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Northern Forests Climate Hub U.S. DEPARTMENT OF AGRICULTURE VULNERABILITY ASSESSMENT OF AUSTIN’S URBAN FOREST AND NATURAL AREAS A report from the Urban Forestry Climate Change Response Framework October 2020 Climate Hub Technical Report NFCH-5 ABSTRACT The trees, developed green spaces, and natural areas within the City of Austin’s 400,882 acres will face direct and indirect impacts from a changing climate over the 21st century. This assessment evaluates the vulnerability of urban trees and natural and developed landscapes within the City Austin to a range of future climates. We synthesized and summarized information on the contemporary landscape, provided information on past climate trends, and illustrated a range of projected future climates. We used this information to inform models of habitat suitability for trees native to the area. Projected shifts in plant hardiness and heat zones were used to understand how less common native species, nonnative species, and cultivars may tolerate future conditions. We also assessed the adaptability of planted and naturally occurring trees to stressors that may not be accounted for in habitat suitability models such as drought, flooding, wind damage, and air pollution. The summary of the contemporary landscape identifies major stressors currently threatening trees and forests in Austin. Major current threats to the region’s urban forest include invasive species, pests and disease, and development. Austin has been warming at a rate of about 0.4°F per decade since measurements began in 1938 and temperature is expected to increase by 5 to 10°F by the end of this century compared to the most recent 30-year average. Both increases in heavy rain events and severe droughts are projected for the future, and the overall balance of precipitation and temperature may shift Austin’s climate to be more similar to the arid Southwest. Species distribution modeling of native trees suggests that suitable habitat may decrease for 14 primarily northern species, and increase for four more southern species. An analysis of tree species vulnerability that combines model projections, shifts in hardiness and heat zones, and adaptive capacity showed that only 3% of the trees estimated to be present in Austin based on the most recent Urban FIA estimate were considered to have low vulnerability in developed areas. Using a panel of local experts, we also assessed the vulnerability of developed and natural areas. All areas were rated as having moderate to moderate-high vulnerability, but the underlying factors driving that vulnerability differed by natural community and between East and West Austin. 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, management of natural areas, and long-term planning. Cover Photo Colorado River in Downtown Austin. Photo Courtesy April Rose, City of Austin Citation Brandt, L.A.; Rottler, C.; Gordon, W.S., Clark, S.L; O’Donnell, L.; Rose, A.; Rutledge, A. King, E. 2020. Vulnerability of Austin’s urban forest and natural areas: A report from the Urban Forestry Climate Change Response Framework. Report NFCH-5. Houghton, MI: U.S. Department of Agriculture, Climate Hubs. 82 p. VULNERABILITY ASSESSMENT OF AUSTIN’S URBAN FOREST AND NATURAL AREAS A report from the Urban Forestry Climate Change Response Framework Leslie A. Brandt, Caitlin Rottler, Wendy S. Gordon, Stacey L. Clark, Lisa O’Donnell, April Rose, Annamarie Rutledge, and Emily King AUTHORS LESLIE A. BRANDT is a climate change specialist with the Northern Institute of Applied Climate Science and the USDA Forest Service. 1992 Folwell Ave., Saint Paul, MN 55108. [email protected] CAIT ROTTLER is a USDA Southern Plains Climate Hub Fellow with the USDA ARS Grazinglands Research Laboratory 7207 W. Cheyenne St., El Reno, OK 73036. [email protected] WENDY S. GORDON is a Founder at Climate Action Texas and Vice-Chair (District 10), Environmental Commission for the City of Austin. [email protected] STACEY L. CLARK was the Natural Resources Conservation Service liaison with the Northern Forests Climate Hub at the time of the writing and is now an Ecologist and Invasive Plants/Pesticide Use Program Manager with the USDA Forest Service Pacific Southwest Region.1323 Club Drive Vallejo, CA 94592. [email protected] LISA O’DONNELL is a Senior Biologist with the Balcones Canyonlands Preserve, City of Austin 3621 Ranch Road 620 South, Austin 78738. [email protected] APRIL ROSE is a Forest Health Coordinator for the City of Austin Community Tree Preservation Division, 6310 Wilhelmina Delco Dr., Austin, TX 78752. [email protected] ANNAMARIE RUTLEDGE is a Climate Change Outreach Specialist with Michigan Technological University and the Northern Institute of Applied Climate Science. [email protected] EMILY KING is an Urban Forester for the City of Austin Community Tree Preservation Division, 6310 Wilhelmina Delco Dr., Austin, TX 78752. [email protected] PREFACE Context and Scope Author Contributions and This assessment is a fundamental component of the Acknowledgements Urban Forestry Climate Change Response Framework Leslie Brandt developed the assessment methodology, project and builds off methods developed for the report structure, and led the writing of chapters 3 and Chicago Wilderness Urban Forestry Vulnerability 4. Cait Rottler and Wendy Gordon led the writing of Assessment (Brandt et al. 2017). This project builds on chapter 2 and assembled climate change projections lessons learned from the Climate Change Response and historical data. April Rose, Lisa O’Donnell, and Framework: a collaborative, cross-boundary approach Stacey Clark led the writing and mapping for chapter 1. among scientists, managers, and landowners to Annamarie Rutledge led the writing for chapter 5 with incorporate climate change considerations into natural help from Emily King, April Rose, and Lisa O’Donnell. resource management. Each project interweaves four components: science and management partnerships, We wish to thank the municipal foresters, park district vulnerability assessments, adaptation resources, and representatives, natural areas managers, and others demonstration projects. This assessment focuses on that participated in the vulnerability and adaptation both the developed and natural areas within the workshops that contributed to this report. We also Austin region. wish to thank Bill Reiner for his assistance with the species list, and the workshop planning committee. We designed this assessment to be a synthesis of the best available scientific information. Its primary goal is to inform those that work, study, recreate, and care about the urban forests and natural areas in the Austin region. As new scientific information arises, we expect that new efforts will need to be undertaken to reflect that acquired knowledge and understanding. Most important, this assessment does not make recommendations about how this information should be used. The scope of the assessment is the urban forest, broadly defined to include both developed and natural settings within the urban landscape. TABLE OF CONTENTS EXECUTIVE SUMMARY VII INTRODUCTION IX CHAPTER 1: The Contemporary Landscape 1 CHAPTER 2: Climate Trends, Projections, and Impacts 17 CHAPTER 3: Vulnerability of Austin’s Trees 29 CHAPTER 4: Vulnerability of Austin’s Urban Forest 51 CHAPTER 5: Management Considerations 61 GLOSSARY OF TERMS 67 LIST OF SPECIES NAMES 69 APPENDIX 1: Natural Community Crosswalk with Texas Parks and Wildlife Vegetation Types 71 APPENDIX 2: Seasonal Climate Trends 73 APPENDIX 3: Modeled Projections of Habitat Suitability 75 APPENDIX 4: Modification Factors For Assessing The Adaptive Capacity Of Tree Species In Urban Areas 77 EXECUTIVE SUMMARY VII EXECUTIVE SUMMARY Chapter 1: The Contemporary Landscape Chapter 2: Climate Trends, This chapter describes the structure and function of Austin’s Projections, and Impacts urban forest, the forces that shaped it, and current stressors. This chapter summarizes what we know about how the This information lays the foundation for understanding climate has changed over the historical record, how climate how shifts in climate may contribute to changes in Austin’s is projected to change over this century, and impacts to trees and urban forests, and how climate may interact with Austin’s urban forest and natural areas. other stressors present on the landscape. Key Points Key Points • Austin has been warming at a rate of about 0.4°F • Austin is composed of two ecoregions: Edwards per decade since measurements began in 1938 and Plateau to the west and Blackland Prairie to the east. is expected to warm by 5 to 10 degrees by the end • Austin’s urban forest is made up of approximately of this century compared to the most recent 30- 34 million trees with a tree canopy covering about year average. 31% of the city. • Austin has been getting slightly wetter on • The majority (92%) of trees are native to Texas, average, but precipitation can vary widely within and the 10 most common trees account for 84% of and between years, and future projections of all trees. precipitation are uncertain. • Natural areas, including agricultural uses, make up • It is highly probable there will be both an increase the majority of total land area (57%), while 39% of in heavy rain events and severe droughts in the total land area is considered developed area and the future decades,