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Unveiling the Connection: How Deforestation Fuels the Growth of Urban Heat Islands

We are seeing this site all over the San Antonio Area. Acres of land are being scalped to make way for development. Thousands of trees are being mowed down. We are also experiencing some of the hottest temperatures on record.

How does the loss of our tree canopy expand the urban heat island effect in Bexar County? The urban heat island effect is defined by the Environmental Project Agency as “urbanized areas that experience higher temperatures than outlying areas. Structures such as buildings, roads, and other infrastructure absorb and re-emit the sun’s heat more than natural landscapes such as forests and water bodies. Urban areas, where these structures are highly concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas. Daytime temperatures in urban areas are about 1–7°F higher than temperatures in outlying areas and nighttime temperatures are about 2-5°F higher. “

We have all been dealing with this incredible heat. July 2023 was the hottest month on record.

In February 2023, The World Economic Forum published the article “Trees for life: Making our cities greener can cut early deaths by a third“. The article makes the case of how “Increasing urban tree cover to 30% would protect the lives of people living in cities” through the many benefits of trees. Trees help keep us cool, remove pollutants from the air and water, help mitigate stormwater, improve our mental and physical health, provide wildlife habitat, and so much more.

Photo courtesy of the World Economic Forum

From the article, “The infographic above shows how temperatures rise in line with the density of urban development. Scientists at the Berkeley National Laboratory say a hot and sunny afternoon can increase the temperature in urban areas by 1-3°C, compared with the air in nearby rural areas.”

What is the current state of our trees? Let’s start at the national level. Reported by “PBS NEWSHOUR Trees across the U.S. face dire threats” shares a study conducted by the Morton Arboretum released in August 2022 showing 1 in 9 tree species in the U.S. are at risk of extinction. Aside from human-caused deforestation, trees face a host of threats including invasive species, deadly diseases, and climate change.

How are the native trees in Texas fairing? This study shows Texas is one of the top 5 states whose native forest is at the most risk of extinction. The most vulnerable native trees in the study are oaks and hawthorns.

The report states “The geographic distribution of threatened and/or at-risk tree species follows a similar pattern as that of native and endemic tree species, with California, Texas, and the Southeast having high concentrations of threatened and/or at-risk tree species.”

How are the trees fairing with the tree canopy in the San Antonio Region? A report published by Global Forest Watch found that from 2001 to 2022, Bexar County lost 10.7 kha of tree cover, equivalent to a 20% decrease in tree cover since 2000. This data does not include any trees lost this year from development, heat, or drought. Another tool available is My City’s Trees, an application that enables anyone to access Urban Forest Inventory and Analysis (FIA) data and produce custom analyses and reports of urban forests — public and private trees where people live, work, and play. Bexar Branches Alliance (BBA) utilizes this information, among others, to identify which communities are at the most risk due to heat island effect, social vulnerability, and equity. This data guides where we are focusing our initial programming efforts. However, the tree inventory is incomplete. The UFIA is a 10-year process that started in 2017. It does not have information on canopy loss from the most recent years of extreme weather and drought.

Where do we go from here? Trees are an intricate part of ensuring the San Antonio region’s resiliency against the extreme heat and drought we have all been experiencing.

Preserving and caring for our existing trees is more important now, than ever before. During the summer of 2022, the Natural Areas Conservancy (NAC) “partnered with 12 cities from the Forests in Cities network to conduct a study focused on quantifying differences in air and surface temperature between types of urban greenspace, with a focus on natural areas. As a result of this study, they found that natural areas are the coolest types of greenspaces in cities. Natural areas were significantly cooler than non-natural and landscaped areas, and forested natural areas had lower air temperatures than areas of landscaped trees by several degrees. In some cities on a hot summer day – it was over 10°F cooler in a forested natural area compared to under landscaped trees just a few hundred feet away in a streetscape. They also found that forests that were higher quality tended to be cooler than those that were more degraded during the warmest point of the day and had lower high-temperature extremes.”

In May of this year, the Natural Capital Project Centered at Stanford University published

The Vibrant Land: The Benefits of Food Forests and Urban Farms in San Antonio (PDF) study. The study shows the benefits of turning unused green space into urban farms or food forests. Tamox Talom at Padre Park and Garcia Street Farm, both located in San Antonio were included in this study. As we look at ways to reforest our tree canopy, preservation of the existing trees is even more important.

We can all do our part by taking care of the trees in our own yards and neighborhoods. We all need water to survive this heat. Our trees are no exception. To learn how to help your trees survive the predicted hotter and drier summer than normal, please check out this video from the Texas Forest Service: Tree Watering Tips – How to Properly Water your Trees. To learn more about any water restrictions in your area, please visit the SAWS Drought Restriction & Watering Rules or the Bexar County Water Control and Improvement District websites for more information. We can all do our part to be a responsible part of the solution.

If you would like to volunteer your time to help Bexar Branches Alliance with our tree planting and preservation efforts, please check out our volunteer opportunities at the Bexar Branches Alliance Event Calendar. We hope to see you among the trees!

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