Functional shift of sycamore maple (Acer pseudoplatanus) towards greater plasticity and shade tolerance in its invasive range
Résumé
What makes an alien plant species a successful invader remains a challenging question in invasion ecology. Although comparing functional traits of exotic species between their native and invasive ranges represents a sensible step when studying invaders, this approach is rarely applied and always disconnected from demographic attributes. In this study, we implemented a cross-continental comparison between native and invasive populations of sycamore maple (Acer pseudoplatanus), a tree native to central Europe and invasive in New Zealand. We analysed individual growth and population structure in addition to physiological, biochemical and metabolic leaf-traits of A. pseudoplatanus in both native (Northern France) and invaded (South Island of New Zealand) ranges under two light regimes (sun vs. shade). We found greater individual growth and shade tolerance in the invasive range. For a given sapling age, both basal stem diameter and height were more than twofold greater in New Zealand compared to France, irrespective of light conditions. At the leaf level, photosynthetic rates were higher along with higher leaf nitrogen content, leaf carbon content and leaf construction costs in New Zealand. However, French populations had significantly greater dark respiration and specific leaf area. Leaves of native trees contained more non-structural carbohydrates and chlorophyll but less soluble proteins than those of invasive trees. Our results indicate faster growth of A. pseudoplatanus in its invasive range compared to the native, which is at least partly due to greater shade-tolerance (i. e. more efficient photosynthesis at reduced costs under shade conditions) and plausibly to greater phenotypic plasticity to light. Functional shifts between the native and the invaded range allow the species to escape the growth-survival trade-off and exhibit greater growth in the invaded range. These shifts may be caused by differences in climatic conditions, biotic environment and/or in genotypes.