مقایسه ی سود و زیان جنگل شهری در مودستو (Modesto) و سانتا مونیکا، کالیفرنیا، ایالات متحده آمریکا

This paper presents a comparison of the structure, function, and value of street and park tree populations in two California cities. Trees provided net annual benefits valued at $2.2 million in Modesto and $805,732 in Santa Monica. Benefit-cost ratios were 1.85:1 and 1.52:1 in Modesto and Santa Monica, respectively. Residents received $1.85 and $1.52 in annual benefits for every $1 invested in management. Aesthetic and other benefits accounted for 50% to 80% of total annual benefits, while expenditures for pruning accounted for about 50% of total annual costs. Although these results were similar, benefits and costs were distributed quite differently in each city. Variations in tree sizes and growth rates, foliation characteristics, prices, residential property values, and climate were chiefly responsible for different benefits and costs calculated on a per tree basis.

Cities need to grow to maintain vigorous local economies, but their ability to grow is influenced by environmental constraints and competition with other regions in terms of quality of life. Research quantifying the benefits of healthy municipal urban forests is showing that trees can mitigate impacts of development on air quality, climate, energy for heating and cooling buildings, and stormwater runoff. Healthy street trees increase real estate values, provide neighborhoods with a sense of place, and foster psychological well-being (Dwyer et al. 1992). Street and park trees are associated with other intangible benefits such as increased community attractiveness and recreational opportunities that make cities more enjoyable places to work and play. The motivation for this study is to provide cities with a comprehensive accounting of municipal forest bene-fits and associated management costs. We build upon previous benefit-cost analyses in Chicago, IL and Modesto, CA (McPherson et al. 1997, 1999a) that were applied to: • assess the adequacy of management programs and justify their funding, • provide baseline information for the evaluation of program cost-efficiency, • highlight the relevance of the urban forest to local quality of life, • develop alternative funding sources through electric utilities, air quality districts, federal or state agencies, legislative initiatives, or local assessment fees.

Prices can be key performance variables because of their effect on the value of urban forest benefits. Price differences between cities were substantial, especially for stormwater, air quality, and aesthetic benefits. Average annual transaction costs for O3 and NO2 were two to three times higher in Santa Monica than Modesto (Table 6). Avoided stormwater runoff reduction benefits were priced four times higher in Modesto than Santa Monica (Table 7). Modesto’s implied values were associated with retention/detention costs of about $ 2/m3, while most of the hydrologic benefits in Santa Monica were priced based on much lower costs for treating sanitary waste ($ 0.48/m3). The relatively large differences in per tree costs and benefits became less when considered on a per capita basis. For example, the difference in per tree annual expenditures ($ 52.82 – $ 28.77 = $ 24.05) is substantially greater than the per capita difference ($ 16.68 – $ 14.39 = $ 2.29). Evaluating benefits and costs on a per capita basis incorporates differences in population densities and tree stocking levels. Santa Monica is four times more densely populated and has 37% fewer public trees per capita than Modesto. Because of more intensive urbanization, trees in Santa Monica may require more intensive care than trees in Modesto. Fewer trees per capita in Santa Monica mean that management costs and benefits are distributed among more people. The benefit-cost analyses resulted in similar management recommendations for both cities: • increase stability of the population to promote longterm net benefits from continuous levels of tree canopy cover • control program and non-program costs to increase net benefits. In Modesto and Santa Monica a large percentage of total benefits were produced by species nearing the end of their life cycles (i.e., Modesto ash, Chinese hackberry, Deodar cedar). Selective removal and replacement is required to avoid substantial short-term loss of canopy cover and to hasten the transition to a more stable population. Although removing large trees may reduce net benefits in the short-term, it provides longterm protection against catastrophic losses that could decimate tree canopy cover and magnify tree removal costs. In Modesto, there was need to discontinue heavy planting of Chinese pistache, already 18% of all trees in the 20–40 cm dbh class. The city nursery should continue to grow and test new tree species and cultivars. Selecting trees that are well-matched to site conditions, will grow as large as possible given space restrictions, and enhance overall diversity are keys to transitioning into a more stable forest. Conflicts between tree roots and hardscape were paramount in Santa Monica due to the large number of cutouts and narrow planting strips. One recommendation was to plant larger-statured trees in front yards where feasible. Another suggestion was to continue experimenting with strategies to reduce conflicts and repair costs such as rubberized “flexible” paving, meandering sidewalks around trees, using structural soils in commercial areas, and deep irrigation to direct roots away from the soil surface (Costello et al. 2000).