When Oskar meets Alice: Does a lack of trade-off in r/ K-strategies make Prunus serotina a successful invader of European forests?

Abstract

Alien plant invasions result from a complex interaction between the species life traits (i.e. ‘invasiveness’) and the recipient ecosystem attributes (i.e. ‘invasibility’). However, little is known about the demographical strategy of invaders and its plasticity among similar ecosystems. To assess the role of demographical attributes and their interaction with soil and light conditions on the durable integration of an exotic invasive tree species into a recipient forest, we analyzed population structure, sexual and clonal reproduction, and growth characteristics of the American black cherry ( Prunus serotina Ehrh.) in a European forest. As seeds, P. serotina is able to enter closed-canopy forests and form a long-living sapling bank, according to the ‘Oskar syndrome’ (no height growth, diameter increment < 0.06 mm year −1). Suppressed saplings typically develop a ‘sit-and-wait’ strategy so that the invader had a head start on native species when a disturbance-induced gap occurs. Once released, suppressed saplings grow rapidly (height growth > 56 cm year −1) to reach the canopy, fill in the gap and produce numerous seeds (6011 per tree on average). During the self-thinning process characterizing the aggrading phase, overtopped saplings die back but subsequently resprout from roots and stumps, going back to ‘Oskar’ stage. This ‘Alice behaviour’ would enable individuals to decrease in size, delay mortality and locally self-maintain in the understories. These results suggest that P. serotina may successfully invade European forests thanks to a combination of traits which fits well the disturbance regime of the recipient ecosystems. It would behave as a shade-tolerant K-strategist in juvenile stages by giving priority to persistence, but as a light-demanding r-strategist once released, by allocating high energy in growth and reproduction. Initial stages of colonisation are weakly affected by soil but strongly by light conditions.