How to manage city trees in a changing climate?

Without sufficient water, urban trees are susceptible to other stressors, including insect pests. Associate Editor, Pieter De Frenne (Ghent University, Belgium) explains how Meineke and Frank’s recent paper, Water availability drives urban tree growth responses to herbivory and warming, provides key research into the management of a vital ecosystem service.

Cities are hot. Not only to taste the best latte macchiato in the fanciest coffee shop, but also when it comes to the temperature of the air. Together, radiative properties of buildings and streets, lower levels of cooling by evapotranspiration of scattered trees and grass, heat produced by traffic and air conditioners, and deviations in moisture and geometry result in consistently higher temperatures in urban, as opposed to rural, sites. This difference can amount to as much as 5 °C or even 10 °C and is referred to as the ‘urban heat island’ phenomenon. This local heat has major implications for city life. Therefore, urban microclimates are now the focus of numerous research projects. For instance, the heat island of Ghent, a city of c. 300,000 inhabitants in Belgium, can be tracked live.

Urban trees - London
Urban trees generally display advanced timing of leaf flush and flowering, and are exposed to a longer growing season. City trees also provide a wide variety of ecosystem services. Photo of city tree in central London, by Pieter De Frenne.

The urban heat island phenomenon often has negative implications for the health of human city inhabitants, especially during summer heat waves. Architects and land planners could therefore, for example, build new care homes for the elderly and childcare nurseries in so-called cold spots near parks or rivers. In addition, vegetation in urban environments is also exposed to higher than average levels of warming (and CO2). An urban setting thus creates the perfect laboratory to further our understanding on the effects of climate change on plants. Compared to forest trees in more natural environments, urban species such as street trees require specific management because they are exposed to such specific conditions. The effects of this include earlier flowering and a longer growing season in city trees. In their recent paper, published in Journal of Applied Ecology, Meineke & Frank research exactly this.

The authors determined how drought, warming and insect pests affect one of the most often planted shade trees in cities in the south-eastern US, the willow oak Quercus phellos. By skilfully combining tree growth data from spatial temperature gradients in the field (the city of Raleigh, in this case) and the growth chamber, they were able to show that water shortages specifically have the potential to reduce tree growth and enhance damage from an insect pest in these urban trees. Remarkably, they find that the urban heat and phloem-feeding insects only reduced growth in saplings that were drought stressed.

Urban trees - Sussex
A willow oak in Sheffield Park and Garden, East Sussex, England. Photo by Daderot from Wikimedia Commons

The recent study by Meineke & Frank illustrates that a key management recommendation to buffer the effects of urban heat and insect pest pressures on city trees is, at least in the willow oak, to avoid drought. Avoiding drought in a city environment is, in principle, possible. Simple adaptation measures that do not require excessive amounts of water, for instance, investing in local drip irrigation near the tree roots, covering the soil surrounding the tree with organic mulches such as woodchips, and growing more drought-adapted provenances or species in the hottest places.

Urban trees deliver a particular set of ecosystem services: they can help to filter out air pollution, store significant amounts of carbon and may even enhance physical and mental human health. If urban trees are becoming more susceptible to stressors such as climate warming, drought and pests, the natural capital of urban vegetation may be threatened. More research on this topic is thus urgently needed, and Meineke & Frank help to fill this gap.

Read the paper, Water availability drives urban tree growth responses to herbivory and warming in Journal of Applied Ecology.

Find out more about the story behind the paper in this post by lead author, Emilie Meineke.

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