Document Type

Working Paper

Publication Date

8-2005

Subjects

Trees in cities -- Oregon -- Portland, Urban forestry -- Oregon - Portland, Forests and forestry

Abstract

Attempts to identify the contribution of street trees to the overall urban forest of a city have been rare and lack consensus on how to measure that contribution – percentage of trees, percentage of canopy cover, or percentage of leaf area. The actual numeric values presented in the literature also vary over a broad range and often are based on estimates, extrapolations from aggregated data, or simply stated with no empirical data referenced. This study was undertaken to evaluate the contribution of street trees to canopy in Portland, Oregon. The study involved both visual and digital analysis of multi-band aerial images, field observation, and GIS analysis involving several ancillary data themes. The strategy was to calculate the street tree component (STC) of the urban forest canopy within a sample of areas in the city and generalize that pattern to the whole city.

The analysis indicates that street trees constitute 3.4% of the whole study area. In commercial / industrial areas the value drops to only 1.1% and in residential areas rises to 4.8%. For the overall study area the STC of the canopy is 17.2%, a value in the middle of the range of values found in the published literature. In commercial / industrial areas we found a value of 23.7% and a value of 16.6% for residential areas.

Residential areas are overall better treed than commercial / industrial areas, but do not rely on street trees to contribute as large a percentage to overall canopy as do commercial / industrial areas. Available planting space in the right-of-way (ROW) is much more nearly constant (or independent of land cover) than the amount of planting space available inside a block. Therefore, the STC tends to vary inversely with the total amount of vegetated area. As the vegetated (read, plantable) area decreases, the ROW becomes a more significant space for planting and the STC increases. Conversely, as the vegetated area increases, the ROW becomes a less significant space for planting and the STC decreases.

For both residential and commercial / industrial areas, the percentage of street tree area within the total canopy varies from locale to locale. The relatively small sampling used in this study suggests that neighborhood age and physical characteristics influence the STC in a given neighborhood, particularly for residential areas. Analysis allowed division of the study area into five residential neighborhood groups that attempt to reflect local differences. A larger / more detailed sampling would allow for improved delineation, perhaps below the level of neighborhood. Three generalized spatial patterns emerge for Portland.

  1. Street tree canopy as a percent of area seems to follow an east-west gradient, with lower values to the east and higher values to the west.
  2. Street trees as a component of the total canopy takes on an inverted-U shape, with higher values in the Inner Eastside and lower values to the east and to the west.
  3. The overall tree canopy category takes on just the opposite pattern – a standard-U – with lower values in the Inner Eastside and higher values to the east and west.

This study represents a useful starting point for understanding the dynamics of an important and highly visible component of the urban forest.

Persistent Identifier

http://archives.pdx.edu/ds/psu/10791

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