| In this paper, we focused on the forest ecosystem in Shaanxi Province. We studied the carbon content of the dominant tree species and carbon density in plots of forest ecosystems. We established storage conversion equations between arbor and shrub and grass, vegetation and litter, aboveground and soil. We alsoestimated the carbon storage and carbon density of forest ecosystem in Shaanxi Province, according to the eighth continuous forest inventory data for Shaanxi Province. We also have presented the features of the spatial distribution of forest carbon storage. In addition, with the 1994, 1999, 2004 and 2009 forest inventory data, we demonstrated the dynamics of Shaanxi Province forest ecosystem carbon stocks in 15 years. The main findings were as follows:(1) The carbon content of leaves, branches, stems and roots in different dominant tree species in Shaanxi Province was ranging from 393.51 to 525.73 g·kg-1.The carbon contents of Betulaceae forests, Quercus wutaishanica forests, Quercus variabilis forests, other oak forests, other hardwood forests, softwood forests, broadleaf mixed forests, Pinus tabulaeformis forests, other pine forests, other coniferous forests and coniferous and broadleaf mixed forests were: 463.88, 426.64, 439.51, 448.64, 436.76, 455.84, 438.42, 495.44, 497.98, 492.81 and 461.36 g·kg-1, respectively.The average carbon content of broadleaf forests was 442.83 g·kg-1, and coniferswas 496.06 g·kg-1. The average carbon content of conifers was generally higher than broadleaf forests. The average carbon content of forest ecosystem in Shaanxi Province was 453.94 g·kg-1.(2) The total carbon storage in forest ecosystems in Shaanxi Province was 769.84 Tg. Forest ecosystem carbon storage in the different components were in the following order: soil layer(69.32%) > arbor layer(27.57%) > litter layer(1.74%) > shrub and herb layer(1.38%). Different types of forest ecosystem carbon storage was ordered : other oak forests(23.52%) > other hardwood forests(21.37%) > Quercus wutaishanica forests(12.90%) > softwood forests(11.90%) > Pinus tabulaeformis forests(8.16%) > Betulaceae forests(6.09%) > Quercus variabilis forests(5.87%) > other coniferous forests(4.23%) > other pine forests(3.95%) > broadleaf mixed forests(1.57%) > coniferous and broadleaf mixed forests(0.45%). The carbon storage of forest ecosystems with different originates were in the order: natural forests(86.93%) > plantations(13.07%). The carbon storage of forest ecosystems with different age stands were in the order: middle-aged forests(28.22%) > young forests(19.80%) > mature forests(18.87%) > nearly mature forests(17.65%) > mature forests(15.46%). The carbon storage of forest ecosystems in different cities were in the order: Hanzhong > Yan’an > Ankang > Shangluo > Baoji > Xi’an > Xianyang > Weinan > Tongchuan > Yulin.(3) The average carbon density of forest ecosystems was 120.43 Mg·hm-2, where the average carbon density of tree layer was 33.20 Mg·hm-2, the average carbon density of shrub and grass layer was 1.66 Mg·hm-2, the average carbon density of litter layer was 2.09 Mg·hm-2, and the average carbon density of soil layer was 83.47 Mg·hm-2. Betulaceae forests in different types of ecosystems had the largest carbon density, which was 212.23 Mg·hm-2; other coniferous forests had smallest carbon density, which was 101.70 Mg·hm-2. Within different origins forests, the average carbon density of natural forests was 128.53 Mg·hm-2, and the average carbon density of plantation was 84.85 Mg·hm-2, which was only 66.01% of natural forests. The overall trends in different age stands of forest ecosystem carbon density was increased with age: the average carbon density of young forests, middle-age forests, near-mature forests, mature forests and over-mature forests were 83.04, 117.13, 137.52, 149.38 and 160.43 Mg·hm-2, respectively. Within Shaanxi Province different cities forest ecosystems, three cities with the largest carbon density were: Xi’an(151.06 Mg·hm-2), Baoji(138.21 Mg·hm-2) and Tongchuan(121.90 Mg·hm-2). The average carbon density of these three provinces were higher than the provincial average carbon density, the average carbon density of other cities were less than the provincial average carbon density. The city with minimum carbon density forest ecosystem was Yulin(91.43 Mg·hm-2).(4) The Shaanxi Provincial forest ecosystem carbon storage in 1994, 1999, 2004 and 2009 were 568.37, 582.52, 647.08 and 769.84 Tg, respectively. In general, it was a gradually increasing trend. The total carbon storage increased 35.45% in 15 years. The average annual growth rate was 2.36%. In the forest with different age stands, the increments of young forest carbon storage was 125.20 Tg, which was 62.14% of total carbon storage, which was also the main source of carbon storage in forest ecosystems increments. In forest with different origins, natural forest carbon storage in 2009 had increased by 29.97% over 1994; plantations had grown by 88.22%. In 15 years, plantation ecosystem carbon storage in forest ecosystem carbon storage has been proportionally growing. In different types of forest, oak forests, other hardwood forests, Pinus tabulaeformis forests, other pine forests and other coniferous forests carbon storage overall had an upward growing trend; softwood forest carbon storage had declined; Betulaceae forests carbon storage had been stable; the average carbon storage of broadleaf mixed forests and broadleaf and conifer mixed forests from 2004 to 2009 had been declining. Oak forests carbon storage in four years were significantly higher than that in other forest types, and the percentage share of the total forest carbon storage was increasing, which is the main carbon sequestration of forest types in Shaanxi Province. Forest ecosystem carbon density of Shaanxi Province in those four years were 115.39, 114.54, 112.17 and 120.43 Mg·hm-2, respectively. In 2009, forest ecosystem carbon storage and carbon density in Shaanxi Province had reached the maximum within 15 years, indicating recent years forest carbon sequestration was increasing in both quantity and quality, and gradually increasing in the carbon sink functionality in Shaanxi Province. |
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