 years. In addition, predawn leaf water potential, fluorescence and soil moisture content were measured at different dates. Results showed that growth was enhanced in the heavy thinned stands due to higher light availability and Q ilex reached greater dimensions than Q pubescens in all treatments. Growth was positively correlated with shrub cover especially in Q pubescens seedlings. Predawn water potentials were lower in the control and for Q pubescens seedlings. However, soil moisture in the 30-50 cm layer was lower during the summer dry period in the heavily thinned stands than in the other stands in relation with a higher soil temperature and a more developed ground vegetation. Therefore, the better performances of oak seedlings observed during this experiment in the more opened stands will not necessarily persist in the future and the moderately thinned stands could constitute a good compromise.