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

peer reviewedAlien 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

Phenotypic variation along urban-to-rural gradients : an attempt to disentangle the mechanisms at play using the alien species **Matricaria discoidea** (Asteraceae)

peer reviewedCities often exhibit higher temperatures, drier soils and greater habitat fragmentation than rural areas, and may thus represent constraining growing environments for plants. This variety of environmental conditions along urban-to-rural gradients might lead to plasticity in plant traits. Urban conditions could affect plant traits such as germination or flower number — key variables for organismal fitness. These proxies of fitness could then potentially experience strong selection in urban environments. Trait variations among individuals are not only due to genotype or the environmental conditions: they can also result from the conditions in which the mother individual developed, i.e. environmental maternal effects. Understanding the impact of city environments on phenotypes might be especially important for alien plant species, as these are often showing exceptionally high density and diversity in cities. However, studies of the sources of alien plant trait variations in urban environments are rare. We conducted a simulated reciprocal common garden experiment using seeds of populations of M. discoidea along urbanization gradients in Belgium. These seeds were sown in growth chambers to study how germination, growth, and flowering traits of the alien species Matricaria discoidea vary in response to temperature (rural or urban) and soil (rural or urban) treatments and the urbanity (percentage of impervious surfaces) of its source population. We found predominant effects of seed mass and germination delay which materialized important environmental maternal effects. We observed a higher germination success and delay for heavier seeds, and a greater number of capitula and dry biomass for plants that germinated earlier. Climate and soil treatments led to plasticity in the majority of the measured traits, with a faster germination and heavier plants in the urban temperature treatment but lighter plants in the urban soil treatment. Our results therefore support the existence of predominant environmental maternal effects and phenotypic plasticity in response to temperature and soil treatments, but no visible evidence of local adaptation

Urban alien plants in temperate oceanic regions of Europe originate from warmer native ranges

peer reviewedWhen colonizing new areas, alien plantspecies success can depend strongly on local environ-mental conditions. Microclimatic barriers might be thereason why some alien plant species thrive in urban areas, while others prefer rural environments. We tested the hypothesis that the climate in the nativerange is a good predictor of the urbanity of alienspecies in the invaded range. The relationship betweenclimate in the native range and the percentage ofartificiallysealedsurfaces(urbanity)attheoccurrences of 24 emerging alien plant species, inEuropean areas with a temperate climate (termedoceanic Europe) was evaluated. We found that alienspecies growing in more urban environments origi-nated from warmer or drier native ranges than found inoceanic Europe. These results have strong conserva-tion implications as climate-warming will likely liftclimatic barriers that currently constrain numerous alien plant species to cities, boosting the role of citiesas points of entry for invasive plant