The role of biotic interactions for the early establishment of oak seedlings in coastal dune forest communities
Résumé
Although forest managers are aware of the need to adapt forest management to climate change, this task is still challenging due to the difficulty for predicting local species and communities responses to climate change. The aim of this paper is to focus on the adaptation of sylvicultural techniques to changes in interactions between tree recruitment and understory species due to climate change. A space-for-time design was used in coastal dune forest communities from the south west of France. The study area is a 240 km-long sand strip covered by forest communities with minimal variation in soil conditions along a natural gradient of increasing water stress. We transplanted seedlings of three oak species of contrasting strategies at both the wet and dry ends of the climate gradient, both in forest and gap plots and with and without understory shrubs. We measured Vapor Pressure Deficit in all treatment conditions. We found strong canopy and climate conditions effects on interactions between understory shrubs and oak transplants. Competition was dominant in the forest plots of the wettest site and facilitation in the gap plots of the driest site. Oak survival without shrubs (but not with shrubs) was strongly related to VPD values, which suggests that the positive effect of shrubs in the most stressful conditions was due to decreased atmospheric stress below their canopy. In contrast, we found that understory shrubs/oak seedlings interactions were weakly affected by oak species functional strategies. Our results provide evidence that future oak regeneration management should take into account changes in interactions with understory shrubs due to climate change. In particular, we recommend conserving understory shrubs in the most stressful sites in order to maintain a sufficient oak regeneration for the long term dynamics of coastal oak forest communities under changing climate.