Performance traits are positively correlated with global invasiveness of exotic trees : two case studies on maples and conifers.

Determining the traits correlated with invasiveness in order to identify potentially invasive species remains a priority in invasion ecology. In trees, an important juvenile relative growth rate (RGR) and specific leaf area (SLA) under optimal conditions have been identified as performance traits promoting invasiveness. We tested the correlation between performance traits and a continuous quantification of invasiveness for two forest species groups: maples (Aceraceae) and conifers (Pinaceae and Cupressaceae). Eight maple species were grown indoor under non-limiting conditions for 8 weeks. Fifteen exotic coniferous species were grown for 10 weeks outdoor, under a shade sail, in southern Belgium. At several harvest times, SLA was measured and seedlings were dried to weigh the biomass and determine RGR. The global invasiveness was quantified using a
combination of the number of regions and countries invaded in GBIF, and the number of citations and risk score in the Global Compendium of Weeds. For conifers, the local invasiveness in Belgian forests was also quantified as the density of regeneration observed in old arboreta. Phylogenetic effects were tested. For maples, as expected, we found a positive correlation between invasiveness and RGR, SLA, relative leaf
production rate and height. For conifers however, RGR was not correlated to global and local invasiveness. Height of the seedlings was the only trait positively associated with global invasiveness, and species with higher SLA, height and number of leaves also had a higher local invasiveness in Belgium. We conclude that invasive maple species, mostly post-pioneer species, could benefit from an acquisitive strategy as already shown for other tree species. For conifers, the combination of traits promoting invasiveness might differ. Invasive conifers in forests might adopt a “sit-and-wait” strategy at the seedling stage.