Carbon Density Of Artificial Forests And Its Influencing Factors In Typical Watershed Of Beijing Mountainous Area

The Soil-gravel Area in North China, with thin soil layer, rough texture and fragile ecological environment, has been the important focus of eco-forestry projects construction in China. In the promotion of key ecological projects, such as Three-north Shelterbelt, Greening Project in Taihang Mountain, Natural Forest Protection Program, etc., large-scale afforestation has been carried out in this area so that the ecological environment has been improved significantly. At the same time, artificial forest also has been played an important role in the absorption and fixation of carbon dioxide and slowing global warming. Five forest stands widely distributed in Hongmenchuan watershed of Miyun county, including two artificial forest with different afforestation ages (Platycladus orientalis and Pinus tabulaeformis), natural shrub, natural secondary forest(Platycladus orientalis)and economic forest (Castanea mollissima), were selected in this study to establish 47 plots (a total of 282 quadrats) to estimate carbon density of tree layers, shrub layers, herbaceous layers and litter layers, as well as carbon density in soil layers and analyze the carbon density status quo, stage and influencing factors of main afforestation tree species. The study will help to evaluate the change direction and accumulation rate of carbon density, and provide scientific basis for the rational utilization and development of land resources in Beijing mountain area. The results are as follows:(1)The carbon density of artificial forest ecosystem (P. tabulaeformis and P. orientalis) significantly increased with the increasing stand age. The average of ecosystem carbon density was 81.79±6.54 t/hm2,94.54±14.47 t/hm2 and 146.61±24.55 t/hm2 in three artificial forests of P. tabulaeformis at the stand age of 35-yr,40-yr and 57-yr old, respectively. Similarly, there were significant differences among three artificial forests of P. orientalis aged 20-yr,40-yr and 57-yr, the mean ecosystem carbon densities were respectively 67.83±7.31 t/hm,89.62±12.59 t/hm2 and 123.23±20.58 t/hm2.(2)The carbon density in different soil layers of P. tabulaeformis plantation increased with the increasing stand age. The soil carbon density was 87.83±5.35 t/hm2 in 57-yr old P. tabulaeformis plantation, which was significantly higher than that in the natural shrub (54.33±9.89 t/hm2),35a (40.11±5.05 t/hm2) and 40a (46.61±9.41 t/hm2) P. tabulaeformis plantations (p<0.05). However, the soil carbon density had no obvious change trend with the increasing stand age in P. orientalis plantation. The soil carbon density in 57-yr old P. tabulaeformis plantation was higher than that in even-aged P. orientalis plantation (53.59±4.67 t/hm), increased by 34.24 t/hm, and soil carbon density showed the same regularity in different soil layers.(3)The mean ecosystem carbon densities in 57-yr old P. tabulaeformis plantation and 57-yr old P. orientalis plantation respectively increased by 84.94t/hm2 (137.73%) and 61.55t/hm2 (99.81%) compared to that in nature shrub. The increased carbon density of P. tabulaeformis plantation mainly derived from vegetation and soil and the contribution accounted for 55.34% and 39.4% with respective value of 47.01 t/hm2 and 33.50 t/hm2. However, the increased carbon density of P. orientalis plantation mainly came from vegetation, increased by 48.92 t/hm2, which contributed 79.48%, and litter also increased more carbon densities, about 13.38 t/hm2.(4)In the study, the multivariate regression equation to carbon density of the artificial forests was established. The result showed the ecosystem carbon density was mainly affected by stand factors (stand age and stand density)and soil properties(soil total nitrogen content, silt content, soil bulk density), which jointly explained 73.3% of carbon density variability. Among them, soil carbon density was affected by stand age and soil properties, which jointly explained 82.3% of carbon density variability, and vegetation carbon density was affected by stand age, soil pH and stand density, which jointly explained 61.5%.