In new research lead by group member Charly Géron, we show that Belgian cities host different exotic plant species than the countryside. Those urban species come from warm and dry native climates and are better adapted to drought and heat. Therefore, they may well become dominant everywhere during summers like this one.
The Mediterranean city
It has long been clear that the city is warmer than the countryside. We call this phenomenon the ‘urban heat island’ effect. The large amounts of paved surfaces in the city absorb a great deal of heat during the day. Also, the buildings layout forms ‘urban canyons’ with decreased wind speed, which easily traps heat accumulated because of the sun but also because of the human activities (such as air conditioners). This is why the hot air envelops the cities like a blanket on hot days. In addition, the soils in the city are often drier than in the surrounding countryside: there is more water runoff, and the extra heat causes faster evaporation.
Scientists have therefore long suspected that this warmer and drier city climate (sharing similarities with Mediterranean conditions in regions with an oceanic climate) would attract species preferring those conditions. This now appears to be the case for exotic tree species, as shown in new research from the University of Antwerp and Liège. The researchers examined where exotic tree species from warmer and drier or cooler and wetter native climates preferred to reside. What they found is that although most exotic species are dominant in the urban fringe – where they have often escaped from gardens – species from warmer and drier climates are by far dominant in the city, and vice versa.
Problem species in the future?
Until recently, trees of warmer origin (they often have a tropical appearance as well) were restricted to the city by necessity, as the climatic conditions of the countryside were simply not sufficient for their survival. However, now that our summers are increasingly dry and hot but also that our winters are not that cold anymore, the right conditions for their development are expanding. This increases the chance that these urban exotics, such as the ‘Tree of Heaven’ or the ‘Princess Tree’, will become more and more present in the countryside too. This spread could then be at the expense of indigenous biodiversity living in the countryside, which is already weakened because of drought and heatwave periods.
Shadow avoids heat stress
Despite the preference of tropical tree species for the urban climate, all species in the study – from warmer or cooler native climates – suffered from heat and sun exposure during heat waves. Alternatively, they all had lower stress values when growing in areas with protection either from buildings or trees. Even if this is surprising as high temperature differences are caused by the presence of urban structure or tall vegetation, the cooling effect of the vegetation is largely altered during droughts. Indeed, the dry vegetation cannot perform evapotranspiration (i.e. cannot sweat anymore which no longer cools down the air) which is visible when walking near crops or sun-scorched lawns, for example. These results suggest that the difference in distribution of alien plant species from different native climates is probably made in winter rather than summer. The city has fewer cold nights in winter than the countryside, which can make all the difference for many species coming from warm environments.