| In order to guide the development of urban forest, improve urban ecological environment, promote the construction of ecological civilization, and improve human welfare, we established the urban forest ecosystem service function valuation system based on the "Specifications for assessment of forest ecosystem services in China"(LY/T 1721-2008), and the characteristics and subjectivity function of the urban forest in Yan’an city. The function indexes of suburban forest, urban green space and street tree were observed, and the value of urban forest ecosystem service function was evaluated by using the principles of forestry, garden, ecology, landscape ecology, meteorology, soil science and so on. The evaluation methods and evaluation parameters in this study will also to lay a foundation for the development of the urban forest ecosystem service function valuation indexes system in arid and semi-arid area, and to provide basic information for rational planning and distribution, tree species selection, and structural optimization of urban forest in Yan’an, and to provide evidence for regional government economic control, sustainable development of urban forestry, and ecological environment security, and to establish ecological compensation mechanism. The results are as follows:(1) According to the field surveying, the total area, built-up area, woodland area, grassland area and forest coverage of study region was 1 545.05 km2, 56.53 km2, 913.10 km2, 146.24 km2 and 59.10 %, respectively. The total area of urban green space was 614.92 hm2, including public park was 51.10 hm2, nursery was 334.19 hm2, green buffer was 53.20 hm2, attached green space was 176.43 hm2, attached green space was 5504.00 hm2, per capita green space was 28.88 m2, and green space ratio was 10.88 %. There were 25979 street trees, belonging to 28 species and 23 genera and 13 families; Sophora japonica and Salix matsudana f. pendula have higher frequencies, which accounting for 50.78 % and 14.81% of the total. There were 246 common greening plants, including 33 coniferous trees species, 99 broadleaved tree species, 72 shrub species, 32 herb species and 10 liana species.(2) The carbon fixation and oxygen release value of urban forest was evaluated by the determination of net photosynthetic rate diurnal variation of main greening species in different seasons and combining with regression models of leaf area. The results showed that mixed forest has the biggest carbon fixation and oxygen release function in suburb, while arbor-shrub-herb type in urban green space. The volume of carbon fixation and oxygen release was 188721.25 ton and 505028.97 ton, respectively, and the value was 226.65 million yuan and 505.03 million yuan, respectively.(3) Annual surface runoff and soil erosion amount were observed through the construction of runoff field in the selected represent forest stands, the value of water conservation and soil conservation was 6982.35 million yuan, which calculated by water balance method, including the value of water regulation, water purification, soil conservation and fertilizer conservation was 3243.03 million yuan, 901.56 million yuan, 691.94 million yuan and 2145.81 million yuan, respectively. The result also showed that broadleaved mixed forest have the largest soil and water conservation function, and level-terrace site preparation could markedly reduce the surface runoff and soil erosion in the forest.(4) Nutrient accumulation function was evaluated base on photosynthetic parameters, leaf area index, and nutrient(N, P, and K) accumulation of greening species. The results showed that: affected by the way of land use, production forest has the largest function of nutrient accumulation, following the shrub after farmland returning; the amount of nutritious substances accumulation annually was 598071.36 ton by city forest, and the value was 116.76 million yuan, contains 401104.23 ton and 100.28 million yuan for N fixation, 46303.26 t and 10.45 million yuan for P fixation, and 150663.87 ton and 6.03 million yuan for N fixation.(5) Based on the detection of heavy metal accumulation, sulfur dioxide absorption, and dust detention of greening trees, and compared the anion providing and reducing noise function by green space with the control, the value of atmospheric and environment purification was evaluated. The total value was 539.10 million yuan, including 538.35 million yuan for suburban forest, 0.75 million yuan for green space, and 7504.66 yuan for street tree. Species that accumulated heavy metal well included Sabina chinensis(MAI=3.89), Juniperus formosana(3.32), Ailanthus altissima(3.06), Sophora japonica(2.56), Salix matsudana f. pendula(2.68) and Ligustrum vicaryi(2.91); took up sulfur dioxide well included Salix matsudana(3.21 mg·g-1), Salix matsudana f. pendula(3.49 mg·g-1) and Fraxinus chinensis(1.21 mg·g-1); and detained dust well included Sabina chinensis(7.23 g·m-2), Juniperus formosana(7.18 g·m-2), Sophora japonica(2.12 g·m-2), Ulmus pumila(3.47 g·m-2), Fraxinus chinensis(2.11 g·m-2), Chaenomeles speciosa(5.36 g·m-2), Amygdalus triloba(10.80 g·m-2), and Syringa oblata(3.70 g·m-2).(6) Based on the diurnal dynamics of the leaf transpiration rate measured in spring, summer and autumn respectively, transpiration water loss and heat absorption of greening tree were obtained, and then translated into the climatic regulation value of urban forest. The results showed that green spaces and street trees in study area could effectively reduce the temperature as well as increase the humidity in urban area. The climatic regulation value was 336.63 million yuan, including 69.85 million yuan for temperature reduction in summer, 25.39 million yuan, 192.93 million yuan and 48.46 million yuan for humidity increase in three seasons, respectively. According to the results, Sophora spp., Salix spp., Ailanthus altissima, Amygdalus persica f. duplex and Ulmus pumila were tree species with better temperature reduction and humidity increase; Ligustrum vicaryi and Chaenomeles speciosa were shrub species with better temperature reduction and humidity increase.(7) Species diversity indices of forests and green spaces in study area were calculated, and then the species conservation value of different forest communities and green spaces were obtained by reference to annual species conservation value. The value of biodiversity conservation of urban forests was 937.49 million yuan. Based on field survey, questionnaire and interview, and also combined with data from government website, statistical yearbooks and related documents, the value of economic function and social function of urban forests was 378.24 million yuan.In summary, the total value of the forest ecosystem services in Yan’an was 10.02 billion yuan. The value of the service functions of ecological unit was in an order of suburban forest > urban green space > street tree, and the value of the service functions of urban forest ecosystem was in an order of water conservation > soil and fertilizer conservation > biodiversity protection > carbon fixation and oxygen release > atmospheric and environment purification > climatic regulation > social function > nutriment accumulation > economic function. |
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