Climatic niche shift of an invasive shrub, a combined modeling and common garden approach from individuals to population levels

Biological invasion is a major component of global change, as invading species can alter the biodiversity and functioning of ecosystems. The assumption that climatic requirements of invasive species are conserved between their native and invaded environment is a key issue to assess the risk of invasion. In this study we assess the climatic niche shift of an invasive shrub (common gorse) comparing native and invasive populations using a combined species distribution modeling (SDM) approach at the world scale, growth chamber and common garden experiments along altitudinal gradient in both invaded and native environments. Climatic shifts of already established populations were assessed using a combination of 8 SDM in native area (Wester Europe) and invasive area (North America, South Africa, Oceania) based on 2381 known populations. Additionally, seed of gorse populations were sampled in native (France, Spain) and invasive populations (Canary, Reunion islands and New Zealand). These seeds were used in two common gardens of seedlings transplant experiments along an altitudinal gradient (sea level, 400, 800 and >1000m) over one year in both native (Spain) and invasive area (New Zealand). Lastly, resistance to high temperature (30°C) and drought of gorse seedlings were assessed over a one-month experiment in growth chamber. The aim of the common garden and growth chamber experiment was to assess the climatic niche of gorse seedlings during the early invasion phases. The world scale approach revealed a climatic niche shift between native European populations and invasive ones. Invasive populations of Australia and South America could invade drier and warmer areas than native populations. Similarly, invasive populations from North Europe and North America, could invade drier and colder areas than native ones. The climatic limit in the establishment phase due to drought and high temperature was confirmed by the growth chamber experiment. Nevertheless, invasive populations showed a higher resistance to elevated temperature and drought than native ones. Such difference could be explained by a higher phenotypic plasticity of invasive populations (e.g. ability to maintain higher root biomass under drought). The phenotypic divergence between native and invasive populations of gorse seedlings was also observed in the common garden experiments. Invasive seedlings showed higher growth rates than native ones, possibly due to higher aboveground biomass allocation under rainy conditions. Nevertheless the extreme conditions in highest altitude sites (where established populations are not observed) weren't limiting for seedlings survival. These results suggested that the propagation of invasive gorse in high altitude areas is not limited by climatic conditions but rather by other mechanisms such as seeds dispersal or biotic interactions. Assessing the climatic niche in native and invasive environments of plant invaders (especially for seedlings) is a key step as early control appears as the most efficient management strategy. Our results describe climatic niche shifts between native and invasive populations which could arise from a higher phenotypic plasticity of invasive seedlings. Detecting climatic niche shifts due to evolutionary changes and plastic genotype in plant invaders is crucial under rapid climate change.