More grass, less asphalt. The best recipe for trees to provide ecosystem services

More grass, less asphalt.  The best recipe for trees to provide ecosystem services

It is no secret that trees planted in urban areas not only provide shade, but also lower the air temperature. However, for these ecosystem services to be optimal, you need to give the trees the best conditions possible. Researchers have discovered that excessive asphalt around trees reduces their environmental benefits. They recommend replacing asphalt with grass to enhance these benefits.

The study focused on horse chestnut trees growing in environments with varying degrees of paving. Image credits: Janina Konarska.

The ecosystem services provided by urban trees are affected by canopy size and density and are limited when trees grow poorly. One of the major stressors that limit their growth in cities is roof paving. Paved surfaces limit rainwater infiltration into the soil, which in turn reduces the trees’ uptake of water and nutrients.

However, studies focusing on the growth and physiological performance of street trees in relation to pavement grade are limited. To address this problem, a team from the University of Gothenburg studied the effect of trunk mulch on oak, horse chestnut and common lime trees at several sites in Gothenburg and Molndal.

Quercus palustris (Pin oak) is native to the wetlands of eastern and central North America. In recent years, it has been planted in Gothenburg. Aesculus hippocastanus (Horse chestnut) is native to the Balkan Peninsula, but is now widespread in Central Europe and North America. Tilia × Europe (Common lime) is native to most parts of Europe.

Against this background, the researchers explored how different surface materials affect these tree species.

Trees vs asphalt

The study was based on various factors, such as crown density, tree growth, and the amount of water released by leaves (through a process called transpiration). All of these factors determine the extent to which a tree affects the local climate. Crown density and tree size determine the shading effect, while transpiration acts as an air conditioner, cooling it.

“Our research shows that an important factor is the amount of space surrounding the tree paved,” Janina Konarska, lead author of the study, said in a press release. “The air over the asphalt surface will be warmer than the air over the grass or soil. The hardened surface also prevents rainwater from reaching the roots, affecting tree growth.

The researchers found that the 20- to 30-year-old trees surrounded by grass were on average 2.6 meters (8.5 feet) taller and had crowns 1.3 meters (4.3 feet) wider than nearby trees with a sedge closer to the trunk. The trees also had crowns that were 61% denser on average, providing twice the cooling. Overall, this suggests that tree ecosystem services can be compromised by poor tree planting design.

There were differences in the extent to which different tree species were affected by the lack of water reaching the roots. Horse chestnut is least affected by paved surfaces, but performs worse than oak and common lime as a cooling plant under better growing conditions. However, the researchers note that while species selection is important, the cooling effect of trees depends more on how close the trunk is to the asphalt.

“You have to do your best when planting trees in urban areas. If it is difficult to create open spaces around trees, it is a good idea to invest in better soil and preferably water the tree. It is important that we take care of “With trees, they are valuable in many ways.”

The study was published in the Journal of Landscape and Urban Planning.

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