Shielded environments reduce stress in alien Asteraceae species during hot and dry summers along urban-to-rural gradients

Urban environments often host a greater abundance and diversity of alien plant species
than rural areas. This is frequently linked to higher disturbance and propagule
pressure, but could also be related to the additional establishment of species from
warmer native ranges in cities, facilitated by the latter's higher air temperatures and
drier soils. A hitherto unresolved question is how stressful the urban environments
become during climate extremes such as heatwaves and droughts. Do such episodes
still favor alien plant species, or set them back? We used in situ measured phenotypic
leaf and development trait responses of the six most widespread alien Asteraceae
species from various native climates along Belgian urban-to-
rural gradients, measured during two unusually warm and dry summers. Urbanization was characterized by three factors: the percentage of artificially sealed surfaces (urbanity, measured
at three spatial scales from in situ to satellite-based),
the vegetation cover and the
sky view factor (SVF, fraction of the hemisphere not blocked by buildings or vegetation).
Across species, either from colder or warmer native climates, we found a
predominant protective effect of shielded environments that block solar radiation
(low SVF) along the entire urban-to-
rural gradient. Such environments induced lower leaf anthocyanins and flavonols indices, indicating heat stress mitigation. Shielded environments also increased specific leaf area (SLA), a typical shade response. We found that vegetated areas had a secondary importance, increasing the chlorophyll
content and decreasing the flavonols index, but these effects were not consistent
across species. Finally, urbanity at the organism spatial scale decreased plant height,
while broader-scale urbanity had no significant influence. Our results suggest that
sealed surfaces constrain alien Asteraceae during unusually warm and dry summers,
while shielded environments protect them, possibly canceling out the lack of light.
These findings shed new light on alien plant species success along urban-to-
rural gradients in a changing climate.