One of the three main tree species in São Paulo, the largest megacity in the Southern Hemisphere, is the tipuana (Tipuana tipu), also known as rosewood or tipu, a tall tree with a large, spreading crown. It tolerates extreme drought and can be seen as a way to strengthen urban climate resilience, according to an article in the journal Urban climate.

The study examined the impact of the 2013-2014 drought on trees in the city’s streets and parks. The period was chosen because of the extreme drought that occurred in summer, usually the rainy season when tree growth is vigorous.

Based on an analysis of growth ring width and carbon cycle processes, the authors concluded that photosynthesis of Tipuana trees accelerates at high temperatures both in different urban microhabitats and during drought periods, so that growth rates increase even under restrictive weather conditions.

“We need to make our cities increasingly resilient in the face of global warming. One way to do this is to plant trees and take good care of the existing trees. We sometimes need to be pragmatic and select species that can respond well to extreme weather conditions in order not to lose the provision of ecosystem services such as carbon sequestration and temperature regulation,” said Giuliano Locosselli, professor at the Center for Nuclear Energy in Agriculture (CENA-USP) of the University of São Paulo and corresponding author of the paper.

According to a report by UN Habitat, 68% of the world’s population is expected to live in urban areas by 2050, meaning the number of urban dwellers will increase by 2.2 billion compared to the current level. On the other hand, cities are increasingly vulnerable to droughts and extreme weather events as permeable green spaces shrink and heat islands spread.

Given these trends, there is increasing discussion about resilient cities, spatial planning, preparedness and rapid response and recovery in the face of climate challenges. On May 5, on the occasion of World Environment Day, the Brazilian government published a decree launching Resilient Green Cities, a program to strengthen the resilience of cities across the country to climate change. Its main objectives are to integrate policies and promote sustainable practices to enhance the ecosystem services provided by urban green spaces.

“The composition of the urban plant cover, including species resistant to different situations, makes the entire city resilient to any extreme event. Our study adds an element that expands its biodiversity. We are also researching other native species,” said Locosselli.

In collaboration with Marcos Buckeridge, Full Professor at the Institute of Biosciences (IB-USP) and last author of the article, and a group of scientists from other institutions, Locosselli has conducted studies on nature-based solutions to mitigate the negative impacts of climate change and pollution by optimizing the ecosystem services of urban forests.

One of these studies used the Tipuana tree model to show that air pollution inhibits tree growth and limits the environmental services provided by trees.

In addition to the Tipuana tree, which is considered an exotic (non-native) species in São Paulo, the other most common tree species in the city are Ligustrum lucidum (shiny privet, a small tree or shrub that can reach 3 m in height and bears white flowers) and Caesalpinia pluviosa (Sibipiruna in Brazil, Momoqui in Bolivia, reaches 28 m, has a round crown and yellow flowers). The number of trees lining the public streets in São Paulo is estimated at about 652,000.

“There is always a very relevant and important discussion about the need to grow native or exotic species. Native species found in nature have important cultural significance and environmental legislation contains incentives to plant and protect them. However, the urban environment is unique and we need to look for resilient alternatives,” said Locoselli.

Characteristics of the urban environment that affect plant health include concrete structures, impermeable soils, air pollution and different microclimates with heat islands and the effects of global warming, exacerbated by greenhouse gas emissions. The average temperature in the city of São Paulo is 4 °C higher than in the past.

The Tipuana tree is native to Bolivia and northern Argentina, but has been ubiquitous in São Paulo for over a century. The species also lines streets in North America, Europe, Asia, the Middle East and Australia.

Carbon isotope analysis

In the study, the scientists analyzed growth rings in street trees and park trees, focusing on δ¹³C (pronounced “delta C 13”), a measure of stable carbon isotopes that reflects processes related to the carbon cycle. Tree rings help estimate the past variability of ecosystem services based on wood microdensity, hydraulic architecture, and carbon and oxygen isotopes, and serve as a reliable indicator to assess physiological processes and trends over time.

They entered the data into a regression tree, a type of algorithm commonly used in urban forest analysis because its results are easy to interpret and thus support decision making. One of the results showed an increase in δ¹³C during drought, which promotes the growth of the Tipuana tree.

“Tipuana trees are part of the cultural heritage of São Paulo and a purely negative view of exotic species may therefore be premature. We have shown that these species can actually be a valuable part of urban biodiversity. Having Tipuana trees in the city is a significant advantage,” said Locosselli.

It remains to be seen whether the species can maintain its tolerance to water stress if a sequence of extreme events occurs, he added.