Climate change is inexorably leading to rising temperatures and longer, more intense droughts in many parts of the world. These global changes are locally exacerbated in urban environments by a phenomenon called the urban heat island. Cities are significantly warmer than their rural surroundings, largely due to the huge concentration of impervious surfaces – such as roads, sidewalks and buildings – that absorb solar radiation during the day and then radiate this heat back into the surroundings.

These heat waves are not just a matter of discomfort, they are also deadly. They have extremely negative consequences for human health, particularly for vulnerable groups such as the elderly and young children, with the 2022 heat wave in Europe, for example, causing more than 60,000 deaths. Vulnerable communities with lower incomes often cannot afford air conditioning, making it essential to find natural ways to cool public spaces.

Urban forests as salvation and the specific challenges of shrinking cities

One of the most effective solutions to mitigate urban heat is the expansion of urban forests. Trees can not only beautify cities, but also provide invaluable ecosystem services: they manage rainwater, sequester carbon, reduce summer electricity consumption by 1.5 to 5.2 % and, thanks to shade, can reduce the average daily temperature in the city by more than 3 °C.

But planting a tree is only the first, and often the easiest, step. Rising temperatures and drought make the survival of a young seedling in the city a huge challenge. This problem particularly affects the so-called „legacy cities“ (shrinking post-industrial cities), which in the past experienced economic booms, but are now suffering from massive population decline and de-industrialization. A typical example is the American city of Dayton, Ohio, which has lost about half of its population since the 1970s, leaving behind more than 9,000 vacant lots and abandoned buildings. Such cities have a critical shortage of financial and labor resources for the necessary long-term care and irrigation of new greenery. While the total tree cover in Dayton is 22.5 %, in poorer neighborhoods it reaches a critical 5 to 8 %.

Extensive field experiment

To find out how to successfully expand green spaces on a limited budget, researchers conducted a large-scale field study in Dayton in the summer of 2024. They planted 640 seedlings of eight native tree species in 20 selected parks across the city. The sites varied in the amount of surrounding impervious surfaces (ranging from 7 % to 65 % within a 500-meter radius), which served as a precise indicator of the degree of urban heat.

The species included in the experiment were: tupela, catalpa bignoni, red maple, tulip lily, sassafras, white oak, western sycamore, and three-thorn clematis. The researchers tested them under four different irrigation strategies: no additional water (control), 19 liters of water every two weeks, 19 liters of water once a month, and the use of 75.7-liter gator bags that were filled once a month and released water to the roots slowly and continuously.

Who survived the concrete hell? Winners and losers

Results after the first season revealed the harsh nature of the urban environment – the overall survival rate of seedlings was only 48 %. However, there were stark differences between species.

The absolute winner of the test was red maple, which achieved a phenomenal survival rate of up to 91 % and maintained its health even in the most concreted locations. It also did extremely well bignonium catalpa (100 % survival in six different highly impervious locations) and three-spined sedge. Gledíčia recorded the highest average height growth during the study (4.58 cm) and trees of this species showed the least visual signs of stress among all. These three species showed that high temperatures did not have a negative impact on them, making them an ideal choice for planting in a demanding urban environment. The increase in trunk thickness (caliber) was minimal in all species during the experiment, with the highest value again recorded by catalpa (0.65 mm on average).

At the opposite end of the spectrum, she ended up tupela (survival only 10 %) and white oak (15 %). However, the researchers add important context: these species had poor health and crown vitality ratings upon delivery from a commercial supplier, which largely determined their low chance of establishment.

The price of survival and the unexpected human factor

Study The study also revealed important insights into irrigation strategies. For most trees, gator bags proved to be the most effective method for survival, growth, and overall health. These bags are ideal for cities with limited staff because they only need to be refilled once a month. But they have one big downside: price. Each bag costs about $30, which would mean an initial investment of almost $20,000 for all 640 trees in the study.

Moreover, another, often underestimated problem comes into play here – the human factor. A significant number of water bags were destroyed by city lawnmowers during the season. The trees themselves faced extreme anthropogenic pressure; in one of the parks, out of 32 planted seedlings, up to 30 were destroyed or completely broken due to careless visitors or maintenance.

What next? Recommendations for modern cities

Based on these findings, experts recommend that cities radically rethink their planting budgets. Since young trees die at a much higher rate in cities than old ones, it is necessary to allocate a much larger share of funds to post-planting care and protection (such as fencing) than to the purchase of seedlings alone.

A stratified approach to forest expansion is also recommended. In poorer neighborhoods that suffer from extreme heat from overdevelopment, only the most resistant species, such as red maple, catalpa, and sycamore, should be planted in the first wave. Although a healthy urban forest should be species-diverse in the long term to resist pest and disease invasions, diversification with more sensitive species should only begin when „pioneer“ resistant trees have created initial shade and at least partially stabilized the local microclimate. Only with such a sustainable and scientifically based approach can cities combat extreme heat and create a healthy environment for their residents. JRi&CO2AI