Functional traits modulate plant community responses to alien plant invasion

It is well known that alien plant invasion modifies the composition and diversity of resident plant communities, yet our ability to predict patterns of vegetation responses to invasion is hampered by a poor understanding of which functional traits make some resident plants more or less vulnerable to invader impacts. For example, resident species may be more likely to persist and coexist with the invader if they display different strategies of resource use (i.e. high niche differences) or if they share similar highly competitive traits (i.e. competitive hierarchy). Here, we used a trait-based approach to 1) specifically test whether species changes in abundance following plant invasion is random or depends on specific functional trait values, 2) identify which functional strategies allow resident species to coexist with the invader and 3) assess to what extent communities where the invader has been removed return to a reference state. To do so, we applied a semi-experimental approach with the annual vine Humulus japonicus, an invasive species in riparian habitats of Southern France. We monitored plant communities seven times over two years in non-invaded reference plots, invaded plots and removal plots in which the invader had been removed at the seedling stage. We quantified species richness and functional richness as well as mean community trait values of seven traits. The comparison of invaded and removal plots highlighted a strong impact of Humulus japonicus on both species and functional richness of riparian communities. Resident species that had a distinct flowering onset from the invader faced less pronounced declines. Specifically, species that flowered before the invader reached high cover, and species that had a short flowering duration had lower risk of declining. In addition, species coexisting with the invader when it reached its maximum cover displayed high stature and high seed mass. These results suggest that both niche differentiation (in terms of flowering strategy) and competitive hierarchies (in terms of height and regeneration strategy) play a role in explaining plant community responses to plant invasion. Finally, while species richness recovered rapidly in removal plots, functional richness remained lower than in non-invaded plots, pointing to a state-shift that would otherwise go undetected. Overall, our study highlights that the analysis of the functional traits of both invaders and resident species, combined with regular monitoring over time of non-invaded reference plots, invaded and removal plots can greatly improve our understanding of the impact of plant invasion on resident communities.