ObjectiveWe studied the tree species composition structure in residential areas within the Fifth Ring in Beijing between the year 2006 and 2016, put forward some suggestions about greening of residential areas, and provided reference for planning and restructuring the urban residential green space in the future.

MethodThe main sites of the study were 23 residential areas within the Fifth Ring in Beijing. Trees and shrubs with the height over 1.5 m were investigated. Main contents of the investigation included species, height, diameter at breast height (DBH), crown diameter, tree amount and pruning extent. We analyzed the density, species richness of trees in 23 sampled plots and frequency, density, coverage, relative frequency, relative density, relative coverage, and average cover area of plants and important values of the 20 dominant species. After that, the related data was compared with that in 2006.

Result(1) All trees in the 23 residential areas involved 34 families, 61 genera and 85 species. There were 9 799 trees in 23 sample plots. Arbors were 6.2 times of shrubs. The average density of trees was 39 plants/ha, which was 27% higher than a decade ago, and the average density of trees increased by 30%; the tree species increased by 39%, and the proportion of shrubs increased to 1.55 times. (2) The species varieties and amount of 20 main tree species changed greatly from 2006 to 2016. Ornamental and edible trees showed an obvious increase. The tree number of Amygdalus persica var. persica f. duplex, Magnolia denudata, Malus × micromalus, Toona sinensis, Platanus hispanica, Ginkgo biloba increased to 7.07, 6.36, 5.53, 4.90, 3.38 and 3.28 times of year 2006, respectively, and they became the new dominant tree species in the green land of residential area. While the numbers of Populus tomentosa, Platycladus orientalis, Ziziphus jujuba var. inermis, Paulownia tomentosa, Pinus bungeana, which were once dominant tree species reduced sharply to 43.95%, 11.74%, 24.09%, 9.34% and 41.24% of year 2006, respectively. At the same time, the number of Sabina chinensis increased to 5.84 times of ten years ago, showing a completely opposite trend to Platycladus orientalis. Sabina chinensis replaced Platycladus orientalis and became the main new coniferous species in the residential area. (3) Compared with year 2006, the use frequency of 20 major tree species in green land of residential area showed an increasing trend. In which, Sabina chinensis, Amygdalus persica var. persica f. duplex, Magnolia denudata, Platanus hispanica increased evidently and were 22.75, 5.27, 4.25 and 3.15 times of ten years ago, respectively. Although the numbers of Ziziphus jujuba var. inermis, Shibataea chiangshanensis and Diospyros kakiwere were comparatively less and even fewer than 2006, yet their use frequencies increased from 0.44, 0.28, 0.48 to 0.61, 0.67 and 0.67, respectively. (4) The overall coverage and mean plant cover area of 20 main tree species in residential greenbelt both decreased, and the overall coverage decreased by 0.007 and the decreasing amplitude was 7.53%. Among which, the coverage of Populus tomentosa, Ailanthus altissima, Robinia pseudoacacia decreased significantly to 30.51%, 15.38% and 50.00% of year 2006, respectively. Due to severe pruning, mean plant coverage area of Populus tomentosa and Ailanthus altissima decreased to 51.57% and 16.85% of ten years ago, respectively. (5) In 2016, the dominant tree species in green land of residential area are Sophora japonica (Ⅳ=0.529), Fraxinus chinensis (Ⅳ=0.321), Sabina chinensis (Ⅳ=0.212), Populus tomentosa (Ⅳ=0.195), Platanus hispanica (Ⅳ=0.182), Toona sinensis (Ⅳ=0.163). The dominance of Platycladus orientalis and Paulownia tomentosa showed a sharp downward trend, and their importance values decreased from 0.155, 0.107 in 2006 to 0.017, 0.002 in 2016, respectively. The dominant position of Sabina chinensis and Platanus hispanica rose significantly, and their importance values increased by 11.16 and 4.67 times, respectively.

ConclusionThe overall afforestation situation has been improved in the residential area. The forest density and tree species have increased significantly, and the species structure is more reasonable and grounded now. The numbers of plant-caused sensitized tree species like Populus tomentosa, Ailanthus altissima, Platycladus orientalis and poor growth adaptability tree species decreased gradually, while the number of popular tree species, including Toona sinensis, Ginkgo biloba, Platanus hispanica, Malus × micromalus increased gradually. However, the overall amount of green space in residential areas has not improved effectively, and the overall coverage has decreased by 4.73%. There is still much room for improvement and potential for green space.