| Forest carbon sequestration is the basic structural and functional parameter of forest ecosystems, and plays a vital role in mitigating the global climate change, and thus, receives an increasing attention in the recent years. However, carbon allocation of forest ecosystem components at the regional scale has less been investigated and with uncertainties. In this study, a regional scale research including totally 212 sample plots were conducted in Gansu Province, northwest China. Several methods, e.g. field investigation, typical sample plots, libratory analysis and up-scaling methods were clustered to quantify the current forest carbon density and storage, and to know the forest carbon sequestration differences between natural forests and plantations at a regional scale. We more focus on the 3 typical natural forests and 3 plantations of different aged forests to quantify the carbon density of different components in these forest ecosystems. A continuous biomass expansion factor(BEF) method was used to estimate the dynamics of the forest carbon density and storage over the past 30 years. Moreover, the effects of species diversity on forest ecosystem carbon density were analyzed along an age chronosequence in both natural forests and plantations of Abies asperata forests. The main conclusions are congregated as follows:(1) Natural forests exhibited a higher forest ecosystem carbon sequestration than that in plantations in Gansu Province. Abies fabri and Quercus aliena var. acuteserrata forest had the highest forest ecosystem carbon density in mature forest, while A. asperata had the highest value in young forest. All of the three natural forest types had the highest vegetation carbon density in mature forest.The ecosystem carbon density of the three different aged forests were ranged between 207.79 t C/ha and 765.01 t C/ha, and followed the sequence of A. fabri forest(549.08 t C/ha)> A. asperata forest(464.33 t C/ha)> Q. aliena var. acuteserrata forest(260.56 t C/ha). Soil organic carbon density of different aged natural forests ranged between 114.14 t C/ha and 559.31 t C/ha, and contributed 44.14~95.02% to the total, which was 4.38 times higher than the vegetation carbon density. Vegetation carbon density was ranged between 18.27~223.10 t C/ha, and accounted for 3.78~53.84% to the total with an average of 30.20%. The carbon density of shrub, grass and litter accounted 0.96%, while fine root contributed 0.63% to the total forest ecosystem carbon density and was higher than that of each understory component.(2) The ecosystem carbon density of plantations varied with forest age class. A. asperata and Larix gmelinii forest had the highest forest ecosystem carbon density in mature forest, while Robinia pseudoacacia forest had the highest value in pre-mature forest. L. gmelinii and R. pseudoacacia forests had the highest vegetation carbon density in pre-mature forest, while that was increased with forest age class in A. asperata forest.The ecosystem carbon density of the three different aged forests were ranged between 69.82 t C/ha and 467.42 t C/ha, and followed the sequence of A. asperata forest(379.33 t C/ha)>L. gmelinii forest(333.07 t C/ha)> R. pseudoacacia forest(92.59 t C/ha). Soil organic carbon density of different aged plantations ranged between 50.91 t C/ha and 392.24 t C/ha, and contributed 56.63~93.86% to the total, which was 6.91 times higher than the vegetation carbon density. Vegetation carbon density of the 3 plantations was ranged between 12.73~159.59 t C/ha, and accounted for 73.98~96.43% to the total. The carbon density of the understory varied with forest ages and types.(3) A vertical distribution pattern of forest ecosystem carbon storage was exhibited at the provincial scale, and soil carbon storage contributed the most to the forest ecosystem.Forest ecosystem carbon storage at the provincial was 634.50 Tg C,soil was the most contributor and stored 446.15 Tg C, which was invariably 2.37 times than the vegetation carbon storage. The vegetation contribution sequence of arbor, shrub, grass, litter and fine root were 27.39%, 0.39%, 0.23%, 0.78% and 0.89%, respectively. Carbon storage in natural forests was 522.18 Tg C, which up to 4.65 times higher compared with plantations. Carbon storage of both natural forests and plantations followed a similar sequence: soil>arbor>fine root>litter>shrub>grass.(4) Carbon density of vegetation, soil organic carbon and forest ecosystem of natural forests were significantly higher than that of plantations at the provincial scale(P<0.001).Mean carbon density of vegetation, soil organic carbon and forest ecosystem of natural forests were 89.53 t C/ha, 250.28 t C/ha and 339.81 t C/ha, respectively. At the provincial scale, mean carbon density of vegetation, soil organic carbon and forest ecosystem of natural forests were invariably 55.11%, 63.84% and 61.44% higher compare with the plantations.(5) Forests play an important role of carbon sink in the past 30 years at the regional scale. Due to the large area of young and middle-aged forests with higher ecosystem carbon density, forests in the province have a dramatic carbon sequestration potential in the future.Forest carbon storage, estimated by the continuous BEF method, increased from 82.16 Tg C in 1979 to 100.66 Tg C in 2011. This approximates an annual increase in net arborous carbon of 0.17% during the period, and the annual carbon accumulation rate was 1.53 Tg C/a during the decade. Carbon storage of middle-aged forest was the major contributor of natural forests, up to 29.61~39.68%, while both young and middle-aged forests contributed 80.22~92.41% to the carbon storage of plantations. Forest carbon density and storage will be improved with more pre-mature forests along the forest succession.(6) Community species diversity indices work as different indicators in carbon density of different of forest ecosystem components in both natural and plantations of A. asperata forests.Species evenness had a positive effect on the carbon density of arbor, shrub, grass, litter and top soil organic carbon density(0-20 cm) in natural forests, whereas species evenness had a positive effect on the carbon density of shrub, grass and 0-50 cm soil organic carbon density in plantations. Considering the fine root mass, species richness can be used as an indicator of fine root mass in natural forests, whereas species evenness represents an effective indicator of such mass in plantations. |
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