Stormy weather was the main cause during the rainy season. Photo credit: Priscilla Cerqueira
In São Paulo, Brazil, the largest and most populous city in Latin America, trees fall every day, but most trees fall during the rainy season, mainly due to temperature, strong winds, and heavy rainfall. When trees fall in the dry season, the main direct cause is not weather, but a lack of management and adequate conditions for street vegetation to survive.
These are the key findings of a study that a group of Brazilian researchers published in the journal Trees — Structure and Function as part of their Urban Trees collection.
The scientists analyzed daily tree fall data and identified around 7,000 occurrences over three years. São Paulo lost 1% of its street trees during this period, an average of 6.2 falls per day. The worst day was December 29, 2014 when 337 tree falls were reported within 24 hours. Ibirapuera Park, the largest green space in the city, had to be closed because of the many trees that the storm had blown over.
Trees are seen as critical to maintaining or improving the quality of the environment in cities. They play an important role in storing and absorbing carbon and helping to mitigate the negative effects of global warming and pollution. They also help reduce flooding by increasing the more permeable soil area and reducing the speed and volume of rainwater runoff by catching some of the rain.
“Of the 652,000 street trees in São Paulo, 7,034 fell in 2013-16. Our analysis showed that the tree fall is driven by precipitation, wind gusts and temperature during the rainy season can have delayed effects, which can lead to a tree fall a few days after the weather event. However, such associations with the climate were not observed during the dry season, although trees fell two-thirds of the days in the season, confirming problems due to poor management and conditions, “the article concludes.
The data used in the study comes from the City of São Paulo Emergency Management Center. The exact location of each tree fall was not known. Researchers focused on correlations between daily weather and tree falls, estimating the immediate effects and effects of rain and wind a few days after a storm.
“When there is heavy rainfall, trees store much more water and their weight increases. In addition, large amounts of water in the soil reduce soil-root friction, making a fall more likely. The weight compensates for the loss of friction and the tree cannot fall in heavy rain. When the soil dries out, its weight decreases without increasing friction and it can fall, “Giuliano Locosselli, researcher at the IB-USP Institute of Botany and first author of the article, Agência told FAPESP.
Co-authors included Marcos Silveira Buckeridge, Professor at the Institute of Biosciences (IB-USP) at the University of São Paulo, and Priscilla Cerqueira, agricultural engineer and head of the Urban Tree Division of the São Paulo City Ministry of the Environment.
According to Locosselli, the results show that the effects of the climate are focused on the rainy season. “Trees fall during the dry season, apparently with no climate-related causes, suggesting lack of care and structural problems that have been around for some time,” he said.
For example, trees are pruned incorrectly, weakening their structure or balance, sidewalks strangle their roots or hinder their development, and often lack space to grow.
“Poorly managed trees fall without any recognizable climate-related causes. Senescent vegetation can fall due to impaired photosynthesis, root support, growth and resistance to pathogens that use the urban climate for reproduction and development, finding trees under poor conditions that can fall for no apparent reason, “Locosselli said.
For Cerqueira, one of the most important aspects of the study is demonstrating the delayed effects of rain and wind in causing tree falls. “The city’s frontline workers see what is happening day in and day out and know that tree falls are reported year-round, but we used the dry season as a base,” she said. “The lagging weather influences are important. Now we have to consider what measures should be taken immediately after stormy rain and wind in order to minimize the risk of tree falls and to be ready when they occur.”
The researchers did not analyze the direct effects of age or pollution on the health of the trees, but Locosselli said the fact that large numbers of trees were planted in São Paulo in the 1950s and 1960s contributed to their vulnerability and likelihood of falling contributes.
With regard to air pollution, an article published in 2019 and co-authored by Locosselli showed that air pollutants inhibit the growth of Tipuana trees (Tipuana tipu), one of the most common tree species in the city, and impair their ecosystem services (more at: agencia.fapesp .br / 30552).
According to the study, the necessary measures to protect street trees in São Paulo include management and planning that take into account species biology, resistance to extreme weather conditions and the characteristics of the local infrastructure.
The authors also recommend implementing tree monitoring programs with tree health indicators and awareness campaigns to gain public support for science-based government decisions.
The city of São Paulo is legally responsible for the management of street trees (Municipal Law 10.365 / 87). This includes planting, protecting tree health, pruning, and replacing trees when necessary. The actual work is outsourced to contractors. The energy supplier is allowed to prune tree tops to make space for overhead lines.
According to Cerqueira, a 20-year municipal tree plan was implemented in September 2020, which aims to improve the city’s resilience to climate change, the provision of shade and aesthetic benefits, and “public satisfaction” through planning and management.
“Several aspects of tree management and planting highlighted in the study are consistent with the problems diagnosed in the city map, which details programs and actions to address each problem,” she said.
The plan is to be reviewed in 2025. For this first five-year period, he sets priorities such as creating a log of fall risk analysis and an emergency management project that includes a review of the agreements signed with contractors and the utility company to reduce pruning and damage to trees.
“The general public is more aware of the importance of trees. The participation of the population has increased. When a tree falls, especially after a storm, we get a lot more calls. We plan to improve the technical material sent to the public, ”said Cerqueira.
With more than 12 million inhabitants, São Paulo has an area of 1,521 square kilometers and a subtropical climate. Approximately 75% of the roads have trees of various native species, including some from the Atlantic Rainforest, as well as exotic species, but most are in more central areas of the city. Planting more trees on the outskirts is one of the goals of the city plan.
Locosselli said he has started a new study with Cerqueira and other colleagues that will analyze georeferenced data that has been available for 30,000 trees in the city since 2012. “We look at where trees are to understand how tree felling is affected by the environment and the environment. The study includes 19 variables such as pollution, sidewalk width and slope, climate and building height before the end of the year, “he said.
A deed of God or just bad management? Why trees fall and how to prevent it
Giuliano Maselli Locosselli et al., Climate Drivers of Tree Felling on the Streets of São Paulo, Brazil, Trees (2021). DOI: 10.1007 / s00468-021-02145-4
citation: Tree falls during the dry season in São Paulo City are due to poor management, study suggests (2021, August 12), accessed August 12, 2021 from https://phys.org/news/2021-08-tree-falls -season-paulo -stadt.html
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