The Investigation And Evaluation Of The Urban Green System In Ya'an City

The ecological environment construction of which the garden afforestation is one of the core issues is the basic characteristics of the modern city construction. Ya'an is located in the west part of Sichuan Basin and belongs to the season wetness climate with central Asia tropics, which plant coverage is the evergreen broad-leaved forest. The construction of the Urban Garden and Greenland there represented and demonstrated for the medium and small towns which shared the same climate and vegetation with Ya'an. This article concentrated on the investigation and evaluation of vegetation fraction estimation, the landscape indices of urban greening, the component species of plant community, site condition and transplant and management. The results were as follows,(1) Till 2005, the urban green coverage, greening rate and average public green land per capital was 23.14%, 22.01% and 5.48m~2/p, respectively, did not reached the criteria of 35%, 30% and 8 m~2/p. By contrast, the percentage of the three indices was 36.4%, 33.6%, and 9.22m~2/p in Chengdu.(2) The landscape of urban green space of Ya'an City was mainly divided into public green space landscape, road green space landscape, residence green space landscape, subsidiary green space landscape and other green space landscape. Among this five green space landscapes, the area of other green landscape was largest, which mainly included scenery area, water resource protection area, outskirt park, forest park and agriculture forest vegetation. The landscape diversity index of study area was low and the proportion of landscape types was uneven. Other green space landscape had the major dominance index. It showed that the main development trend of urban green space was other green space landscape. Other green space landscape was also the key point of Urban-Rural Integration greenery for future. The dominance index of public green space landscape was low and it was possible that it would develop better in future. Base on the analysis of landscape fragmentation and landscape isolation, the researcher come to conclusion that, because of the large area of old urban district and high building density, residence green space landscape and subsidiary green space landscape become to be such landscapes that had great amount of patch and small patch area. Due to the lack of large-scale green space, there was disadvantage to conservation of city biodiversity. The fractal dimension of green space landscape showed that the spatial pattern of green space landscape in Ya'an City was complicate and unstable.(3) Based on the investigation, the amount and component of street trees and its soil condition in Ya'an were studied in this paper. The results indicated that: The cover index of both street of the north river and streets along river exceeded to the national standard (20%), with numerical value being more than 40% and 22%, respectively. While the value of main street and slip street of the south fiver were only 8% and less than 11% respectively, which were lower than the national standard (15%). As far as the relative coverage of species of the trees concerned, Ficus virens was the major tree in the main street and streets along river, while in south fiver section, London Planetree were much more than any others and Needle Fir was just in the second place. As a whole, the relative amount of evergreen broadleaf was fewer and its kind was simple. So "Ficus virens＋Native trees of Evergreen broadleaf" was proposed to make the local color and ecological function harmoniously. The topsoil was covering with new soil and its quality was good, but some topsoil did not communicate with the subsoil. The soil capacity was small and dashed with some flotsams. All of these were the main limiting factor for the process of tree's growing.(4) The abundant degree of arbor and shrub had been incremented in public green space, residence green space and subsidiary green space, the advantage of green complex structure have become a consensus. It is will be the main direction in urban greening that the applied enlargement of local trees and herbs.