Urban Ecosystems and Social Dynamics
Urban Ecosystems and Processes
Urban Ecosystems and Social Dynamics
City of San Francisco, California Street Tree Resource Analysis
Street trees in San Francisco are comprised of two distinct populations, those managed by the cityâ??s Department of Public Works (DPW) and those managed by private property owners with or without the help of San Franciscoâ??s urban forestry nonprofit, Friends of the Urban Forest (FUF). These two entities believe that the publicâ??s investment in stewardship of San Franciscoâ??s urban forest produces benefits that outweigh the costs to the community. Hence, the primary question that this study asks is whether the accrued benefits from San Franciscoâ??s street trees justify the annual expenditures?
This analysis combines results of a citywide sample inventory with benefit-cost modeling data to produce four types of information (Maco 2003):
1. Resource structure (species composition, diversity, age distribution, condition, etc.)
2. Resource function (magnitude of environmental and aesthetic benefits)
3. Resource value (dollar value of benefits realized)
4. Resource management needs (sustainability, pruning, planting, and conflict mitigation)
â?¢ Based on the FUF inventoried sample of 2,625 trees, there were an estimated 98,534 (Â±9,677) street trees in San Francisco. Publicly managed trees accounted for 18.5% (18,234 Â±2,779) of the total, while privately cared for trees comprised the remaining 81.5% (80,301 Â±9,634).
â?¢ While San Francisco is on par with the statewide average of 104 trees per street mile, there are many opportunities to increase the resource extent. Approximately 127,500 sitesâ??56% of all street tree-planting sitesâ??were unplanted, ranging from 28% to 74% among districts.
â?¢ Citywide, the resource represented 115 different tree species and diversity was high. However, several districts were dominated by few species and lack of diversity should be of concern to managers.
â?¢ Having the most numbers, leaf area, and canopy cover, Victorian box and London plane were found to be the two most important street trees in San Francisco.
â?¢ Age distribution varied by district, but citywide the street tree population was immature, lacking adequate numbers of functionally mature trees.
RESOURCE FUNCTION AND VALUE
â?¢ Because of San Franciscoâ??s moderate summer weather, potential energy savings from trees are lower than those that would be found in warmer inland locations. Electricity and natural gas saved annually from both shading and climate effects totaled 651 MWh and 1,646 Mbtu, respectively, for a total retail savings of $85,742 ($0.87/tree).
â?¢ Citywide, public trees sequestered 611 tons of the greenhouse gas carbon dioxide. The same trees offset an additional 71 tons through reductions in energy plant emissions. Private trees had an annual net sequestration rate of approximately 1,660 tons and reduced emissions by another 186 tons. The combination of these savings was valued at $37,907 ($0.38/tree) annually.
â?¢ Annual air pollutant uptake by tree foliage (pollutant deposition and particulate interception) was 12.5 tons combined. The total value of this benefit for all street trees was $189,375, or about $1.92/tree.
â?¢ Because of the relatively small avoided hydrocarbon emissions benefit at power plants due to energy savings, and the fact that many species have high biogenic volatile organic compound (BVOC) emission rates, trees had a negative net impact on avoided pollutant emissionsâ??causing more harm than good. The net cost of avoided and BVOC emissions was valued at approximately $135,000, or $1.37/tree.
â?¢ The ability of San Franciscoâ??s street trees to intercept rainâ??thereby avoiding stormwater runoffâ??was substantial, estimated at 13,270,050 ft3 annually. The total value of this benefit to the city was $467,000. These values ranged by district and population subset. Citywide, the average street tree intercepted 1006 gallons, valued at $4.73, annually. Thus, street trees were found to provide a particularly important function in maintaining environmental quality of San Franciscoâ??s important water resources.
â?¢ The estimated total annual benefit associated with property value increases and other less tangible benefits was approximately $6.9 million, or $70/tree on average. London plane ($146/tree), blackwood acacia ($110/tree), and Chinese elm ($361/tree) were on the high end, while New Zealand Christmas tree ($40/tree), evergreen pear ($23/tree), and maidenhair tree ($23/tree) averaged the least benefits.
â?¢ Overall, annual benefits were determined largely by tree size, where large-stature trees typically produce greater benefits. For example, average small (lemon bottlebrush), medium (New Zealand Christmas tree), and large (blackwood acacia) broadleaf evergreen trees produced annual benefits totaling $16, $44, and $125 per tree, respectively.
â?¢ The street tree resource of San Francisco is a valuable asset, providing approximately $7.5 million ($77/tree) in total annual benefits to the community. However, approximately the same amount is spent on their care.
Other important findings:
â?¢ Fort Collinsâ?? municipal forest is healthy and well-stocked. Seventy-five percent of the trees are in excellent or good condition, 20% are in fair condition, and 5% are poor, dying, or dead. Approximately 66% of all streetside planting sites are filled with trees.
â?¢ Benefits total $2.17 million ($70/tree) with greatest value for aesthetic benefits/increased property values ($1.6 million, $52/tree) and reduced stormwater runoff ($404,000, $13/tree). Building shade, cooler summertime temperatures, and decreased winter winds attributed to street and park trees produce energy savings valued at $112,000 ($4/tree). Smaller benefits result from atmospheric carbon dioxide reduction ($25,000, $2/tree) and improved air quality ($18,000, $1/tree).
â?¢ These findings indicate that the Cityâ??s trees are providing important aesthetic, health, and environmental benefits to residents.
â?¢ Costs total $997,000 ($32/tree) with 41% of this amount for pruning ($405,000). Mature tree care accounts for 70% of total costs.
â?¢ Green ash is the most abundant street tree species. It accounts for 22% of all street trees and produces 17% of all street tree benefits. Other important species by virtue of their size and numbers are Honeylocust, American elm, Hackberry, Siberian elm, Littleleaf linden, and Silver maple.
â?¢ Large, old trees (> 24 inch dbh) account for 11% of the population and produce 22% of all benefits ($144/tree). American elms on streets alone account for 23% of total annual benefits from old trees. Siberian elm, Silver maple, and Plains cottonwood along streets together account for another 31% of the remaining benefits. Although these dominant species have proven to be relatively long-lived, they are being phased-out of the population. Insect and disease problems, brittle wood and intensive pruning requirements make them unsuitable for planting in large numbers. Intensive inspection and maintenance are necessary to insure that these problems do not jeopardize tree health, public safety, and the sizable benefits that these trees produce.
â?¢ The City is planting a myriad of large-stature trees that are more suitable than the species they replace. These new plantings include varieties of White ash, Oak, Maple, and Linden. As a result, the forest is becoming more diverse, and ultimately, more stable.
Years of Research: 2003
Funded by: USDA Forest Service, Pacific Southwest Research Station, Center for Urban Forest Research, Davis, CA.
Cooperators: City of San Francisco, and (FUF) Friends of the Urban Forests.
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