Detroit region urban forest vulnerability assessment and synthesis: a report from the Urban Forestry Climate Change Response Framework

Abstract

As the climate changes over the 21st century, the people, trees, and green spaces of Detroit, Michigan will face physical, biological, health, and indirect impacts. This assessment evaluates the vulnerability of the urban forest in the Detroit region. We synthesized and summarized information on the contemporary landscape, provided information on past climate trends and projected future climates, and illustrated climate impacts on a range of topics. We used models of habitat suitability for trees native to the Detroit area and used projected shifts in plant hardiness zones and heat zones to understand how less-common native species, nonnative species, and cultivars are projected to tolerate future conditions. We also assessed the adaptability of planted and naturally occurring trees to stressors that are not included in habitat suitability models such as drought, flooding, wind damage, and air pollution.

The summary of the contemporary landscape identifies major stressors currently threatening urban trees and natural areas in the Detroit region. Major current threats to the urban forest in the region include urban heat islands, invasive species and diseases, vacant land, soil and water contamination, air pollution, and social and economic inequality. Detroit has been warming at a rate of about 0.4 °F per decade since 1960 and the average temperature is projected to increase by 5 to 13 °F by the end of the century compared to the mean for 1980 through 2009. Precipitation in Detroit has been increasing by 0.95 inch per decade since 1960 and spring precipitation is projected to increase, while other precipitation projections vary by season and climate scenario. Extreme heat and heavy precipitation events are expected to increase in intensity and become more frequent. By the end of the century, Detroit is projected to shift from hardiness zone 6 to 7 or 8, and from heat zone 5 to a zone between 7 and 9, depending on the climate change scenario.

Species distribution modeling of native trees projects that suitable habitat will decrease for about a third of the tree species, increase for 13 percent of species, and remain stable for 13 percent of species, while 26 percent of native species evaluated for habitat suitability may be able to gain suitable habitat. In terms of adaptive capacity for planted or developed conditions, 27 percent of native species evaluated received a high adaptability score, 57 percent received a medium adaptability score, and 16 percent received a low adaptability score. For natural areas (both native and naturalized), 45 percent of species received a high adaptability score, 46 percent received a medium adaptability score, and 9 percent received a low adaptability score. Under the low climate change scenario, most Detroit tree species (51 percent) fell into the low-moderate vulnerability category. However, more species received higher vulnerability ratings under the high climate change scenario; 14 percent were categorized as low vulnerability, 21 percent as low-moderate vulnerability, 20 percent as moderate vulnerability, 35 percent as moderate-high vulnerability, and 10 percent as high vulnerability. These projected changes in climate and their associated impacts and vulnerabilities will have important implications for urban forest management, including the planting and maintenance of street and park trees, equity and environmental justice efforts, and long-term planning from partnerships to green infrastructure.

Citation

Rutledge, Annamarie; Brandt, Leslie A.; Peters, Matthew P.; Baroli, Madeline J.; Foen, Fai; Harrington, Anita; Grantham, Katherine. 2024. Detroit region urban forest vulnerability assessment and synthesis: a report from the Urban Forestry Climate Change Response Framework. Gen. Tech. Rep. NRS-218. Madison, WI: U.S. Department of Agriculture, Forest Service, Northern Research Station. 172 p. https://doi.org/10.2737/NRS-GTR-218.