According to a study in the journal Urban forestry and urban greening In São Paulo (the capital of the state of São Paulo in Brazil), the factors that most affect and increase the risk of tree failure 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 in streets with buildings of five or more stories is double the average for the city as a whole, the researchers found. It is also above average in neighborhoods that originated more than four decades ago. It is highest for trees taller than 9.58 m and planted on sloping sidewalks. Newer neighborhoods with lower buildings have 37% fewer cases.
The researchers analyzed 26,616 records of tree falls in the city’s 96 districts over an eight-year period. São Paulo lost about 4% of its 652,000 street trees between 2013 and 2021, with a share ranging 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 world’s largest cities. The methodology and findings will be used in tree management and urban planning programs.
“We used an artificial technique aimed at practical application of the results and were able to set monitoring targets by identifying the height of buildings and trees that increase the risk of tree failure,” said Giuliano Locosselli, corresponding author of the paper. He is a researcher at the Center for Nuclear Energy in Agriculture (CENA-USP) of the University of São Paulo and the Environmental Research Institute (IPA) of the State Government of São Paulo.
For Priscilla Cerqueira, a co-author of the paper and a technical advisor in the Urban Tree Division of the city’s Department of Green Space and Environment, the findings 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 responsible for tree management so that priorities can be more accurately determined based on the calculated risk of tree failure,” she told Agência FAPESP.
Since a municipal law was passed in 2020 that allows the pruning of trees without publication of the permission for this in the government gazette, the technical staff has expanded the service to entire streets or blocks. “We are implementing the unified system and database that the municipal urban tree management plan calls for,” Cerqueira said.
Influence
São Paulo is the largest city in South America and is in the top five in the world by population according to the Human Settlement Program of the United Nations. Like other megacities, it suffers from the effects of climate change due to heat islands, impenetrable surfaces and environmental pollution. Nature-based solutions and ecosystem services provide relief from these urban problems.
Trees and other vegetation contribute to carbon sequestration, while also reducing global warming, absorbing air pollution and protecting the city from flooding by increasing permeable surfaces and the ability to intercept stormwater runoff.
The main threats to trees include verticalization, which creates ‘urban canyons’ – rows of tall buildings that affect the microclimate by altering local wind speed and patterns of turbulence, pollution spread and shade.
These changes affect important biological processes such as evaporation and assimilation of trees, growth and survival, and contribute to premature tree failure, financial and material losses and even the risk of death for the inhabitants of the city. The impact of urban canyons on tree growth and stability explains the increased risk in areas with tall buildings.
“The mechanisms behind tree failure in urban canyons are poorly understood. Tree health is influenced by many factors in these areas. Verticalization increases risks in the medium to long term as these effects accumulate,” Locosselli said.
One of the solutions available is to plant plant species that are better adapted to the type of environment in urban canyons.
Methodology
The researchers analyzed data from GeoSampa, an online open-source digital mapping platform containing information about health, education and other aspects of the city of São Paulo, including its trees. The platform collects data from boroughs and municipal management systems related to critical events and civic relations.
The study focused on seven variables; age of the neighborhood, height of the building and tree, coverage of the canopy, width and slope of the sidewalk and slope of the site.
The height of trees and buildings has been measured in a LiDAR survey available on GeoSampa. LiDAR (Light Detection and Ranging) is an airborne system that models terrain three-dimensionally by recording laser light pulses as they bounce off objects on the ground and return at the speed of light.
Tree and building height are determined based on the delay between emission and reception of the pulses. Resolution can be up to 1 meter. LiDAR is often used to map topography and vegetation.
The data was processed using an artificial intelligence method called Classification and Regression Trees (CART). According to Locosselli, the algorithm was easy to implement with an accuracy of 82% and the methodology can be applied to other cities, although few in Brazil have detailed information about the height of buildings and tree falls.
“We have done previous research to understand tree falling in São Paulo and assess the influence of climate factors on this process,” he said. “Here we wanted results that could be used by the municipal government to produce tree management guidelines and reduce risk. We prioritized scientific rigor and quality in pursuit of practical results.”
According to an article published in 2021, including by Locosselli, tree fall in São Paulo during the dry season correlated with poor management and a lack of adequate conditions for the survival of street vegetation.
“We wanted to end with guidelines for everyday use. The project involved collaboration between academics and public administrators to support decision-making and contribute to public policy formulation,” Cerqueira said.
A new municipal law on tree management came into effect in April. The city’s 2021-24 target plan calls for the planting of 45,000 trees per year.
Dry-season tree fall in São Paulo City is due to poor management, study suggests
More information:
Rodrigo Manfra et al, The average height of the surrounding buildings and the age of the neighborhood are the main predictors of tree failure in the streets of São Paulo/Brazil, Urban forestry and urban greening
(2022). DOI: 10.116/j.ufug.2022.127665
Quote : The risk of falling urban trees in São Paulo is influenced by the height of the building and the age of the neighborhood, study shows (2022, September 29) retrieved September 29, 2022 from https://phys.org/news/ 2022-09-urban-tree-falls -paulo-height.html
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