Background/Question/Methods Many exotic invasive species exhibit contrasted behaviors between their native and introduced range. Forest ecosystems have long been considered more resistant to invasion than more disturbed habitats, but invaders establishment is longer lived more durably in the latter. Successful forest invaders often show a novel set of life-history traits, which enables them to competitively exclude native neighbors. In particular, forest tree species show a trade-off between traits that promote the fast colonization of empty spaces (e.g. high fecundity, fast growth) and traits that allow long-term persistence (e.g. shade-tolerance). This is the rationale behind Pianka’s r/K strategy scheme, which considers that no plant species can be successfully adapted to both high dispersal and long persistence. However, this traditional view is challenged by many invasive tree species, which can seemingly shift from one strategy to another. Here we ask whether the American black cherry (Prunus serotina Ehrh.) successfully invades European temperate forests because it is less constrained by the r/K trade-off. We studied population structure, growth, leaf traits, sexual and vegetative reproduction of P. serotina. This species was introduced in Europe in the 17th century and is now considered as the most noxious forest invader in western Europe. We compared growth attributes of P. serotina and the most shade-tolerant native species, the Common beech Fagus sylvatica L.
Results/Conclusions P. serotina behaves as a K-strategist when juvenile: under a closed canopy, saplings exhibit a low growth rate, so that the species builds a long-lived sapling bank, accumulating several generations of offspring (Oskar Syndrome). In contrast, P. serotina behaves as an r-strategist when adult: it grows up to the canopy more rapidly than natives, reaches sexual maturity very early (≥7 yrs-old) and produces thousands of seeds, despite having a short life-span (<150 yrs in general). Interestingly, when P. serotina fails to reach the canopy before closure, the stem dies back but individuals actively resprout from the stem base and numerous root suckers emerge, which stop their growth and join the dormant sapling bank. By doing so, the species adapts its size to light availability, but still preempts space (Alice syndrome). These results demonstrate that a lack of trade-off between r/K-selected life-history traits may explain the invasive success of P. serotina, especially over the native late-successional, shade-tolerant F. sylvatica. A similar strategy may explain the invasion success of other shade-tolerant woody species in closed-canopy forests (eg Acer platanoides, Rhamnus cathartica, Quercus rubra).
