Woody invaders from contrasted climatic origins distribute differently across the urban-to-rural gradient in oceanic Europe – Is it trait-related?

Alien plant species invasion depends on biotic and abiotic conditions that can represent environmental barriers as compared to their native range conditions. Specifically, little is known about how alien plant species distribute along the urban-to-rural gradients based on their native climatic conditions, and how environmental conditions along these gradients could influence intraspecific trait variation. We studied the distribution of eight woody alien plant species from contrasted native range climates along urban-to-rural gradients in European areas with a temperate climate (hereafter termed oceanic Europe). During two consecutive summers and in the Belgian part of oceanic Europe, we then measured their intraspecific trait variation using the nitrogen balance index (NBI), chlorophyll content, flavonols index, specific leaf area (SLA) and internode space. Urban-to-rural gradients were characterized by a system of local climate zones (LCZ), the percentage of artificially sealed surfaces (urbanity) and the sky view factor (SVF). We found that the distribution of studied species in the LCZ classes was highly dependent on the climate of their native range, with species from warm climates occurring more in the most urban areas while the ones from cool climates preferred the more rural or natural areas. However, their intraspecific trait variation was not related to the LCZ class in which they grew, nor to their native climate. Instead, we found a surprisingly consistent effect of shielded environments (low SVF) along the entire urban-to-rural gradient on leaf and development traits. Such environments induced a lower leaf flavonols index and higher NBI and SLA, suggesting a shade response and possibly lower heat and drought stress. Our results show that although woody alien plant species from warmer or cooler native climates distributed differently along the urbanization gradient in oceanic Europe, they did not show contrasted intraspecific trait variation. Nevertheless, our findings highlight that even if the woody alien plant species from cooler native ranges are currently more present in the most natural areas, special attention should be paid to woody alien plant species from warmer native ranges that are yet restricted to the most urban areas and could potentially have severe impacts in the future when the barriers to their spread weaken with climate change.