Are non-native plants adapting to city life?

With chapter 4 now published, the now finished PhD of Charly Géron has created quite the storyline on how non-native plants are invading our urban environments!

In his first paper, we found out that alien plant species in European cities originated from warmer and drier native ranges, as they are often much better at ease in the warm and often drier climates in our cities than many native plants. The urban heat island in action!

Pauwlonia tomentosa, one of our studied tree species, at ease on a rooftop

In the second ‘Géron et al.’, we dove deeper into the performance of alien plants in the city, with the help of 6 species from the Asteraceae family, one of the families of plant species with the most global invaders. A hitherto unresolved question was 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? The results were a tad surprising: all studied species, regardless of their climate of origin and thus their love for heat or cold, showed fewer signs of stress during a heatwave when shaded (by nature or concrete alike).

Interestingly, this positive effect of shading was found again when focussing on woody invaders, again both for species from warmer ánd from cooler origins. Interestingly, however, we did see a clear contrast in WHERE along the urban-rural gradient species were growing: those from warmer origins preferred the city, those from cooler origins the countryside. Yet despite this clear segregation, all of them were remarkably less stressed when trees or buildings provided them with shade during one of Western Europe’s recent hot and dry summers!

So now, there is Géron et al. number 4, and it takes us even deeper into the story. We wondered if the strong variation in environmental conditions along the urban-rural gradient (hot/dry vs. cool/moist) would result in natural selection within a species: would populations from urban environments perform better in warm and dry conditions than their nephews from the same species yet coming from cool environments? Pineapple* weed (Matricaria discoidea), a tiny Asteraceae plant, came to the rescue.

Pineapple weed in its favorite environment: cracks! (c) Charly Géron

We harvested seeds from several populations from urban and rural environments across Belgium, and put them in growth chambers with either urban or rural climate and/or soils. With a smart experimental design (‘a simulated reciprocal common garden experiment’, in scientific terms), we could disentangle performance differences between the different populations and, very important, if these were due to plasticity, natural selection, or an influence of the mother plant.

Racks of tiny test tubes with seeds of pineapple weed – ready to be subjected to urban climate conditions in the growth chamber. Working with small plants is definitely convenient from the space-perspective! (c) Charly Géron

So, what did we find here? First of all: things are pretty complicated in the realm of population genetics, as tons of things interact with each other. Secondly: most of the observed variation in the growth chamber was related to the conditions of the mother plant (working through e.g. seed size), while our little pineappleweeds from the same populations also showed substantial plasticity when subjected to different environments. The ‘holy grail’ we were looking for – local adaptation, populations changing genetically as a result of the urban environment – could however not be found.

And thus: are non-native species adapting to a city life? No clear genetic adaptation could be observed in our case, at least nothing that resulted in a different performance. But that’s just our little stone on the pile of science – others might find other things in other environments, as so often in ecology.

The 4 papers in a row:

Géron et al. 2021a. Biological invasions

Géron et al. 2021b. Ecology and Evolution

Géron et al. 2022a. Urban forestry & urban greening

Géron et al. 20202b. Plant ecology

*Pineapple weed does smell quite nicely like pineapple!

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