天然林保护工程区近20年森林植被碳储量动态及碳汇(源)特征

分析近20年来天保工程区森林植被碳储量的动态变化及碳汇（源）特征,以期为我国天然林保护的政策制订和措施实施提供数据支撑。利用天然林资源保护工程区6-9次森林资源连续清查数据,把森林植被划分乔木林、灌木林、竹林、疏林地、散生木、四旁树,基于行业标准的生物量模型和碳计量参数、采用生物量加权平均法等方法,估算整个工程区和各省的森林植被总碳储量;对乔木林分起源、龄组、优势树种（组）估算碳储量和碳密度;量化森林植被总碳储量和乔木林碳储量随时间变化的消长,明确其碳汇/源特征。研究结果表明：6-9次清查,天保工程区森林植被总碳储量分别为2999 TgC、3254 TgC、3585 TgC和4097 TgC,年均增长率为1.65%、1.96%和2.70%;碳储量集中分布于我国东北和西南区域,其中四川碳储量最高,4期碳储量均占天保工程区总量20%以上;乔木林碳储量是森林植被碳储量的主体,每期占比均稳定在80%以上,其中天然林比例由94.67%下降至90.28%,人工林比例稳步上升,但到9次清查时其碳密度仍低于天然林50%;不同龄组间,中龄林碳储量最高,近熟林碳储量增长最快,碳密度从幼龄林到过熟林逐渐上升,4期趋势一致;乔木林中纯林碳储量占60%以上,大部分树种（组）碳储量和碳密度随时间推移而增加。7-9次清查,天保工程区森林植被总固碳量（当期相对于前期）分别为255.33 TgC、331.46 TgC和511.53 TgC,对全国森林植被总碳汇量的贡献由8次连清的53.78%上升到9次的67.46%,其中,乔木林对全国乔木林碳汇的贡献为68.71%;天保工程区内天然林对乔木林碳汇的贡献为75.90%;不同清查期,乔木林各龄组的碳汇变化较大,幼龄林和中龄林碳汇占比明显上升,近熟林和过熟林下降,9次清查时各龄组碳汇量大小顺序为：中龄林 > 近熟林 > 幼龄林 > 成熟林 > 过熟林;不同清查期,各个优势树种的碳汇/源表现不一,总体上,混交林的碳汇比例最大,到9次清查时,阔叶混交林和针阔混交林对乔木林碳汇的贡献分别为62.59%和17.23%,纯林中柏木碳汇贡献最大,为5.43%。天保工程区森林植被总碳储量随时间稳步增长,乔木林是总碳储量的主体,天然林是碳汇的主要来源,天然林保护增强了我国天然林碳汇的碳汇功能,促进了人工林碳汇作用提升,未来天保工程区碳汇潜力很大。

Dynamic changes of forest vegetation carbon storage and the characteristics of carbon sink (source) in the Natural Forest Protection Project region for the past 20 years

In order to provide data support for policy formulation and measures implementation of natural forest protection in China, the dynamics of carbon storage and carbon sink (source) characteristics of forest vegetation in the natural forest protection project (NFPP) region were estimated and evaluated in the last 20 years. Based on data from 6-9th Chinese National Forest Inventory (CNFI) of the NFPP region, the forest vegetation was divided into arbor forest, shrubbery, bamboo, open forest, scatted trees, and "four-side" trees. By means of tree biomass model and related parameters to carbon accounting from forestry industry standard and biomass conversion and expansion factor from the weighed biomass average method, total carbon storage of forest vegetation were estimated in the NFPP region and provincial level. The carbon storage and carbon density of the arbor forest were evaluated by origins, age groups, and dominant tree species (groups). The changes of total carbon storage of forest vegetation and carbon storage of arbor forest with time were quantified to clarify carbon sink (source) characteristics. The results showed that the total carbon storage of forest vegetation in the NFPP region in 6-9th CNFI was 2999 TgC, 3254 TgC, 3585 TgC, 4097 TgC, respectively, with average annual growth rates of 1.65%, 1.96%, and 2.70%. Carbon storage was concentrated in the regions of northeast and southwest of China, and Sichuan province had the highest carbon storage which accounted for more than 20% of the total amount of NFPP region in 6-9th CNFI. The arbor forest carbon storage was the main body of the forest carbon storage, which accounted for more than 80% of the total forest carbon storage in every inventory with little variation in the proportion. Further, the proportion of natural forest carbon storage in arbor forest decreased from 94.67% to 90.28%. The proportion of plantation forest was gradually increasing, but the carbon density was lower than half of natural forest in the 9th CNFI. Among different age groups, the middle-aged forest had the highest carbon storage, and near-mature forest had the fastest growth rate in carbon storage. The carbon density gradually increased from young-aged forest to over-mature forest and the tendency was the same in 4 periods. Pure forest carbon storage accounted for more than 60% that of arbor forest, and the carbon storage and carbon density of most tree species (groups) increased over time. In the 7-9th CNFI, the total carbon sequestration of forest vegetation in the NFPP region (current period relative to the previous period) were 255.33 TgC, 331.46 TgC, and 511.53 TgC, respectively. They contributed to the total forest carbon sink in China from 53.78% in the 8th CNFI to 67.46% in the 9th CNFI. In particular, arbor forest contributed 68.71% to the national arbor forest carbon sinks while natural forest in the NFPP region contributed 75.90% to arbor forest carbon sinks in the NFPP region. The arbor forest carbon sinks of each age group had relatively great variation with different CNFIs. The proportion of carbon sinks in young-aged forest and middle-aged forest increased significantly while near-mature forest and over-mature forest decreased. The order of carbon sinks in each age group in the 9th CNFI was middle-aged forest > near-mature forest > young-aged forest > mature forest > over-mature forest. The carbon sink (source) performance of every dominant tree species were different in different CNFIs. In general, the mixed forest had the largest proportion of carbon sinks. In the 9th CNFI, the contribution of mixed broad-leaf forest and mixed conifer-broadleaf forest to the carbon sink of arbor forest were 62.59% and 17.23%, respectively. In pure forest, cypress contributed the most to carbon sink, accounting for 5.43%. The total carbon storage of forest vegetation in the NFPP region had steadily increased over time. Arbor forest was the main body of total carbon storage and natural forest was the main source of carbon sinks. The natural forest protection enhances the carbon sink function of natural forest in China, and promotes the carbon sink function of plantation. The carbon sinks in the NFPP region will have great potential in the future.