Fast height growth is key to non-native conifers invasiveness in temperate forests

Many exotic conifer species have been introduced for wood production in temperate regions. Some of these species can display an invasive behavior and negatively impact native ecosystems. It is therefore crucial to identify potentially invasive species before they are widely planted. Seedling high relative growth rate (RGR) and specific leaf area (SLA) have been associated with enhanced invasiveness of trees in previous studies. However, it has been mainly demonstrated for light-demanding species in disturbed habitats, less for shade-tolerant species in closed forests. Here, we investigated the link between seedlings growth traits of 15 exotic conifer species and invasiveness considered at both global and local scale. Seedlings were grown outdoor, under a shade net, and harvested after 4 and 10 weeks. RGR, SLA, Shoot–Root ratio, shoot relative growth rate, relative height growth rate (RHGR) and relative needles production rate were measured. We developed a continuous approach to position each species along a gradient of invasiveness. Local invasiveness consists of a value based on regeneration densities and dispersal distances observed in forest arboreta in Southern Belgium, and is therefore related to the ability of species to invade closed forest ecosystems. Global invasiveness was calculated based on the GBIF Database and the Global Compendium of Weeds, and encompasses all potentially invaded habitats. It appeared that RHGR was positively related to both local and global invasiveness, while SLA was positively related to local invasiveness only. However, RGR was not significantly related with local nor global invasiveness. This study indicates that preferential investment in rapid vertical growth associated with fast resource acquisition is a strategy enhancing invasiveness of non-native conifers, also in closed, shaded temperate forests.