Composition and Estimated Value of Selected Benefits of Trees in Parking Lots in Raleigh, NC

Abstract

Trees are often planted in parking lots to reduce the environmental and visual impacts created by these paved surfaces. I used a two-phase cluster sampling scheme to assess the composition of trees within parking lots in Raleigh, NC, USA. These parking lots contain discrete unpaved areas, called inclusions, which I classified into four types by size and shape: inclusions smaller than 1,000 ft2 were categorized as compact “islands†or linear “slivers†, and inclusions larger than 1,000 ft2 were categorized as compact “chunks†or linear “rows†. I measured more than 1800 trees in 502 inclusions within 110 parking lots to determine the overall composition of trees and how this composition varied among different inclusion types.

Raleigh’s parking lots were found to contain 44,000 ± 24,000 trees (with 95% confidence). These trees were estimated to provide benefits valued at more than one million dollars annually. Inclusions containing trees that appear to have existed before construction (“preserved†inclusions) were found to have more trees, canopy, and basal area per acre than other (“designed†) inclusions. Significant differences in tree composition and the proportion of trees meeting Raleigh’s mature shade tree size goals were associated with the size and shape of the inclusion used. A disproportionately high number of willow oaks and trees in larger inclusions were observed meeting Raleigh’s size goals for mature shade trees. Additionally, using GIS, parking coverage was found to vary greatly between different zoning classes, with the greatest coverage in commercial and industrial areas. These results indicate that the distribution and coverage of parking lots in Raleigh, and the composition and resulting benefits of trees within these lots, may depend upon planning and design decisions made before a parking lot is constructed. This study provides a methodology for assessing the composition of trees in a city’s parking lots, which allows comparisons to be made among different types of parking lots, inclusions, and trees. While few studies have addressed the composition of trees in parking lots, this area of research has both practical and scientific value. With additional research, we may better understand how to maximize the ecosystem services provided by trees in limited growing spaces, and use this knowledge to design parking lots with improved environmental impacts.