How Climate change threatens survival of urban trees?

Climate change is putting trees in cities and towns at grave risk. More than half of the tree species in the cities studied in the Global Tree Inventory by the researchers are threatened as the climate gets hotter and drier.

In a study published today in Nature Climate Change, a team of scientists from Australia and France have examined the impacts of projected changes in temperature and precipitation over the coming decades on 3,129 tree species in 164 cities in 78 countries.

If no action is taken, two-thirds of the trees and shrubs in cities around the world will be at risk by 2050, with serious implications for climate action and quality of life in urban settings.

Why urban trees are under threat?

The researchers assessed the potential impact of global warming, in the form of projected future temperatures and rainfall, on trees planted along streets and in parks in 164 cities in 78 countries.

"A surprising finding was that about half of the tree species in each city examined are already experiencing climate changes that put them at risk," says Study lead author Dr Manuel Esperon-Rodriguez from Western Sydney University’s Hawkesbury Institute for the Environment.

“By 2050, an average of 65% of the species in each city will be at risk. This study is important globally because it identifies species at risk and also indicates which species are likely to be climate resilient."

In the UK, the researchers looked at five cities: Belfast, Birmingham, Bristol, London and York, with trees in York, London and Birmingham expected to be among the worst affected in the future.

Trees in tropical countries such as India, Niger, Nigeria and Togo are particularly vulnerable to climate change.

According to the researchers, it is pretty common for the species to already be planted in cities under stressful climatic conditions. On the other hand, certain cities, especially the richest ones, can spend money to water trees in times of drought and mitigate the impact of climate change. In the future, the risks will increase, as well as the maintenance costs.

“The risk is projected to be higher in low-latitude cities, such as New Delhi and Singapore, where all urban tree species are vulnerable to climate change. These findings help assess the impacts of climate change to secure the long-term benefits provided by urban forests,” the study authors wrote.

Risks in Australia are higher

In Australia, reduced rainfall will be the most common stress on urban trees, but rising temperatures will also be a major factor, especially in Darwin.

By 2050, the proportion of urban tree species that could be at risk from projected temperature increases in Australian cities is very high. Among the major cities with urban plantation inventories, those with high percentages at risk include: Cairns 82%, Melbourne 93%, Perth 95%, Hobart 95%, Sydney 96%, Canberra 98% and Darwin 100%.

Common native species, including manna gum, swamp gum, yellow box, narrow-leaved peppermint, blackwood and brush box, and well-loved introduced species, such as jacaranda, oaks, elms, poplars and silver birch, are among the trees that could be at risk in Australia.

By risk, we mean that these species could be experiencing stressful weather conditions that could affect their health and performance. However, we could buffer the risk to these species by providing water or creating other microclimatic conditions. Additionally, urban trees may exhibit plasticity in traits that govern survival, growth, and environmental tolerance, which may help them adapt to local environmental conditions.

Why are city trees so important?

Trees make cities more comfortable places to be with their cooling effect: they provide shade in hot weather and cool the air by releasing water vapour from their leaves. For every 10% increase in the urban tree canopy, ozone is reduced by between 3% and 7%.

Trees have also been shown to remove carbon from the air, absorbing it and storing it as cellulose in their trunks, branches and leaves (a process known as sequestration). Planting trees remains one of the most profitable ways to extract carbon dioxide from the atmosphere.

Independent studies have shown a consistent 5-15% increase in property values on tree-lined streets, proving that trees increase the value of commercial and residential properties.

Buildings increase wind speed as the wind is forced to travel further around them. Trees significantly reduce wind speed up to a distance of 10 times their height. In towns and cities trees help to prevent flooding by absorbing rain water

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