| The construction of high-quality urban forest is an important way to effectively alleviate the problems and improve the quality of urban ecological environment. In the process of urban forest construction, the existing theoretical and technical reserves in arid zones are far from satisfying the actual need. There is an urgent need for a series of theoretical and practical explorations regarding the selection, distribution and configuration of tree species; quality improvement of urban forest and green spaces (greening); and even the spatial configuration technical system of urban forest. Structure configuration is of great significance to improving the ecosystem service functions that the urban forest provides. In this paper, the suburban forest and urban green spaces in Yan’an were investigated based on extensive survey in sample plots, biological and ecological property analysis of tree species, laboratory tests, questionnaire survey, and data collection. The survey data were quantitatively analyzed using the analytic hierarchy process, SPSS R cluster analysis, quantitative theory (Ⅰ), path analysis and artificial neural networks. The quality evaluation standard and index system of urban forests in arid and semi-arid regions was proposed to scientifically evaluate the urban forest quality in Yan’an, and the factors influencing public willingness to pay (WTP) for urban forest construction were identified. Suitable tree species were selected for urban forest construction in the study area, laying a foundation for structure configuration of tree species. Based on quality evaluation of urban forests, quantitative analysis of public WTP, and tree species selection, we summarized the structure configuration scheme of suburban and urban forests in Yan’an, as well as the corresponding operation and management measures. The main research results are as follows:(1) According to the urban forest quality comprehensive index F, the quality of (sub)urban forests in Yan’an can be ranked into three grades:23.33% samplers as high quality (Ⅰ, F=0.93～0.46),56.67% samplers as medium quality (Ⅱ, F=0.42～0.33) and 20.00% samplers as low quality (Ⅲ, F=0.30～0.27). As can be seen, the (sub)urban forests in Yan’an are generally of medium quality, the sub-urban forests are generally of high quality and the city urban forests are generally of low quality. The proportion of Yan’an urban forests quality is 1:3:1.The F value was positively correlated with the stand characteristics (R2=0.167, p＜0.0001), site condition (R2=0.623,p＜0.0001), service function (R2=0.252,p<0.005) and type (R2=0.187, p<0.0001) of urban forests in Yan’an. The binary quadratic curve of F with elevation and slope was extremely significant (R2=0.2267, p=0.0007), and the fitting equation was z=1.40-0.21x-0.46y -0.002749x2-0.000828y2. From -10° to 25°slope, the F value showed great fluctuations with increasing altitude. Specifically, the F value first rose from the lowest point of 0.27 (980 m) to the highest point of 0.93 (1080 m), then decreased and slightly rebounded, and lastly leveled off after a decrease. With 25-50°slope, the F value decreased as altitude increased from 980 m to 1040 m; an opposite variation trend was observed in the F value with increasing altitude from 1040 m to 1200 m; the F value slightly decreased before leveling off with increasing altitude above 1200 m. As can be seen, the urban forests quality on the shady and gentle slpoe are generally of high quality; the quality of urban forests have large change on the sunny and steep slope.(2) The regression relationship between 11 factors of urban forests in Yan’an and public WTP for urban forest construction was significant (R=0.595, F=46.637>F(11,936) a=o.ooo= 18.596). Among these factors, forest scenic spot had the largest standardized regression coefficient (1.207) and partial correlation coefficient (0.232), with an extremely significant impact on public WTP (p<0.01). Factors following forest scenic spot included tourists’ intrinsic properties (0.384), purpose and demands (0.356), travel mode (0.308) and sensory evaluation (0.244), which also had extremely significant impacts on public WTP (p<0.01). In the 11 factors, only revisit times and willingness to prose increase had no significant impact on public WTP (p>0.05).According to the numerical theory (Ⅰ) and path analysis, the 11 influencing factors of public WTP were arranged in the following order:forest scenic spot (1.207)> tourists’ intrinsic properties (0.384)> purpose and demands (0.356)> travel mode (0.308)> sensory evaluation (0.244)> urban environment (0.178)> monthly income (0.138)> place of residence (0.112)> revisit times (0.098)> communication network (0.063)> willingness to price increase (0.028). Twenty percent of the samples were analyzed by multilayer perceptron training. The results showed that objective factors affected public WTP to different degrees: 81.82% reached the significant level (p<0.05), including 54.55% at the extremely significant level (p<0.01). Subjective factors, such as objective and demand, also strongly influenced public WTP.(3) Tree species of urban forests in Yan’an were comprehensively evaluated by measuring drought resistance (leaf structure, diurnal variation of water potential, photosynthetic rate, diurnal variation of transpiration rate, photosynthetic water use efficiency and major water physiological parameters), cold resistance (peroxidase activity, soluble protein content and malondialdehyde content), and pollution resistance (anti-SO2 pollution, heavy-metal adsorption and dust-catching capacities). The following 15 species exhibited outstanding performance: Sophora japonica var. japonica (0.1421), Platycladus orientalis (0.1344), Sophora japonica (0.1265), Prunus serrulata (0.0924), Salix babylonica (0.0912), Ailanthus altissima (Mill.) Swingle (0.0860), Ligustrum vicaryi (0.0719), Salix matsudana Koidz. (0.0657), Prunus cerasifera cv. Atropurpurea (0.0581), Pinus tabulaeformis (0.0545), Amygdalus triloba (0.0257), Picea asperata (0.0221), Amygdalus persica (0.0125), Robinia pseudoacacia (0.0097), and Chaenomeles speciosa (0.0070).(4) The stand density of urban forests in Yan’an was extremely significantly correlated with soil available phosphorus content (p<0.01) and significantly correlated to soil organic matter and total phosphorus contents (p<0.05). There was no significant correlation between stand density and soil pH, available potassium, total nitrogen, total potassium, nitrate nitrogen, or ammonium nitrogen. Margalef index and total phosphorus were significantly positively correlated (p<0.05), while Shannon-Wiener index, Pielou index and Simpson index were no significant correlation with soil nutrient.Soil moisture condition is the dominant factor for dividing the types of site conditions. On this basis, we proposed to divide the stand sites using the combinations of external features of terrain factors. According to the results of plot survey, the structure of most stands located in sunny flat top, sunny uphill steep, and shady downhill gentle-slope areas need to be adjusted, the same as stands located in sunny downhill steep and sunny uphill gentle-slope areas. There is generally no need to adjust the structure of stands at sunny downhill or shady uphill gentle-slope areas.(5) Based on tree species selection and site condition analysis, we proposed the structure configuration of urban greening in Yan’an:Landscape forest should be appropriately adjusted with increased evergreen coniferous species to enhance the landscape effect. In areas such as Baota, Fenghuang and Qingliang Mountains, it is recommended to configure Quercus variabilis, Robinia pseudoacacia, Platycladus orientalis, Pinus tabulaeformis, Sophora japonica, and Amygdalus davidiana. Public greenfield should be configured with Sophora japonica f. pendula, Sophora japonica, Salix babylonica, Prunus cerasifera, Cerasus yedoensis, Amygdalus triloba, Juniperus formosana, Syringa oblate, Chaenomeles speciosa, and Ligustrum lucidum. Memorial greenfield, which holds a particular important position in the political, cultural and other aspects in Yan’an, should be configured with Sophora japonica, Amygdalus triloba, Prunus cerasifera, Salix matsudana, Robinia pseudoacacia, Syringa oblate, Juniperus formosana, Sabina chinensis, and Sophora japonica f. pendula. Special affiliated greenfield are mostly located in schools, which can be configured with Sophora japonica f. pendula, Prunus cerasifera, Cerasus yedoensis, and Amygdalus triloba.For the structure configuration of suburban forests in Yan’an: Protection forest should be configured with different species according to various terrain characteristics. Wind-and cold-resistant, drought-tolerant tree species are recommended for ridge and hill top, Platycladus orientalis, Ailanthus altissima. Ridge slope should be configured with slope-protection forest and planted with slope-protection grass, Sophora japonica, Amygdalus persica. For slope-protection forest of ridge and gully slopes, mixed forest of arbor and shrub is a good option, Sophora japonica f. pendula, Koelreuteria paniculata, Ligustrum vicaryi, Rosa chinensis.(6) The basic guarantee for urban forest development is proper selection of operation direction and rational configuration of forest stands according to natural conditions, resource situation and current economic development in the city. The important task of silvicultural work in Yan’an is to establish sound forest spatial structure by timely rejuvenation of urban forest stump, transformation of inefficient forest, regeneration of tree species, strengthening of stand tending management, and protection of forest soil litter layer. |
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