Tree and impervious cover in the United States

doi:10.1016/j.landurbplan.2012.04.005

Landscape and Urban Planning

Volume 107, Issue 1, July 2012, Pages 21-30

https://doi.org/10.1016/j.landurbplan.2012.04.005 Get rights and content

Abstract

Using aerial photograph interpretation of circa 2005 imagery, percent tree canopy and impervious surface cover in the conterminous United States are estimated at 34.2% (standard error (SE) = 0.2%) and 2.4% (SE = 0.1%), respectively. Within urban/community areas, percent tree cover (35.1%, SE = 0.4%) is similar to the national value, but percent impervious cover is significantly higher (17.5%, SE = 0.3%). Tree cover per capita in urban areas averaged 377 m²/person, while impervious cover per capita averaged 274 m²/person. Percent tree cover in urban/community areas tends to be significantly higher than in rural areas in several predominantly grassland states, with the greatest difference in Kansas (+17.3%). Most states in more forested regions exhibited a decrease in tree cover between urban/community areas and rural lands, with greatest difference in Kentucky (−37.9%). These changes in tree cover varied significantly among states, illustrating the roles of urban development patterns, management/planning interactions, and the natural environment on creating cover patterns exhibited in urban areas. Understanding these forces and patterns can lead to better planning and management activities to optimize the mix of tree and impervious cover to sustain urban functions while enhancing environmental quality and human health in urban areas.

Highlights

► Percent tree cover in the conterminous US is 34.2% (266.7 million ha). ► Percent impervious cover in the conterminous US is 2.4% (18.4 million ha). ► Percent tree cover in urban/community areas is 35.1% (14.6 million ha). ► Percent impervious cover in urban/community areas is 17.5% (7.3 million ha). ► Tree cover varies between urban and rural land by state and natural vegetation type.

Introduction

Tree canopy and impervious surface cover affect ecosystem services provided by a landscape. These cover elements play particularly important roles in cities and towns in terms of their impacts on the physical and socio-economic environment, and, consequently, human health and well-being in these areas. Trees not only provide numerous economic and ecosystem services and values to a community, but also incur various economic or environmental costs. Trees supply ecosystem services associated with air and water quality, building energy conservation, moderation of air temperatures, reductions in ultraviolet radiation, and many other environmental and social benefits (e.g., Dwyer et al., 1992, Kuo and Sullivan, 2001, Nowak and Dwyer, 2007, Westphal, 2003, Wolf, 2003). Costs associated with trees can be both economic (e.g., planting and maintenance, increased building energy costs) and environmental (e.g., pollen, volatile organic compound emissions) (Nowak & Dwyer, 2007).

Likewise, impervious cover plays an important role in the landscape, particularly in developed areas. These surfaces, such as roads, buildings, sidewalks, and parking lots, facilitate transportation and provide shelter, but also can negatively impact the environment. Increased impervious surface area can enhance local temperatures and heat islands effects (Heisler and Brazel, 2010, Oke, 1989), which consequently affects building energy use, human comfort and health, ozone production, and pollutant emissions.

Impervious surfaces impede water infiltration rates (Hamilton and Waddington, 1999, Pitt and Lantri, 2000), and reduce percolation, water table levels, and stream baseflow regimes (Faulkner et al., 2000, Lerner, 2002). Removal of forest cover and/or increased impervious area due to urbanization is known to increase stream flow and peak runoff in streams (Leopold, 1968, National Research Council, 2008). These changes in stream flows can lead to flooding, soil erosion, and sedimentation in streams (Anderson, 1970, McMahon and Cuffney, 2000, Paul and Meyer, 2001, Rose and Peters, 2001, Urbonas and Benik, 1995). As the volume of urban stormwater runoff increased throughout the United States from the increase in impervious surfaces, the quality of surface runoff has degraded substantially (U.S. EPA, 1983). Poorer water quality and increased temperatures due to impervious surfaces can significantly impact human health.

Quantifying tree and impervious cover within the United States is important for understanding the magnitude, distribution and variation of these landscape attributes nationally. By quantifying these cover attributes, better estimates of the impacts of these landscape cover elements can be ascertained, and improved landscape planning and management can be initiated. Understanding how cover types vary among states can also support the development of optimal cover recommendations to sustain natural ecosystem functions in urban areas.

Various estimates of tree and impervious cover in the United States have been made in the past, often based on satellite data. The most recent estimates of tree and impervious cover estimates are based on the 2001 National Land Cover Database (NLCD) that provides free, 30 m resolution, percent tree canopy and percent impervious cover values for the conterminous United States derived from circa 2001 Landsat 7 imagery (MRLC, 2010). However, NLCD estimates of tree cover tend to underestimate tree cover, relative to photo-interpretation estimates, in the conterminous United States by an average of 9.7% (SE = 1.0%) and impervious cover by 1.4% (SE = 0.4%) with underestimates varying by region (Nowak & Greenfield, 2010).

Google Earth^® imagery provides a good means to assess overall cover as it offers nearly complete coverage of the conterminous United States with interpretable aerial images. The purpose of this paper is to determine the magnitude and variation in tree and impervious surface cover among states using aerial photo-interpretation, and to quantify how these cover types vary overall and within and among rural and urban/community defined areas.

Section snippets

Methods

To determine the percent tree and impervious cover in the United States, photo-interpretation of Google Earth^® imagery was conducted. This interpretation was done in various stages based on the area being analyzed: (a) a sampling of the urban/community area in the conterminous United States was conducted, (b) the conterminous United States was interpreted within 65 NLCD mapping zones (Nowak & Greenfield, 2010) to determine cover in rural areas, and (c) an analysis of rural and urban/community

Results

Tree cover in the conterminous United States is estimated at 34.2% (SE = 0.2) or 266.7 million hectares (659.0 million acres). Urban/community areas, which occupy 5.3% of the land area, have comparable tree cover at 35.1% (SE = 0.4) or 14.6 million hectares of tree cover (36.2 million acres) (Table 1). Including Alaska and Hawaii, national tree cover in urban/community areas is estimated at 35.8% (SE = 0.4) or 17.7 million hectares of tree cover (43.7 million acres). Statewide, percent tree cover is

Tree cover

Tree cover in the conterminous United States is an important landscape element, covering over 1/3 of the nation and occupying 266.7 million hectares. Even within urban lands where people and impervious surfaces are concentrated, tree cover is still a dominant element, covering approximately the same percent of land cover (35.0%). A previous urban tree cover study for the conterminous United States, based on advanced very high resolution radiometer (AVHRR) data and Landsat thematic mapper data (

Conclusion

Percent tree and impervious cover differ significantly among states. Urban development increases the amount of impervious cover, but can increase or decrease tree cover depending upon the local environment and human interactions. Urban development in predominantly grassland states tends to increase percent tree cover in contrast to rural lands. Urban development in forested regions tends to decrease percent tree cover relative to rural lands, but the reduction varies among states. These

Acknowledgments

Funding for this project was provided, in part, by the U.S. Forest Service's RPA Assessment Staff and State & Private Forestry's Urban and Community Forestry Program. The use of trade, firm, or corporation names in this article is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S. Department of Agriculture Forest Service of any product or service to the exclusion of others that may be suitable. We thank Mike Boarman,

References (32)

H. Faulkner et al.
Problems of quality designation in diffusely polluted urban streams – The case of Pymme's Brook, North London

Environmental Pollution

(2000)
D.J. Nowak et al.
Tree and impervious cover change in U.S. cities

Urban Forestry and Urban Greening

(2012)
D.J. Nowak et al.
Measuring and analyzing urban tree cover

Landscape and Urban Planning

(1996)
D.G. Anderson
Effect of urban development on floods in Northern Virginia

(1970)
J.F. Dwyer et al.
Assessing the benefits and costs of the urban forest

Journal of Arboriculture

(1992)
Dwyer J. F., Nowak D. J., Noble M. H., & Sisinni, S. M. (2000). Connecting people with ecosystems in the 21st Century:...
C.D. Elvidge et al.
Global distribution and density of constructed impervious surfaces

Sensors

(2007)
Google Inc.
Google Earth

(2011)
G.W. Hamilton et al.
Infiltration rates on residential lawns in Central Pennsylvania

Journal of Soil and Water Conservation

(1999)
G.M. Heisler et al.
The urban physical environment: temperature and urban heat islands

F.E. Kuo et al.

Environment and crime in the inner city: Does vegetation reduce crime?

Environmental Behavior

(2001)

D.J. Leopold

Hydrology for urban land planning: A guidebook on the hydrologic effects of urban land use

(1968)

D.N. Lerner

Identifying and quantifying urban recharge: A review

Hydrogeology Journal

(2002)

B.W. Lindgren et al.

Introduction to Probability and Statistics

(1969)

I.L. McHarg

Design with nature

(1992)

G. McMahon et al.

Quantifying urban intensity in drainage basins for assessing stream ecological conditions

Journal of the American Water Resources Association

(2000)

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Published by Elsevier B.V.

Tree and impervious cover in the United States

Abstract

Highlights

Introduction

Section snippets

Methods

Results

Tree cover

Conclusion

Acknowledgments

Environmental Pollution

Urban Forestry and Urban Greening

Landscape and Urban Planning

Effect of urban development on floods in Northern Virginia

Assessing the benefits and costs of the urban forest

Journal of Arboriculture

Global distribution and density of constructed impervious surfaces

Sensors

Google Earth

Infiltration rates on residential lawns in Central Pennsylvania

Journal of Soil and Water Conservation

The urban physical environment: temperature and urban heat islands