| Forests in Gansu province play an important role in safeguarding national ecological security as the ecological barrier of Qinghai-Tibet Plateau,the ecological barrier of Loess Plateau–Sichuan and Yunnan province,and the Sand Control Belt of Northern China.However,as influenced by the economic development and the environmental constraints,the ecological environment pressure continues to increase in this area.Ecological problems such as the destruction of natural vegetation,soil erosion and land desertification are still the key"bottleneck"restricting sustainable development of economy and society in Gansu province.In view of the important role of forests in climate regulation,soil and water conservation and carbon sequestration etc.,it is very crucial to protect forests and improve service function of forest ecosystem to overcome the"bottleneck"of development and guaranteeing ecological security in this region.As a consequence,an intensive research on the potential,dynamics and influencing mechanism of carbon sequestration in major forest types is significant for evaluating the service function of forest ecosystem and ecological security in Gansu province.In this study,8 major forest types,including natural forests of Picea asperata,Abies fabri,Quercus spp.and artificial forests of Robinia pseudoacacia,Larix gmelinii,Pinus tabuliformis,Populus spp.,P.asperata?only for Chapter 5?were collected in the areas.Based on the data of sample plot inventory,the vegetation carbon sequestration status,potential and rate of major forest types,the ratio of below/above-ground carbon density in tree layer and its affecting factors were researched.The influencing factors of vegetation carbon density in major forest types were discussed.The characteristics of carbon sequestration between the two different ecosystems?natural and plantation forest of P.asperata?were thoroughly analyzed.Furthermore,according to the data of 8th national forest continuous inventory and anticipated target of the forestry modernization of Gansu province,the future dynamics of carbon stock in the seven major forest types were estimated.The results showed that:?1?The vegetation carbon sequestration status,potential and rate in major forest types.There were significant differences in carbon sequestration capacity among the major forest types as well as different age classes within the same forest type.The natural forest of P.asperata and A.fabri had the highest carbon density,with the average of 110.72 and 135.28 t hm-2,respectively,while the R.pseudoacacia plantation had the lowest carbon density,with the average of 26.01 t hm-2.There were significant differences in vegetation carbon sequestration potential among different forest types.The carbon sequestration potential of different age classes in major forest types were as follows:natural forest of P.asperata,A.fabri and Quercus spp.were16.78–189.01,46.58–208.31,43.83–118.77 t hm-2,respectively,and artificial forest of R.pseudoacacia,L.gmelinii,P.tabuliformis and Populus spp.were 0–36.53,7.81–106.59,9.35–49.17 and 11.03–61.86 t hm-2,respectively.The maximum vegetation carbon sequestration potential appeared in young forest except for L.gmelinii;and the minimum vegetation carbon sequestration potential appeared in mature forest except for R.pseudoacacia,L.gmelinii.During certain growth stage,the vegetation carbon sequestration rate in natural forest of P.asperata,A.fabri and artificial forest of Populus spp.,R.pseudoacacia,L.gmelinii showed negative values,with the value of-1.74,-0.21,-1.59,-5.86 and-2.35 t hm-2 a-1,respectively,which indicated that these forests acted as net carbon emissions.Different forest types attained the fastest carbon accumulation rate in different growth stages,P.asperata and Populus spp.appeared in growth stage I,A.fabri,R.pseudoacacia,and L.gmelinii appeared in growth stage II,while Quercus spp.and P.tabuliformis appeared in growth stage III.?2?The ratios of below/above-ground carbon density in tree layer and its influencing factors.The ratios of below/above-ground carbon density in tree layer?R/S?varied greatly among different forest types,ranging from 0.17 to 0.26,which was the highest in R.pseudoacacia forests and was the lowest in P.asperata forest.There was no significant difference in R/S among different age classes in the same forest ecosystem.The R/S values showed significant negative correlation with stand age,average diameter at breast height?DBH?and aboveground biomass of trees and showed significant positive correlation with stand density;however,there were no significant correlation between R/S values and the average tree height.In general,the R/S of the 7 types forest showed no significant variation trend with the increase of mean annual precipitation?MAP?.The R/S values of different forest types showed different trends with the increasing MAP if divided the 7 forest types into different groups according to different criteria.The R/S value increased significantly with the increase of mean annual temperature?MAT?if the 7 forest types considered as a whole.The belowground carbon density of tree layer had a significant linear positive correlation with abovegroud biomass in every forest type.?3?Main factors influencing vegetation carbon density in the major forest types.The main factors affecting the vegetation carbon density in every forest type were different.Stand age,MAP and MAT were the main factors in P.asperata forest.Stand density and ratio of soil organic carbon content to total nitrogen content in 0–10 cm soil layer?C/N?were the main factors in natural forest of A.fabri.Stand density and forest age were the main factors in natural forest of Quercus spp..Stand age,MAT and altitude were the main factors in R.pseudoacacia plantation.Stand age and MAT were the main factors in L.gmelinii plantation.Stand age,MAP,slope position and aspect were the main factors in P.tabuliformis plantation.Slope was the only main factor among selected factors affecting vegetation carbon density in Populus spp.plantation.?4?The characteristics of carbon sequestration between the natural and artificial P.asperata forest ecosystems.The carbon density of tree layer in natural forest of P.asperata was greater than that in artificial P.asperata forest.The carbon density of shrubs,herbs and fine root had no significant difference between the two forest types.The carbon density of litter and dead trees in natural P.asperata forest were greater than that in artificial P.asperata forest in the whole.Carbon density of soil layer?0–100 cm?in natural and artificial forest were 151.50–459.79 and 242.25–367.80 t hm-2,respectively,and there were no significant difference between the two forest types.Although the biomass carbon density of both the two forest types increased obviously with stand age,natural P.asperata forest(23.43–278.62 t hm-2)was significantly greater than that of artificial P.asperata plantation(17.13-165.98 t hm-2).The ecosystem carbon density of natural and artificial P.asperata forest did not increase with the stand age,and there was no significant difference between them.Both the natural and artificial forest of P.asperata could accumulate carbon rapidly during growing mature stage,however,compared with natural forest,artificial forest of P.asperata showed a greater ability to accumulate carbon,which suggested that artificial forest of P.asperata could sequestrate more carbon with extended rotation period.?5?The future carbon stocks of major forest types were predicted.The vegetation carbon density of the major forest types will increase significantly from 2010 to 2050,and L.gmelinii forest shows the biggest increase rate in afforestation area.The carbon stock of Quercus spp.forest will increase by 17.71 Tg,followed by R.pseudoacacia forest,which will increase by 17.61 Tg,however,the carbon stock of A.fabri forest will only increase by 3.66 Tg.The increase rate in carbon stock of L.gmelinii plantation is the largest,which will increase by 12.81 times from 2010 to 2050.The total carbon stocks of 7 forest types will increase from 92.41 Tg in 2010 to 169.03 Tg in 2050,with average annual carbon sequestration of 1.92 Tg.From the perspective of increasing the carbon potential of forest ecosystem,the afforestation area of R.pseudoacacia should be reduced and the afforestation area of L.gmelinii should be increased appropriately in the future.Meanwhile,the management of existing natural forest of Quercus spp.as well as the protection of natural forests should be enhanced. |
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