Street tree canopy is especially problematic to assess because of differences that exist in street widths and configurations. After reviewing existing methods and experimenting with various methods, we recommend canopy cover at the edge of pavement (CCEP) as a standard for assessing street tree canopy. CCEP can be measured on almost all types of roads either from aerial imagery or by ground survey. Furthermore, CCEP values are related to the amount of shading that streets receive, and the "canopied" effect that is obtained when trees arch over streets. In the two photos below, the street on the left has a low CCEP value whereas the street at the right has a high CCEP value. Both streets are in Vacaville, CA.
Variations of the line intercept method are used to measure CCEP from aerial images. Using the hybrid line/point method, the evaluator lines up a single row of dots or a finely divided ruler along the visible edge of the pavement. The evaluator counts the number of dots or ruler divisions that fall on tree crowns and the total number of dots in the sampled segment. Percent CCEP is then calculated as:
% canopy cover = 100 x (points falling on tree canopy/total number of points along sampled lines)
While this method is easiest to apply on relatively straight sections of road, it is possible to apply the method on moderately curved roads by using a flexible plastic rule held on edge.
On very curved roads and those with very low or high canopy cover, it may be more efficient to use the standard line intercept method. In this method, the lengths of all tree canopies that intersect the line of the edge of pavement are measured with a ruler, planimeter, or digitizer. Percent CCEP would be calculated as for the line intercept method, i.e.,
% canopy cover = 100 x (length covered by tree canopy / total length of sample)
CCEP can also be readily evaluated by using a foot survey. To estimate CCEP in a foot survey, the evaluator walks along the edge of the pavement. At evenly spaced points, e.g., every 3 steps, the evaluator notes whether canopy is present directly over the point at the edge of the pavement. Hand tally counters can be used to keep track of the total number of sample points and the number of sample points with tree canopy overhead. A length of 1/2 inch diameter PVC pipe, lightweight measuring pole, densitometer (a small instrument with a level that allows one to sight a point directly overhead), or similar tool can be used to help project a line vertically from the edge of pavement upward to increase the accuracy of the evaluation. The percent CCEP is calculated using the following formula:
% CCEP = 100 x (points with canopy cover/total number of points)
One advantage of measuring CCEP as part of a ground survey is that it allows one to examine correlations between CCEP and other tree or site characteristics. The evaluator can collect data on site or tree characteristics at the same time that canopy is being assessed. The level of CCEP can be affected by tree species selection, tree age, planting position, and pruning practices, and it may be of use to know which factors are the most important in your community.
Evaluation example: Street tree canopy measured using two methodsWe measured percent canopy cover at edge of pavement (CCEP) in two different subdivisions in Vacaville, California. One subdivision was constructed around 1950 and the other was completed in 1975. We assessed CCEP from 1:2,400 aerial photography taken in 1973 and 1980, 1:1,200 aerial photography taken in 1978, and by ground surveys in 1990 and 2001 along typical residential streets in these areas. Street segments analyzed were about 0.5 mile in length. Percent CCEP values (averaged for both sides of each street) for the different evaluation dates are shown below. It should be noted that even though several curb/sidewalk configurations were present along different portions of the first site (no sidewalk, sidewalk adjacent to curb, treelawn between curb and sidewalk), they did not affect the determination of CCEP.
Subdivision completed in 1950 (Peach
Subdivision completed in 1975 (Andrea Drive; left photo
at top of page)
These data reflect changes in street tree selection and planting practices between 1950 and 1975 as well as the effects of tree maintenance practices. Part of Peach Tree Avenue was planted by the developer with Modesto ash (Fraxinus velutina 'Modesto') street trees. These are fast-growing trees that produce a large crown. Trees were planted relatively close to the sidewalk, and consequently a high CCEP developed in these areas relatively quickly. Other portions of the same street were planted on an ad hoc basis by homeowners, and CCEP levels in these areas were relatively low. Canopy cover on Peach Tree peaked about 30 years after development as the Modesto Ash trees reached mature size. Subsequently, poor maintenance practices by many homeowners, particularly severe topping, have led to a decline in tree condition and CCEP. Between 1990 and 2001, a number of mature trees were removed, causing further reductions in CCEP. Notably, CCEP on the south side of Peach Tree Ave. has dropped from 34% in 1990 to 25% in 2001.
In the Andrea Drive subdivision, developers provided a tree for each house to be planted by the owners. Not all owners complied and/or some plantings were unsuccessful, and as a result, street tree canopy is quite sporadic. Many of the trees were species that develop only medium-sized crowns. Because many of the trees were also planted in the centers of the front yards rather than near the street, most tree crowns barely reach the edge of pavement even though the trees are approaching their mature spread. Thus, while CCEP has increased from 8% in 1990 to 20% in 2001, it is unlikely to increase much further unless additional trees are planted. CCEP 26 years after development on Andrea Drive is well below that seen 23 years after development on Peach Tree Avenue.
Street tree canopy provides a variety of benefits, including reductions in the urban heat island effect and resultant energy conservation, longer pavement life, "traffic calming", reduced hydrocarbon emissions from cars parked along streets, and enhanced property values. If a community has the objective of developing and maintaining street tree canopy through comprehensive planning and appropriate ordinances, evaluations of CCEP help provide the needed information on the status of street tree canopy and the consequences of past management actions.
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