Patterns of plant species invasion along urban-to-rural gradients

The highest rates of introduction of alien species in Europe have been recorded in the last 25 years, and alien plant species are mainly brought in through horticulture, agriculture or contamination of commercial goods. Only a small portion of the alien plant species pool can establish and develop in the introduced range. These low success rates are notably due to the presence of developmental and reproduction barriers. These barriers can be significantly weakened with climate change or disturbances from anthropogenic activities, however, increasing the risks of plant invasion.
Alien plant species represent opportunities to study range expansions and plant response to new environmental conditions, especially when studied along environmental gradients. Urban-to-rural gradients came into view only recently for this purpose, despite their high variation in climatic conditions and anthropogenic influences at all spatial scales. In western Europe, urban-to-rural gradients exhibit temperature variations at scales ranging from kilometers to meters, due to the urban heat island effects (UHI) and microclimatic effects of urbanization. Moreover, due to the presence of anthropogenic surfaces and materials, western European cities have a lower relative humidity and urban soils are drier than rural ones.
Cities represent communication nodes, are subject to anthropogenic disturbances and display a variety of growing conditions; therefore, a high density and diversity of alien plant species are present in urban areas. However, several aspects of alien plant species invasion along urban-to-rural gradients are still not fully understood. The hypothesis that alien plant species from warmer native ranges establish preferentially in urban areas has been formulated, but has never been formally tested. The phenotypic response of alien plant species along the urbanization gradient has been investigated mainly at the interspecific level and at rather coarse temporal and spatial scales, with often inconclusive results. Finally, the study of the evolutionary processes of alien plant species in relation to urbanization is still in its infancy.
In this thesis, I study the relationships between alien plant species invasion and urbanization from the regional scale to the level of the plant. Focusing on western Europe, I investigate inter and intra-species variability using various species at different stages of invasion. Additionally, I integrate temporal and spatial scales in order to better highlight distribution, phenotypic response and evolution of alien plant species in relation to urbanization. Research questions are answered using a wide range of methods: from species distribution modelling over in-situ measurements of leaf and developmental traits to experimental reciprocal common gardens in-situ and in controlled conditions.
I found that all alien plant species, regardless of their native climatic conditions, shared a comparable intraspecific phenotypic response to summer conditions along urban-to-rural gradients. The phenotypic response of alien plant species was only related to variables depicting the urbanization at the plant scale, and not by those at a coarser resolution. While alien plant species development was constrained by urban conditions, these restrictions seemed to be mitigated by the protection of shielded environments in which they experienced less light, heat and drought stress. The protection by shielded environments did not depend on whether the skyline consisted of buildings or vegetation, nor did it depend on climatic conditions during the growing season or on the location of these shielded areas along the urbanization gradient.
Furthermore, I unveiled a complex interplay of plasticity, environmental maternal effects and local adaptation as sources of the phenotypic response of alien plant species. A critical role of seed mass and related environmental maternal effects was particularly apparent in urban conditions, which could represent strategies to persist in heterogeneous city environments. Finally, I found a high importance of native climatic conditions for the distribution of alien plant species along urbanization gradients at coarse spatial resolutions. While alien plant species, regardless of their climate of origin, were highly present in the central part of the urbanization gradient corresponding to detached houses with garden, their distribution patterns diverged towards the urban and rural ends of the gradient. Alien plant species from warmer or drier native climates were more present in urban areas, whereas species from cool and wet native climatic conditions had expanded more in the rural zones.
The results of this PhD indicate that alien plant species distribute differently along the urban-to-rural gradient in oceanic Europe with species from warmer or drier native climates being more present in urban environments while species from cooler or wetter native climates are more present towards rural areas. The contrasted distribution of alien plant species according to their native climatic conditions was not related to their phenotypic response to summer conditions. Rather, the phenotypic response of alien plant species along urbanization gradients was similar, despite their different climatic affinities and depended on conditions at the local scale. This stresses the necessity to account for very local parameters when studying urbanization effects on alien plant species and the need to have access to fine resolution data of environmental conditions such as temperature. The high importance of seed variables and related environmental maternal effects for alien plant species phenotypic response suggests that the study of seedlings of alien plant species in relation to urbanization might unveil processes regarding their contrasted distribution along urban-to-rural gradients. Cities host new combinations of native and alien plant species which are subject to rapid and drastic changes. Better understanding the response of the urban flora to urbanization is crucial, especially in the ongoing era of globalization which sees the homogenization of biotas and because urban areas are where the majority of the world's human population live.