Characteristics Of Carbon Stock And Its Spatial Differentiation In The Forest Ecosystem Of Sichuan

Global climate change has been concerned about worldwide because climate change characterized by global warming and atmospheric CO₂ enrichment is changing the structure and function of terrestrial ecosystem as well as threatening the survival and safety of human beings. Furthermore, estimate of carbon stock in national or regional vegetation and soil, as a crucial important scientific basis in the implementation of international environmental conventions, has been an important field in climate change science, as a consequence of which numerous reports on national and regional carbon stock in vegetation and soil have been published in the past three decades. However, the shortcomings of forest resource inventory, soil survey, Holdridge life zone, and satellite data in estimating carbon stock in vegetation and soil still limited the measurement precision of carbon density and stock in regional forest ecosystem with complex landforms and diverse forests and soils, implying that more detailed measurements on carbon density and stock in vegetation and soil of the forest ecosystem are necessary for the region with complex landforms and diverse forests in order to obtain more precise estimates of regional forest carbon stock. Sichuan province, located at the upper reaches of the Yangtze River and in the eastern Qinghai-Tibet Plateau honored by the third pole in the world, is the principal part of the second largest forest region in China as well as the key region of projects on natural forest resource protection, conversion of cropland to forest and grassland and protection forest in the upper and middle reaches of the Yangtze River. Besides the irreplaceable and important roles in conserving water and soil, nursing biodiversity and global carbon cycle, Sichuan forest is also an ideal natural lab for studying the spatial differentiation on regional forest ecosystem carbon stock due to the diverse vegetation and soil combinations, clear vertical climate differentiation and complex landforms. As yet, far less information is available on the characteristics of carbon stock in the forest ecosystem and its spatial differentitation at the regional level in Sichuan, which is difficult to make needs of forest carbon cycle in Sichuan province and China. Carbon density and stock in the forest ecosystem and its spatial differentiation in Sichuan were studied to provide an important scientific basis on carbon sequestration potential in Sichuan forest, employing the methods of measurements on carbon contents in trees and soils, and biomass with forest resource inventory and GIS technology, Consequeathy, the techniques of the regionalized forest carbon management were put forward based on the spatial differentiation of carbon density and stock in the forest ecosystem in Sichuan.Average carbon content in trees of Sichuan forest was 51.09%, varying from 46.75% to 54.89%. Among this, average carbon content was 52.82% (ranging from 50.50% to 54.89%) and 49.37% (ranging from 46.75% to 50.50%) in coniferious and broadleaved forests, which was higher than 50.00% and 45.00%, respectively. It is suggested that carbon stock and the potential of carbon sequestration in Sichuan forest be underestimated by the transform coefficients of 0.50 or 0.45.The equation w=avb, fitted by multiple objectives of biomass and tree volume, was confirmed as the optimum model to express the relationships between tree volume and biomass in Sichuan forest. Here the carbon stock in trees of Sichuan forest was estimated based on the method of carbon content in the forest stand in combination with regional biomass-volume model, and in turn carbon stock in the vegetation was 478.25 TgC in Sichuan forest in 2004, which was 11.56% and 1.73% higher than those estimated by the common transfer coefficient of 45.00% or 50.00%, indicating that the method of estimating the carbon stock in regional forest by measuring tree carbon content in combination with regional biomass-volume model is important to improvt the precision of estimating regional forest carbon stock.Carbon stock in trees of Sichuan forest increased 172.88 TgC from 305.37 TgC in 1974 to 478.25 TgC in 2004 by 1.51% of annual increment rate over the past 30 years, implying that Sichuan forest played the role of carbon sink in global carbon cycle.The total organic carbon stock in the forest ecosystem was 2926.81 TgC in Sichuan, and the carbon stock was 2394.26 TgC in soil layer, 478.25 TgC in tree layer, 41.14 TgC in litter layer and 13.16 TgC in shrub and grass layer, accounting for 81.80%, 16.34%, 1.41% and 0.45% of the total, respectively, indicating that forest soil was the greatest organic carbon stock. Although carbon stock in litter layer was lower than that in tree and soil layers, organic carbon in this layer was very important since litter layer is one of the most important ecological interfaces in terms of mass cyle and energy transfer between vegetation and soil as well as the source of soil organic carbon pool. Consequently, the formation mechanism on forest soil carbon sink is difficult to be revealed completely as well as carbon stock in the forest ecosystem would be underestimated if organic carbon in litter layer was neglected.Carbon stock varied greatly with the forest stands. Coniferous forest dominated by fir (Abies) and spruce (Picea) trees had the greatest carbon pool with 1384.09 TgC. which accounted for 47.29% of the total carbon stock. Carbon stock was 327.56 TgC and 309.21 TgC in the forest ecosystem dominated oak (Quercus) and other hardwood trees and in the softwood forest ecosystem, accounting for 11.19% and 10.56%of the total carbon stock in Sichuan forest, respectively. In total, carbon stock in the former three ecosystems was 2020.86 TgC, which accounted for 69.05% the total carbon stock, resulting from the relative higher forest area and carbon density. The carbon stock in the rest forest ecosystems only accounted for 30.95%. The results indicated that the forest originating from natural forest was main contributor to carbon pool in Sichuan forest, while the artificial forest had the relative lower carbon stock.Carbon stock in Sichuan forest ecosystem distributed mainly in the region with sharp slope (>25°) and high elevation(3000～4000m), resulting from relative low human activities in these zones. Carbon stock in the forest ecosystem at the zone with >25°slope was 1598.22 TgC, accounting for 54.61% of the total in Sichuan forest. Similarly, carbon stock in the forest ecosystem at the zone with 3000-4000 m elevation was 1313.24 TgC, which accounted for 44.87% of the total carbon in Sichuan forest ecosystem. In order to protect and increase the carbon stock in Sichuan forest, the first measure is to reduce the human disturbance to forest resulting from that the forest is easy to be damaged and the degraded forest ecosystem is difficult to be restored since the region with sharp slope and high elevation is the typical fragile ecological and environment belt.Average carbon density in the forest ecosystem was 232.81 MgC·hm^-2 in Sichuan, and the corresponding carbon density was 190.45 MgC·hm^-2 in soil layer, 38.04 MgC·hm^-2 in tree layer, 3.27 MgC·hm^-2 in litter layer and 1.05 MgC·hm^-2 in shrub and grass layer, respectively, all of which were lower than those of average carbon density in China. Average ratio of soil to vegetation carbon density was 4.96, ranging from 3.44 to 17.41 in Sichuan forest ecosystem, indicating that Sichuan forest had lower vegetation carbon density and higher potential of carbon sequestration.Carbon density in the forest ecosystem had clear spatial differentiation with the zonalities of longitude, latitude and altitude in Sichuan, and also showed obvious speciality and complexity because of the uplift of Qinghai-Tibetan plateau and the disturbance of human beings in densely populated area (such as the plain and low-mountain and hilly area). Soil organic carbon density in Sichuan forest ecosystem increased with longitude and altitude (0-4000m a.s.l.), and decreased with the latitude, which was in agreement with the results worldwide resulting from the budget of soil organic carbon driven by hydrothermal dynamics caused by the increases of longitude, latitude and altitude. Meanwhile, carbon density in trees of Sichuan forest ecosystem also increased with longitude and altitude, and decreased with the latitude, which was not in agreement with the result of "carbon density in trees decreased with the increases of longitude and altitude" worldwide, resulting from that carbon density in tree layer increased with the longitude and the elevation resulted from the vertical zonality, the disturbance of humana activities and their interactions, while the reason that carbon density in tree layer increased with the altitude was the decline of human disturbance intensity with the elevated altitude. The results indicated that the speciality and complexity of spatial differentiation on carbon density and stock in Sichuan forest ecosystem caused mainly by the disturbances of human beings, which also meant that the measures of forest conservation and degraded forest ecosystem restoration in combination with the projects of natural forest resource protection and conversion from farmland to forest (grass) could significantly increase the potential of forest carbon sequestration in Sichuan.An effective approach to increase the capacity of carbon sequestration in Sichuan forest was to implement regionalized forest management concluded from spatial differentiation of carbon density and stock in the forest ecosystem in Sichuan province. The hilly and plain area dominanted by artificial forest and a few secondary forests should adopt the method of low-benefit forest improvement in combination with the forest conservation and fast-growing plantation construction in order to enhance the potential of carbon sequestration. The low mountain area dominanted by natural secondary forest and a few plantations around the basin should adopt the forest management in combination with forest conservation and high-efficiency plantation construction in order to stabilize and enhance the capacity of carbon sequestration. The subalpine and alpine region dominanted by fir and birch forests should further strengthen the natural forest consersvtion.