You don’t have to lose pavements to grow trees in cities

A recent study by Arianna Grassi and colleagues, published in Urban Forestry & Urban Greening, investigated the impact of different types of soil sealing on the communities of arbuscular mycorrhizal fungi colonising the roots of two shade trees commonly found in urban areas: Celtis australis and Fraxinus ornus. The researchers found that impermeable pavements, such as monolithic asphalt, caused shifts in the composition of arbuscular mycorrhizal fungal communities associated with the roots of these trees and impacted the percentage of mycorrhizal root length.

The research could help protect trees in urban environments. The soil beneath our feet is home to a vast and intricate web of organisms, including arbuscular mycorrhizal fungi (AMF), which form a symbiotic relationship with plant roots. These fungi play a crucial role in plant nutrition and health, enhancing their ability to absorb nutrients and water from the soil. Unfortunately, urbanisation and impermeable pavement materials have led to soil sealing, disrupting this vital ecosystem.

In their article, Grassi and colleagues cite research showing that Tilia, Linden or Lime trees suffer in areas with low fungal diversity. The authors state: “To the best of our knowledge, no information is available about the effects of pavements suitable for water sensitive urban designs, such as permeable and porous pavements, on AMF communities actually colonizing the roots of shade trees.”

Grassi and colleagues tackled the problem by examining the roots of European Hackberry and South European Flowering Ash growing in Vertemate con Minoprio, a little way north of Milan in Italy. The trees were planted in 2012, and in 2020, the pavements over the trees were removed. That allowed the biologists to take root samples from the trees and see what fungi had colonised them.

Grassi and colleagues observed a fungi community similar to the fungi in the unpaved soil when the root zone was covered with permeable pavements. This provides valuable information that can be used to reduce the disturbance caused by specific types of soil sealing on AMF symbionts, ultimately benefiting urban tree health.

The study identified 45 different types of arbuscular mycorrhizal fungi, with Sclerocystis and Septoglomus as the most abundant phylotypes, accounting for 84% of the genetic sequences revealed. The predominance of Sclerocystis species in the roots of both tree species under impermeable pavements indicated their high and unexpected tolerance towards harsh environmental conditions. These species could be used as arbuscular mycorrhizal fungal inocula. Inoculating the soil with these fungi, specifically selected for their proven resilience in paved sites, would allow planners to exploit their ability to boost biogeochemical processes fundamental for energy fluxes and plant nutrition and health.

By revealing how different pavements impact the fungi that symbiotically nourish trees, this study provides guidance for creating urban infrastructure that fosters tree health and longevity. Permeable pavements, in particular, could help sustain diverse, robust fungal communities essential for supporting trees in the often challenging conditions of cities. With more trees, cities can enjoy cooling, pollution reduction, and other benefits these fungi-tree partnerships provide. 

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Grassi, A., Pagliarani, I., Cristani, C., Palla, M., Fini, A., Comin, S., Frangi, P., Giovannetti, M., Turrini, A. and Agnolucci, M. (2023) “Effects of pavements on diversity and activity of mycorrhizal symbionts associated with urban trees,” Urban Forestry & Urban Greening, 83(127916), p. 127916. Available at: https://doi.org/10.1016/j.ufug.2023.127916.