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

Cities 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.