Acer Negundo invasion along a successional gradient: early direct facilitation by connative pioneers and late indirect facilitation by conspecifics

Biotic interactions are major drivers of biological invasion. However, the roles of direct and indirect positive and negative interactions at the different phases of an invasion process have rarely been addressed because most studies have focused on a particular stage of invasion. We analyzed the respective role of direct and indirect interactions during the colonization and establishment phases of the invasion of Acer negundo, a North-American tree species which has invaded European floodplains. In the Middle Rhone floodplain (France), where Acer forms monodominant stands at the ecotone between native early- and late-successional communities, we set up an experiment along a natural successional gradient. In the three communities we transplanted seedlings of the invasive and of three natives within the forest and in experimental gaps and with and without herbaceous species. We also quantified the effects of the tree canopies and of understory communities on light, available nitrogen and soil moisture. We made two main predictions: (i) native pioneer tree species directly facilitate Acer, and (ii) adult Acer have a higher competitive effect on all other species than native overstory species. Consistent with our first hypothesis, we observed strong direct facilitation for the survival of Acer seedlings in the early-successional Salix community, which was likely due to the mitigation of flooding disturbance. In contrast to our second hypothesis adult Acer did not have higher direct competitive effects on the other species than late-successional native Fraxinus. However, we found significant indirect facilitation of Acer canopies on the growth of Acer seedlings. This facilitative effect occurred for the invasive within its own community because adult Acer significantly reduced the abundance of highly competitive herbaceous competitors which decreased belowground competition. We conclude that the relative importance of direct and indirect interactions were highly variable during the invasion process, but that direct facilitation by native species was predominent during the colonization phase of Acer and indirect facilitation by adult Acer canopies was predominant during the establishment phase