Page 54 - Mapping the Nation: Taking Climate Action

53 could extrapolate temperature readings on a larger scale to see what heat looks like around town and compare it to the tree canopy." The result was an interactive web map showing that morning temperatures were higher in dense urban areas close to the city center than in other areas. Results exposed how concrete structures that absorb solar heat during the day and radiate it at night can be seven degrees hotter than outlying areas in the day and five degrees hotter at night. Strategies to mitigate the effects of urban heat islands include white roofs; more crosswalks so that people don't have to walk as far; more bus shelters; and, of course, more trees. The Effect of Trees on the Lives of Residents Planting more trees in an underserved area starts a positive chain reaction: more trees mean more canopy; more canopy means more shade; more shade means less heat; less heat means lower energy bills and more outdoor activity. Therefore, more trees result in improved health and quality of life. Trees create fresh air while also cleaning some pollutants. The greenery is appealing, which draws people outside where they can move around and be more social. A recent study even found that street trees are present where fewer people take medications to deal with depression. Trees also actively cool areas in a process that's similar to perspiration. "The scientific term is evapotranspiration," Halter says. "I noticed it on a superhot day in July at midday, and suddenly these trees started to cry or sweat, as if it was raining. The trees are taking up water from the ground, then it goes up to the leaves, and then the tree rains on itself and the water goes back into the soil. It's kind of a breathing, liquid-to-gas process." This analogy is fitting since forests are often called the lungs of the earth, but most people don't experience the process so directly. "Trees cool the environment—you can actually feel it," Halter said. This measurable benefit is often referred to as an ecosystem service. Trees also help protect areas from increasingly severe storms—especially important in a place like Austin, which experiences frequent cycles of drought and flooding. Tree roots draw in rainwater and keep the soil from washing away. The leafy limbs slow heavy raindrops before they hit the ground, so the soil is less prone to erosion. Selecting the Right Trees In 2014, the US Forest Service (USFS) conducted an inventory of trees in Austin to help understand tree canopy in detail and also assess the carbon sequestration capacity of trees. USFS analysts determined that in Austin, there are currently 33.8 million trees, which store about 1.9 million tons of carbon dioxide. Researchers found that every year, the trees remove about 92,000 tons of carbon as well as 1,253 tons of air pollutants and reduce residential energy costs by $18.9 million. The inventory included a species review, finding that the most common trees are Ashe juniper, cedar elm, live oak, sugarberry, and Texas persimmon. Ashe juniper, Halter said, is "the number one cause of tree allergens in Austin—but it's also the tree species with the greatest air quality impact because it's an evergreen species, and Ashe juniper trees act as year-round air filters. Ashe juniper also captures the most stormwater runoff and sequesters the most carbon in our urban forest. It's a weird dichotomy—the tree that's disliked the most is also the tree that's helping us the most." Like all urban forestry specialists, Halter and his colleagues in Austin have a difficult management task dealing with weather and infestations. Oak wilt, a fungal disease that is spread by beetles and gets into the root system, can