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

Charly Géron,Arnaud Monty,Ivan Nijs,Jonas J Lembrechts

doi:10.1002/ece3.7872

Charly Géron, Arnaud Monty + Show 2 more

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https://doi.org/10.1002/ece3.7872

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Abstract

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.

Highlights

Urbanization currently affects the whole world, and about 70% of the population is expected to live in urban areas by 2050 (Gross, 2016; Terama et al, 2019)
With our analyses of alien Asteraceae phenotypic trait variations during two consecutive hot and dry summers in Belgium, we showed that, across all species, the sky view factor (SVF) and the vegetation cover were the
Significant relationships were limited to a negative effect of the organism scale urbanity on the height of the alien plants, in both field seasons and across most species

Summary

| INTRODUCTION

Urbanization currently affects the whole world, and about 70% of the population is expected to live in urban areas by 2050 (Gross, 2016; Terama et al, 2019). Overheating due to solar radiation can be reduced in areas with very low SFV, these locations have reduced airflow and higher solar reflection, which increases heat trapping (Steeneveld et al, 2011) Another factor that significantly impacts both the intra-urban microclimate and the temperature differences between rural and urban environments is the amount of vegetated surfaces (Tan et al, 2016). A notable exception is the study by Zipper et al (2017), who found higher UHI-induced evapotranspirative demand during midsummer in Madison, United States of America Such studies are rare, and little is known about how rural versus urban and intra-urban microhabitat differences influence alien plant species phenotypic responses, especially during heatwaves and droughts which are predicted to be more common and severe under climate change in general, and in particular in urban environments (Hamdi et al, 2015; Rosenzweig et al, 2018). | 10615 species show phenotypical variation along urban-to-rural gradients, and at which scales? (ii) Do the urbanization proxies, that is, the urbanity at different scales, the vegetation cover and the SVF, interact in determining these phenotypic responses? (iii) Are urbanization effects on phenotypic responses modulated by the vegetation cover in the alien plants’ surroundings?

| METHOD

Findings

| DISCUSSION