Trees Under Siege
My grandmother’s neighborhood in northwest Atlanta is a veritable paradise of ancient beech trees. In the winter, long after leaves have fallen off of the oaks, sweet gums, and maples, the golden companions of silver beech bark are still hanging on. When my grandparents bought the house in 1968, the beech trees were there, growing from the rich soil, through which the Chattahoochee River flows. Beech drops now sprout up beneath them. They set their roots near old beech trees and slough energy from their beech tree hosts, unable to photosynthesize themselves. Birds that my grandmother watches from her living room light upon beech branches and eat a supper of beech nuts. A squirrel lays its nest in the crook of the trunk.
Someday, this rhythm will change. Research shows that hardiness zones - the climate areas that scientists put together to define suitable habitats for plant species - are shifting up the globe at a fast pace. In 1960, my grandmother’s neighborhood and its beech trees were sitting pretty in hardiness zone 7a. Fifty years later, in 2010, the zone had shifted to 8a. Figure 1 shows this shift; what was once north Florida is now Atlanta. The beech tree outside of my grandmother’s house is no longer suited to survive in this human-induced hotbox.
Figure 1: Research conducted by Brian Stone and Kevin Lanza at the Georgia Institute of Technology shows the hardiness zone shift for metro Atlanta from 1961-70 to 2001-10.
If the trend repeats itself in the next fifty years, Atlanta is projected to lose 37% of its native tree species. The ecological consequences for this loss could be devastating. Species relationships that have been in place for centuries will fall apart, leaving native consumers - those creatures that feed off of photosynthesizing plants - to search for new resources. Humans will suffer, too.
As global temperatures rise, heat, which is already the leading cause of death by natural disaster, will increasingly become a health risk. In cities like Atlanta, where the urban heat island effect takes hold, trees are repeatedly shown to be a key factor in mitigating rising temperatures. But low-income, minority communities are especially lacking when it comes to green space. Mechanisms like redlining deem certain areas of cities, typically those populated by people of color, unsuitable for investment and affordable mortgages. The result: neighborhoods with less ownership, more built environments, and fewer trees. Redlined neighborhoods are on average 2.6 degrees Fahrenheit warmer than their non-redlined counterparts. In some cities, this difference climbs to 7 degrees. A map of redlined districts in 1950’s Atlanta is shown in Figure 2.
Figure 2: A map of redlined districts in 1950s Atlanta. Neighborhoods are ranked in terms of “residential security” with the least desirable shaded in red and the most desirable in green.
Many cities are recognizing this disparity and making a concerted effort to plant trees in historically redlined neighborhoods. The question remains: What trees should they plant? In the Atlanta of the future, a whole host of native species will no longer be suited for the ever-creeping hardiness zone. Plant a sugar maple on the eastside of Atlanta now, and it will be sick or dead in fifty years, well before its natural lifespan has been reached. On the other hand, planting a park full of citrus trees might last fifty years, but what impact will that have on native ecology?
Here is the good news: there is still overlap. Common tree species like pecans, red maples, and eastern redbuds are among some of the native tree species that will still be around in fifty years. The white oak that my dad planted with his father in the 70’s will still be dropping acorns in the front yard. Indeed, the beauty of ecosystems is that species most often depend on a myriad of food sources.
In the process of regreening cities to combat global warming and the urban heat island effect, city planners would do well to consider how the changing climate will impact the health of the species that they plant. Furthermore, they should do so in concert with a concern for native ecology. As for those of us that are not making decisions for the city, we could all take a moment to appreciate the trees outside of our windows at the very least and, hopefully, grow inspired to do everything we can to avoid the climate change that is threatening them.
Field, Julia Marie. Urban Tree Canopy Governance and Redlined Neighborhoods: an Analysis of Five Cities, Massachusetts Institute of Technology, 1 Jan. 1970, dspace.mit.edu/handle/1721.1/127588.
Givens, Darin. “Atlanta's Patterns of Segregation and Where They Originate.” Medium, Medium, 5 Sept. 2017, daringivens.medium.com/atlantas-patterns-of-segregation-and-where-they-originated-2b13b89092af.
Hoffman, Jeremy S., et al. “The Effects of Historical Housing Policies on Resident Exposure to Intra-Urban Heat: A Study of 108 US Urban Areas.” MDPI, Multidisciplinary Digital Publishing Institute, 13 Jan. 2020, www.mdpi.com/2225-1154/8/1/12.
Lanza, Kevin, and Brian Stone. “Climate Adaptation in Cities: What Trees Are Suitable for Urban Heat Management?” Landscape and Urban Planning, Elsevier, 17 May 2016, www.sciencedirect.com/science/article/pii/S0169204615002443.
Lanza, Kevin. Urban Heat Islands and the Georgia Tech Climate Network. serve-learn-sustain.gatech.edu/urban-heat-islands-and-georgia-tech-climate-network.
Moran, Barbara. “Mapping Project Explores Links Between Historic Redlining And Future Climate Vulnerability.” Mapping Project Explores Links Between Historic Redlining And Future Climate Vulnerability | Earthwhile, WBUR, 6 Mar. 2021, www.wbur.org/earthwhile/2021/03/05/haverill-merrimack-climate-redlining-maps.