According to a study reported in the journal Urban Forestry and Urban Greening, the factors that most influence and increase the risk of tree failure in São Paulo (the capital of the state of São Paulo in Brazil) are the height of nearby buildings and the age of the neighborhood. Sidewalk width and tree height are the next most important variables.
Tree failure on streets with buildings of five or more stories is twice as common in the city as a whole, the researchers found. It is also higher than the average of the districts created more than four decades ago. This is maximum for trees taller than 9.58 m and planted on sloping sidewalks. New neighborhoods with lower buildings had 37% fewer cases.
Researchers analyzed 26,616 tree fall records in 96 city districts over eight years. São Paulo lost about 4% of its 652,000 street trees between 2013 and 2021, with proportions varying from 0.59% in the southern part of the city to 17% in the center.
This is the first study to evaluate a comprehensive urban tree failure dataset for one of the largest cities in the world. The methodology and findings will be used in tree management and urban planning programs.
“We used an artificial technique that focuses on the practical application of the results and was able to set monitoring targets by identifying building and tree heights that increase the risk of failure of wood,” said Giuliano Locosselli, corresponding author of the article. He is a researcher at the University of São Paulo’s Center for Nuclear Energy in Agriculture (CENA-USP) and the Environmental Research Institute (IPA) of the Government of São Paulo.
The study is supported by FAPESP through four projects.
For Priscilla Cerqueira, a co-author of the article and a technical advisor of the Urban Tree Division of the Department of Green Areas and the Environment of the city, the results will contribute to a more effective and efficient assessment of tree health in São Paulo . “The data can be shared with our own technical staff and the contractors in charge of wood management so that priorities can be set more precisely based on the calculated risk of wood failure,” he told Agência FAPESP.
Since the municipal law was passed in 2020 allowing tree pruning without publishing the authorization to do so in the official gazette of the city, the technical staff has extended the service to the entire street or block. “We implemented the integrated system and database called the municipal urban tree management plan,” said Cerqueira.
effect
São Paulo is the largest city in South America and one of the top five in the world in terms of population, according to the United Nations Human Settlement Program. Like other megacities, it suffers from the effects of climate change due to heat islands, impermeable surfaces and environmental pollution. Nature-based solutions and ecosystem services provide relief from urban problems.
Trees and other plants contribute to carbon sequestration, while also reducing global warming, absorbing air pollution and strengthening the city against flooding by increasing permeable surfaces and the capacity to block water flow- rain.
The main threats to trees include verticalization, which creates “urban canyons” – rows of tall buildings that affect the microclimate by changing local wind speeds and turbulence patterns, spreading pollution and shade.
These changes affect important biological processes such as evapotranspiration and assimilation of trees, growth and survival, contributing to early tree failure, financial and material losses, and even the risk of death of those city dweller. The effect of urban canyons on the growth and stability of trees helps explain the increased risk in areas with tall buildings.
“The mechanisms behind tree failure in urban canyons are not well understood. Tree health is affected by many factors in these areas. Verticalization increases the risks in the medium to long term, because this effects are cumulative,” said Locosselli.
One of the available solutions is to plant species that are better suited to the type of environment found in urban canyons.
procedures
The researchers analyzed data from GeoSampa, an online open-source digital mapping platform that contains information on health, education and other aspects of the city of São Paulo, including its trees. The platform collects data from borough councils and municipal management systems related to critical events and citizen relations.
The study focuses on seven variables; neighborhood age, building and tree height, canopy cover, sidewalk width and slope, and terrain slope.
Tree and building heights were measured with a LiDAR survey available in GeoSampa. LiDAR (Light Detection and Ranging) is an airborne system that models the terrain in three dimensions by recording pulses of laser light as they bounce off objects on the ground and return at the speed of light.
The height of the tree and building is determined based on the lag between the emission and reception of the pulses. The resolution can be up to 1 meter. LiDAR is often used to survey topography and vegetation.
The data is processed using an artificial intelligence method called Classification and Regression Trees (CART). According to Locosselli, the algorithm is easy to implement with 82% accuracy, and the method can be used in other cities, although few in Brazil have detailed information on building height and tree fall.
“We did the previous research to understand the tree fall in São Paulo and look at the influence of climate factors on this process,” he said. “Here we want results that can be used by the municipal government to create guidelines for tree management and reduce risks. We prioritize scientific strength and quality in search of practical results.
According to a paper published in 2021, also by Locosselli and others, the fall of trees in São Paulo during the summer is related to poor management as well as the lack of adequate conditions for the maintenance of the street plants.
“We want to end up with guidelines for daily use. The project involves collaboration between academics and public administrators to support decision-making and contribute to the formation of public policy,” said Cerqueira.
A new municipal law governing tree management came into force in April. The city’s 2021-24 Target Plan calls for planting 45,000 trees annually
