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Carbon storage in grasslands of China
Journal of Arid Environments (2002) 50: 205–218 doi:10.1006/jare.2001.0902, available online at http://www.idealibrary.com on
Carbon storage in grasslands of China
Jian Ni%wz %
Max Planck Institute for Biogeochemistry, Carl Zeiss Promenade 10, P.O. Box 100164, D-07701 Jena, Germany wLaboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Xiangshan Nanxincun 20, 100093 Beijing, China (Received 15 September 2000, accepted 26 June 2001) Carbon storage in grasslands of China was estimated by the carbon density method and based on a nationwide grassland resource survey finished by 1991. The grasslands in China were classified into 18 types, which are distributed mostly in the temperate region and on the Tibetan Plateau, and scattered in the warm-temperate and tropical regions. Based on the median estimate, vegetation, soil and total carbon storage of grasslands in China were 3?06, 41?03 and 44?09 Pg C, respectively. Vegetation had low carbon storage and most carbon was stored in soils. Of the four types of regions that have grasslands, alpine region (54?5%) and temperate region (31?6%) hold more than 85% of the total grassland carbon (in both vegetation and soils) in China. Considering specific types within these two regions, three grassland types, alpine meadow (25?6%), alpine steppe (14?5%) and temperate steppe (11%) constituted more than half of all carbon stored in China’s grasslands. In general and regardless of regional vegetation types, steppes (38?6%) and meadows (38?2%) made up more than 2/3 of total grassland carbon. The carbon storage in alpine grasslands may have a significant and long-lived effect on global C cycles. This study estimated more carbon storage in vegetation and less in soils than previous studies. The differences of grassland carbon between this study and two previous studies were due probably to four reasons, i.e. different estimation methods, different classification systems of grasslands, different areas of grasslands, and different carbon densities. China’s grasslands cover only 6–8% of total world grassland area and have 9– 16% of total carbon in the world grasslands. They make a big contribution to the world carbon storage and may have significant effects on carbon cycles, both in global and in arid lands. # 2002 Elsevier Science Ltd. Keywords: carbon storage; carbon density; vegetation area; soil carbon; alpine grasslands; temperate grasslands; China; arid lands
Introduction and literature Grasslands are one of the most widespread vegetation types worldwide, covering 15 million km2 in the tropics and a further 9 million km2 in temperate regions, together z Corresponding author. Tel: +49 3641 64 3743; Fax: +49 3641 64 3789; E-mail: [email protected] 0140-1963/02/020205 + 14 $35.00/0
# 2002 Elsevier Science Ltd.
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nearly one-fifth of the world’s land surface (Scurlock & Hall, 1998). Natural grasslands both in tropical and in temperate regions play a significant role in the regional climate and global carbon cycle (Hall & Scurlock, 1991; Hall et al., 1995; Sala et al., 1996). Carbon storage and carbon sinks in the vegetation and soils of grasslands in temperate and tropical regions were previously studied based upon different data and methods at a regional to global scale, particularly within the context of global change (e.g. Lieth, 1978; Hall & Scurlock, 1991; Long & Hutchin, 1991; Thornley et al., 1991; Parton et al., 1993, 1995; Fisher et al., 1994, 1995; Hall et al., 1995; Tate et al., 1995; Scurlock & Hall, 1998). However, carbon storage may have been underestimated, especially in the soils and in the tropics (Scurlock & Hall, 1998). Recent studies of grassland carbon suggested that natural grass ecosystems might be responsible for a substantial proportion (as much as 20% or more) of total terrestrial production, and cautiously proposed that these biomes might already constitute an annual sink of about 0?5 Pg carbon (Scurlock & Hall, 1998). Grasslands in China cover vast, continuous areas of the northern temperate regions with a continental arid climate, the Tibetan Plateau at very high elevations, and small, disjunct areas of the warm temperate and tropical regions controlled by the monsoon climate (Editorial Committee for Vegetation of China, 1980; Hou et al., 1982). Several estimates of grassland distribution are now available at the national level, i.e. 355 million ha (DAHV & CISNR, 1994) and 399 including 354 million ha of major 17 grassland types (DAHV and GSAHV, 1996) according to the national grassland resource survey, 360 million ha based on the Map of Grassland Resources in China at 1:4M scale (CISNR, 1996), 349 million ha by the Map of Land Use in China at 1:4M scale (Wu, 1991), and 406 million ha by the Vegetation Map of China at 1:4M scale (Hou et al., 1982). The areas of grasslands in China are similar regardless of the data source, about 40% of the total territory of China (Chen & Fischer, 1998). The vast area and wide distribution of Chinese grasslands implies that they would have significant effects on regional climate and regional to continental carbon cycles. There is relatively little research on carbon storage and carbon cycle in grasslands of China (Fang et al., 1996; Feng et al., 2001; Ni, 2001). Some studies present carbon estimates from all vegetation types in all of China, but they only included the carbon storage in grasslands at the national level (Fang et al., 1996; Ni, 2001). Total biomass (above- and below-ground) estimates of Chinese grasslands were calculated by Fang et al. (1996) based on areal extent, the range of forage yield and above-below-ground biomass ratio. However, carbon of grasslands estimated by the biomass method is not completely accurate because of the lack of information on the biomass component of grassland vegetation, especially below-ground biomass that was estimated only by the ratio of above- to below-ground biomass (Fang et al., 1996). Ni (2001) also calculated the carbon storage of Chinese ecosystems including grasslands using the carbon density method, but the classification of grasslands derived from 1960 to 1970s investigations is very simple. On the other hand, strong human activities in the last two decades have had an impact on Chinese grasslands (distributions and areas). These may influence the accuracy of the carbon estimation (Ni, 2001). The areal extent of vegetation is a critical factor influencing the estimate of carbon in terrestrial ecosystems, regardless of the method used. Ni (2001) has demonstrated that different spatial resolutions of vegetation, and subsequently different estimated vegetation areas could impact on the calculations of carbon storage within ecosystems. Differences in vegetation classification also affect the calculation of area and determination of carbon density (Ni, 2001). After the two phases of long-term investigation of grassland resources in China from 1949 to 1978 and from 1979 to 1990s (Chen & Fischer, 1998), more detailed classification and more accurate estimates of the areal coverage of grasslands in China are now available (DAHV & CISNR, 1994; DAHV & GSAHV, 1996). Data on Grassland Resources in China provided detailed descriptions and data on grasslands based on a nationwide grassland
CARBON STORAGE IN GRASSLANDS OF CHINA
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resource survey in 1991 (DAHV & CISNR, 1994). Therefore, it is now possible to use the carbon density method in combination with the improved classification scheme and new estimates of grassland area to more accurately calculate carbon storage in Chinese grasslands. Then, the ‘new and better’ carbon storage estimate of China’s grasslands will be compared to previous studies, with a specific focus on temperate and alpine grasslands.
Material and methods Grasslands in China Data on grassland type, total area, utilizable area, forage yield, dominant species and distributions (Table 1) were obtained from a specialized statistical book Data on Grassland Resources of China (DAHV & CISNR, 1994). These data were collected and sorted systematically from all Chinese counties and provinces (autonomous regions and municipalities) excluding Taiwan, based on a 10-year nationwide grassland resource survey completed in 1991. There are four grassland classification schemes in China, including (a) 18 vegetation types in the national grassland survey (DAHV & CISNR, 1994), (b) more than 50 types in the Map of Grassland Resources in China (CISNR, 1996), (c) three types in the Map of Land Use in China (Wu, 1991), and (d) more than 40 types in the Vegetation Map of China (Hou et al., 1982). Chen & Fischer (1998) digitized and simplified the Map of Grassland Resources in China at 1 : 4M scale (CISNR, 1996). They used a 17-type grassland classification scheme, excluding the tropical dry shrubtussock with savanna, which covered only a very small area (Chen & Fischer, 1998). In the current work, the DAHV & CISNR classification scheme of grasslands (18 types) was used (Table 1) for the following reasons. First, the 18 grassland types have enough information to characterize Chinese grasslands. Second, they match the grassland areas and other characteristics in the national grassland survey (DAHV & CISNR, 1994). Finally, they are simplified from the more comprehensive Map of Grassland Resources in China (CISNR, 1996). The grasslands of China are generally and mainly distributed in the temperate and alpine regions in north and west China, respectively. The temperate meadow–steppe, steppe, desert–steppe, as well as part of lowland and mountain meadow are distributed in the northern region (Table 1). Some extend to north-eastern (temperate meadow) and north-western (temperate steppe–desert and desert) regions. The Tibetan Plateau has unique alpine meadow–steppe, steppe and desert–steppe types, which are quite different from the temperate meadow, steppe and desert (Table 1). In east and south parts of China, the warm-temperate and tropical tussocks, shrubs and savannas are sparsely distributed, and deciduous and evergreen forests dominate (Table 1). Carbon density Biomass carbon densities from Olson et al. (1983) and soil carbon densities from Zinke et al. (1984) were used in this paper to give a range of low, median and high carbon densities for every grassland type (Table 2). These values were multiplied by the utilizable areas to obtain the carbon storage of grasslands in China (Table 2). Areas of sediment, road, bare land and river have been excluded from the estimate of utilizable area (DAHV & CISNR, 1994), and therefore more accurately reflect the total utilizable grassland areas in China.
Utilizable area (ha)
Forage yield (kg1 ha)
Ecosystem complexes (Olson et al., 1983)
Dominants
Temperate meadow– steppe
14,519,331
12,827,411
1465
Cool grassland
Temperate steppe
41,096,571
36,367,633
889
Cool grassland
Temperate desert–steppe
18,921,607
17,052,421
455
Cool grassland
6,865,734
6,011,528
307
41,623,171
35,439,220
284
Very cold Tibet meadows and parklands Cool grassland
Leymus chinense, Stipa baicalensis, Filifolium sibiricum, Carex, Festuca, Artemisia, Lespedeza Leymus chinense, Stipa grandis, Stipa Krylovii, Stipa bungeana, Agropyron cristatum, Cleistogenes squarrosa, Artemisia frigida, Artemisia, Caragana, Ephedra Stipa klemenzii, Stipa brevifolia, Stipa glareosa, Stipa goboca, Artemisia, Caragana Stipa purpurea, Stipa capillacea, Festuca, Carex
9,566,006
7,752,078
195
Alpine desert
Alpine meadow– steppe Alpine steppe Alpine desert–steppe
Stipa purpurea, Festuca subsessiliflora var. basiplumosa, Poa, Artemisia, Carex Stipa glareosa, Stipa purpurea, Caragana, Artemisia, Ceratoides compacta
Distribution North-east and northwest China North China
North and north-west China West China West China West China
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Area (ha)
Grassland
208
Table 1. Information on grasslands in China (from DAHV & CISNR, 1994). Grassland was defined as coverage of steppe community greater than or equal to 5%; canopy index of tree community less than or equal to 0?3; and canopy index of shrub community less than or equal to 0?4. The grassland area was summarized by areas measured from the grassland map at a 1:50,000 or 1:100,000 scale, differing from region to region. The utilizable area equals total grassland area minus areas of sediment, road, bare land and river. The forage yield is a mean value of the 10-year production, estimated by measuring above-ground standing crops in fresh weight in the field during the highest yield period in summer and autumn. The fresh weight was converted to dry weight based on grassland type-based dry/fresh weight ratio (DAHV & CISNR, 1994). Every grassland classification was assigned to an ecosystem complex (from Olson et al., 1983) in order to obtain their carbon densities in vegetation and soils (Olson et al., 1983; Zinke et al., 1984). More details can be found in Data on Grassland Resources of China (DAHV & CISNR, 1994)
10,673,418
9,140,926
465
Cool semi-desert
Temperate desert
45,060,811
3,060,4131
329
Desert and semidesert
Alpine desert
7,527,763
5,592,765
117
Warm-temperate tussock
6,657,148
5,853,667
1643
Sand desert: scrub/ herb or barren Warm/hot grassland
Warmtemperate shrub-tussock
11,615,910
9,773,518
1769
Warm/hot grassland with relatively more shrub, trees
Tropical tussock
14,237,195
11,419,999
2643
Warm/hot grassland
Stipa glareosa, Salsola abrotanoides, Ceratoides latens, Reaumuria soongarica, Caragana, Artemisia Sympegma regelii, Iljnia regelii, Ephedra, Nitraria sibirica, Haloxylon, Artemisia, Caragana, Zygophyllum, Calligonum, Halocnemum strobilaceum Ceratoides compacta, Ajania, Artemisia Spodiopogon sibiricus, Bothriochloa ischaemum, Themeda triandra var. japonica, Eulalia pallens, Carex, Artemisia Bothriochloa ischaemum, Themeda triandra var. japonica, Miscanthus sinensis, Vitex negundo var. heterophylla, Ziziphus jujuba, Lespedeza, Carex, Artemisia Miscanthus floridulus, Miscanthus sinensis, Heteropogen contortus, Cymbopogon distans, Imperata cylindrica var. major, Arundinella hirta, Ischaemum ciliare, Dicranopteris dichotoma
North-west China North-west China
West China Central east and central south China Central east and central south China
Central south and south China
CARBON STORAGE IN GRASSLANDS OF CHINA
Temperate steppe–desert
209
210
Table 1Fcontinued.
Grassland
Area (ha)
Utilizable area (ha)
Forage yield (kg ha)
Tropical 17,551,276 shrub-tussock
13,447,569
2527
Tropical dry 863,144 shrub-tussock with savanna Lowland 25,219,621 meadow
639,429
2719
21,038,409
1730
16,718,926
14,923,439
1648
Alpine meadow
63,720,549
58,834,182
882
Swamp
2,873,812
2,253,714
2183
Warm/hot grassland with relatively more shrub, trees
Dominants
Miscanthus floridulus, Miscanthus sinensis, Heteropogen contortus, Imperata cylindrica var. major, Arundinella hirta, Ischaemum ciliare, Rhododendron, Melastoma Warm/hot grassland Heteropogen contortus, Pinus with relatively more yunnanensis, Acacia shrub, trees farnesiana, Flacourtia indica Heath and moorland, Calamagrostis angustifolia, maritime scrub with Achnatherum splendens, meadows Sanguisorba officinalis, Carex, Deyeuxia angustifolia, Aeluropus littoralis var. sinensis Maritime/montane grass Festuca ovina, Deyeuxia arundinacea, Arundinella chenii, Stipa aliena, Carex, Poa Kobresia pygmaea, Kobresia Very cold Tibet meadows and parklands humilis, Kobresia capillifolia, Kobresia schoenoides, Kobresia littledalei, Festuca rubra, Polygonum macrophyllum, Polygonum viviparum, Carex, Rhododendron Carex muliensis, Phragmites Swamps/shrub/herb marshes australis, Scirpus triqueter, Kobresia
Distribution Central south and south China
South China Whole China J. NI
Mountain meadow
Ecosystem complexes (Olson et al., 1983)
North and west China West and south-west China
North, west and central China
Table 2. Carbon density (from Olson et al., 1983; Zinke et al., 1984) and carbon storage (low: L, median: M and high: H) in vegetation and soils of grasslands in China. Every grassland type was assigned to an ecosystem complex (see Table 1) described in Olson et al. (1983), and then the carbon density in vegetation and soils of each ecosystem complex at three levels (low, median and high) was obtained from Olson et al. (1983) and Zinke et al. (1984). The carbon densities multiplied by the utilizable area (see Table 1) give the carbon storage of grasslands in China
Grassland
M
H
Vegetation carbon storage (Pg C)
L
M
H
9?5
11?2
13?3
L
M
H
Soil carbon storage (Pg C)
Total carbon storage (Pg C)
L
M
H
L
M
H
0?13 0?19 0?26
1?22
1?44
1?71
1?35
1?63 1?96
1
1?5
2
0?5 0?5 0?5 0?5 0?5 0?3 0?2 0?02 0?5 1
1 1 1 1 0?8 0?6 0?4 0?05 1 1?6
2 2 4 2 1?5 1 1 0?2 2 3
11?6 7?2 15?7 14 14 7 4?1 14 10 10
12?3 8?7 18?2 17 17 8 6?2 17 13 13
13 10?2 20?7 19 19 9 8?3 19 14 14
0?18 0?09 0?03 0?18 0?04 0?03 0?06 0?00 0?03 0?10
0?73 0?34 0?24 0?71 0?12 0?09 0?31 0?01 0?12 0?29
4?22 1?23 0?94 4?96 1?09 0?64 1?25 0?78 0?59 0?98
4?47 1?48 1?09 6?02 1?32 0?73 1?90 0?95 0?76 1?27
4?73 1?74 1?24 6?73 1?47 0?82 2?54 1?06 0?82 1?37
4?40 1?31 0?97 5?14 1?12 0?67 1?32 0?78 0?61 1?08
4?84 1?65 1?15 6?38 1?38 0?79 2?02 0?95 0?82 1?43
0?5 1 1
1 1?6 1?6
2 3 3
11 11 6?7
14 14 7?3
15 15 7?9
0?06 0?11 0?23 0?13 0?22 0?40 0?01 0?01 0?02
1?26 1?48 0?04
1?60 1?88 0?05
1?71 2?02 0?05
1?31 1?61 0?05
1?71 1?94 2?10 2?42 0?06 0?07
1 0?5 0?5 1?5 0?64
1?5 1 1 3 1?15
2 2 4 6 2?37
9?5 15?7 15?7 11?6 11?02
11?2 18?2 18?2 12?3 13?16
13?3 20?7 20?7 13 14?73
0?21 0?07 0?29 0?03 1?67
0?36 0?17 0?06 0?35 0?06 0?05 0?12 0?00 0?06 0?16
0?32 0?15 0?59 0?07 3?05
5?46 2?08 1?48 7?44 1?59 0?91 2?85 1?07 0?94 1?66
CARBON STORAGE IN GRASSLANDS OF CHINA
L
0?42 2?00 2?36 2?80 2?21 2?67 3?22 0?30 2?34 2?72 3?09 2?42 2?87 3?39 2?35 9?24 10?71 12?18 9?53 11?30 14?53 0?14 0?26 0?28 0?29 0?30 0?34 0?43 7?08 34?52 41?03 46?37 36?18 44?09 53?44 211
Temperate meadow–steppe Temperate steppe Temperate desert–steppe Alpine meadow–steppe Alpine steppe Alpine desert–steppe Temperate steppe-desert Temperate desert Alpine desert Warm-temperate tussock Warm-temperate shrub-tussock Tropical tussock Tropical shrub-tussock Tropical dry shrub-tussock with savanna Lowland meadow Mountain meadow Alpine meadow Swamp Mean carbon density and total carbon
Soil carbon density (kg m2)
Vegetation carbon density (kg m2)
212
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Results According to the carbon densities in vegetation and soils (Table 2) and the utilizable areas (Table1), the low, median and high values of carbon in vegetation, soils and total carbon storage for each grassland type are given in Table 2. Based on the median estimate, vegetation, soil and total carbon of grasslands in China were 3?06, 41?03 and 44?09 Pg C, respectively. Vegetation has low carbon storage and most carbon is stored in soils (Table 2). The alpine meadow has the highest carbon storage both in vegetation and in soils, covering 25?6% of total carbon in grasslands of China (Table 2). The alpine steppe (14?5%) and temperate steppe (11%) also have higher carbon storage (Table 2). Together, these three grassland types make up more than half of all of the carbon being stored in China’s grasslands. This is due to these three grasslands having the highest utilizable areas and higher carbon densities (Table 1). The warm temperate and tropical tussocks as well as swamp, however, have the lowest carbon because of their small areas and lower carbon densities (Table 2). In terms of the grassland type, steppe has the highest carbon storage in vegetation and soils, totaling 17?03 Pg C (38?6%), with meadows storing 16?83 Pg C (38?2%). These two types constitute more than 2/3 of the total carbon stored in grasslands of China (Fig. 1). Other three grassland types, desert, tussock and swamp have a carbon storage of only 10?22 Pg C, less than 1/3 of the total carbon of grasslands in China (Fig. 1). Based on regional grassland distribution, the alpine region has the highest carbon storage, 24?03 Pg C, making up 54?5% of the total carbon (Fig. 2; Table 2). The temperate region also has a large area and therefore a high carbon storage of 13?94 Pg C (31?6% of total carbon) (Fig. 2). Together, these two regions constitute more than 85% of all the carbon stored in grasslands of China (Fig. 2). In warmtemperate and tropical regions, however, the grasslands have very low carbon storage (Fig. 2). Discussion Grasslands play a very important role in modeling the biospheric feedbacks of atmospheric CO2 increase and climate changes (Hall et al., 1995; Thornley & Cannell, 1997). It has long been suggested that the majority of the ‘missing sink’ for carbon in the global CO2 balance may be located in temperate and tropical regions (e.g. Tans et al., 1990). Therefore, the role of grasslands, with large area both in tropics and in temperate regions, should not be overlooked in global biogeochemical cycles (Hall & Scurlock, 1991; Thornley et al., 1991; Parton et al., 1993, 1995; Fisher et al., 1994, 1995; Hall et al., 1995; Tate et al., 1995; Scurlock & Hall, 1998). In China, grasslands are mostly distributed in temperate regions (the semi-arid and arid north China) and in alpine regions (the high and cold west China) (Hou et al., 1982; DAHV & CISNR, 1994). The largest proportion of carbon is stored in alpine and temperate grasslands (Table 2; Fig. 2). The importance of temperate grassland carbon storage is well known (e.g. Thronley et al., 1991; Hall et al., 1995; Tate et al., 1995; Sala et al., 1996), but the importance of alpine carbon storage is less appreciated. In grassland ecosystems carbon is stored mostly in soils, where C turnover times of the bulk soil carbon are relatively long (Hall et al., 1995; Tate et al., 1995; Matthews, 1997). 95% of the carbon stored in alpine grasslands of China are stored in soils (Table 2), which is 55?6% of the total carbon storage in China’s grassland soils (Fig. 2). Similar to high-latitude tundra ecosystems (Hobbie et al., 2000), the high elevation of alpine regions results in low temperature and low evapotranspiration in alpine region. Therefore, soil carbon is difficult to decompose and mostly remains in soils for a long time. If climate change and CO2 increase were to would impact on soil carbon, especially in cooler regions (Hall et al., 1995), changes of
CARBON STORAGE IN GRASSLANDS OF CHINA
213
Figure 1. Utilizable areas and median carbon stored in grasslands of China by grassland type. Steppe includes temperate meadow–steppe, temperate steppe, temperate desert–steppe, alpine meadow–steppe, alpine steppe and alpine desert–steppe. Desert includes temperate steppe–desert, temperate desert and alpine desert. Tussock includes warm-temperate tussock, warm-temperate shrub-tussock, tropical tussock, tropical shrub-tussock and tropical dry shrub-tussock with savanna. Meadow includes lowland meadow, mountain meadow and alpine meadow. Swamp includes swamp grassland.
alpine grassland carbon storage may have a significant and long-lived effect on global C cycles. Fang et al. (1996) and Ni (2001) calculated the grassland carbon of China based on the biomass and organic carbon matter method and the carbon density method, respectively, when they estimated carbon storage of terrestrial ecosystems in China (Table 3). Compared to the carbon storage of grasslands in this paper, considering the meadow and tussock as steppe ecosystems (Table 3), the sort for vegetation carbon is Fang o this paper o Ni and that for soil carbon is this paper oNi oFang. In total, the carbon storage in grasslands of China estimated by this study was lower than those calculated by Ni (2001) and Fang et al. (1996). The differences of grassland carbon between this study and two previous studies are probably due to the following four reasons. First, the classification of grasslands was quite different. Fang et al. (1996) used a very coarse classification of eight types (Table 3). Ni (2001) classified the grasslands
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Figure 2. Utilizable areas and median carbon stored in grasslands of China by region. Temperate region includes temperate meadow–steppe, temperate steppe, temperate desert– steppe, temperate steppe–desert, temperate desert, lowland meadow and swamp. Warmtemperate region includes warm-temperate tussock and warm-temperate shrub-tussock? Tropical region includes tropical tussock, tropical shrub-tussock and tropical dry shrub-tussock with savanna. Alpine region includes alpine meadow–steppe, alpine steppe, alpine desert– steppe, alpine desert, mountain meadow and alpine meadow.
into 11 types (Table 3) based on the vegetation map of China (Hou et al., 1982). In this study, however, 18 grassland types were used based on the nationwide grassland survey (DAHV & CISNR, 1994). The differences in classification result in the differences in area and carbon density, subsequently carbon stored in grasslands of China. Second, authors used the different areas of grasslands, i.e. 569?9 million ha (Fang et al., 1996), 405?87 million ha (Ni, 2001) and 355?3 million ha (298?97 for the utilizable area) in this study. This is a very important factor that influences the estimation of carbon (Ni, 2001). The live vegetation area of grasslands of Fang et al. (1996) was derived from statistics of land use resources and an agricultural atlas of China from the 1980s. The soil area was calculated from all soils in which grasslands are able to grow (Fang et al., 1996). Clearly, the live grassland area was not accurate enough and soil area was overestimated. Thus, Fang’s grassland area is much higher than those of other calculations (Table 3). The grassland area from Ni (2001) was calculated from The Vegetation Map of China
CARBON STORAGE IN GRASSLANDS OF CHINA
215
Table 3. Areas and carbon storage in Pg C of grasslands in China estimated by different authors
Grassland
Total Area Vegetation Soils carbon carbon (106 ha) carbon
Steppe*
430?66
1?02
63?44
Desert Swamp Subtotal
128?24 11?00 569?90
0?01 0?20 1?23
3?52 7?78 74?74
Steppe z
220?11
2?66
32?78
Desert} 152?57 Swamp11 33?19 Subtotal 405?87
1?12 0?87 4?66
16?86 4?08 53?72
Steppe
115?45
1?20
15?83
Meadow Tussock Subtotal
94?80 41?13 251?38
1?05 0?55 2?81
Desert Swamp Subtotal
45?34 2?25 298?97
0?18 0?07 3?06
Estimated method
Reference
64?46 Biomass and organic Fang et al., carbon matterw (1996) 3?53 7?98 75?97 35?45 Carbon density, median estimatez 17?98 4?95 58?38
Ni (2001)
This study
15?78 5?56 37?17
17?03 Carbon density, median estimate 16?83 6?11 39?98
3?58 0?28 41?03
3?76 0?34 44?09
*Meadow–steppe, typical steppe, desert–steppe, alpine steppe, tussock, savannas and grasslands in desert area. zArid shrublands/steppe, temperate savannas (grass-scrub), temperate steppe, temperate deserted steppe, alpine steppe and alpine meadows. }Temperate desert, tundra and alpine desert. 8Bogs/mires of cool or cold climate, wetlands, swamps and marshes. wFang et al. (1996) classified the grasslands in China into eight types: Meadow-steppe, typical steppe, desert–steppe, alpine steppe, tussock, savannas and grasslands in desert regions, desert, and swamp. They used the biomass multiply areas to product vegetation carbon, and the soil organic carbon content, areas, mean depth and mean voluminal weight to product the soils carbon. zNi (2001) used the same method as this study (carbon densities multiply vegetation areas) to obtain the vegetation and soils carbon in grasslands of China, but the grassland classification and carbon density of every type were quite different.
(Hou et al., 1982). This value was derived from the 1970s grassland distributions and therefore potentially overestimates the actual grassland area compared with today because of long-term human disturbances. The utilizable area used in the current study excluded areas of sediment, road, bare land and river from the total grassland area (DAHV & CISNR 1994). It is more accurate than other areal calculations such as the statistical data from provinces (Fang et al., 1996) and the potential area of grasslands from the vegetation map of China (Ni, 2001). If the relatively accurate grassland area of this study (355?3 million ha) was used with methods of Fang et al. (1996) and Ni (2001) to recalculate the carbon storage in grasslands of China, the new estimates would both be lower than those estimated by the two previous studies. In other words, Fang et al. (1996) and Ni (2001) overestimated the carbon storage in China’s grasslands. Third, the biomass field measurements had many uncertainties and were not accurate because they were averaged from only a few samples in grasslands (Fang et al., 1996). Specifically, the measurements of below-ground biomass were very few and were calculated by the ratio of above- to below-ground biomass (Fang et al.,
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1996). This approximation may lead to very low values of vegetation carbon in grasslands of China estimated by Fang et al. (1996), compared with the other two estimates (Tables 2 and 3). The fourth cause of differences in carbon storage estimates relates to carbon density. In this paper and also in Ni (2001), carbon densities in vegetation and soils were estimated using common values from global ecosystems (Olson et al., 1983; Zinke et al., 1984). Although these values have general global averages and are used largely by ecologists studying carbon of terrestrial ecosystems, they are not perfectly suited to Chinese grasslands. However, more accurate carbon densities in vegetation and soils of Chinese grasslands have not been available till now. Only soil organic carbon of some grassland types in some places was reported (Fang et al., 1996; Feng et al., 2001). The common carbon densities worldwide, therefore, are still the useful values in estimating grassland carbon at present. In summary, Fang et al. (1996) underestimated the vegetation carbon and overestimated the soil carbon of grasslands in China. Ni (2001) also overestimated both the vegetation and soil carbon. The two previous studies also overestimated the total carbon storage in grasslands of China. The estimation of carbon in this study might be more credible because of more accurate grassland areas and more reasonable vegetation classification scheme than the two previous studies. What proportion do China’s grasslands contribute to the world grassland carbon storage? It is not easy to answer this question because estimates of the amount of grassland carbon worldwide vary, specifically with respect to soil carbon (Hall et al., 1995; Scurlock & Hall, 1998). The grassland ecosystem complexes (cold and warm grass/shrub, desert and semidesert, swamps, marshes and bogs/mires, alpine ecosystems) have 50?4 Pg C in vegetation in the area of 5155 million ha, based on the carbon densities (Olson et al., 1983). The temperate thorn steppe, cool temperate steppe, tropical desert bush, warm desert, cool desert and wetlands have 435?7 Pg C in soils in the area of 3510 ha, based on soil surveys (Post et al., 1982). From these estimates, China’s grassland carbon is 6?1% of the world grassland live carbon, 9?4% of the global grassland soil carbon, and 9?1% of total grassland carbon. By comparison, China’s grasslands constitute 5?8–8?5% of world grassland area. Prentice et al. (1993) also estimated the present world grassland carbon when they modeled global terrestrial carbon storage using the carbon density method from Olson et al. (1983) and Zinke et al. (1984). The warm grass/shrub, cool grass/shrub, hot desert and semidesert combined have 27?9 Pg C in vegetation, 250?5 Pg C in soils and 279 Pg C in total carbon, with a total area of 4160 million ha (Prentice et al., 1993). Using these estimates, carbon storage in China’s grasslands is 10?9% of the world grassland live carbon, 16?4% of the world grassland soil carbon and 15?8% of the total grassland carbon in ca. 7?2% of the total world grassland area. Therefore, China’s grasslands cover only 6–8% of the total world grassland area and have 9–16% of the total carbon in the world grasslands. They make a big contribution to the world carbon storage and may have significant effects on carbon cycles, both in global and in arid lands within a global context. The research was funded by the National Natural Science Foundation of China (NSFC No. 39970154), the China Ministry of Science and Technology (G1999043507), the Alfried Krupp von Bohlen und Halbach-Stiftung, the Max-Planck Society (MPS) and the Max-PlanckInstitute for Biogeochemistry (MPI-BGC), Germany. The author would like to thank two anonymous reviewers for their thoughtful comments and useful information on an earlier version of the manuscript. Thanks are also due to Dr Karen E. Kohfeld for her improvement of English language of this paper.
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Carbon storage in grasslands of China
Jian Ni%wz %
Max Planck Institute for Biogeochemistry, Carl Zeiss Promenade 10, P.O. Box 100164, D-07701 Jena, Germany wLaboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Xiangshan Nanxincun 20, 100093 Beijing, China (Received 15 September 2000, accepted 26 June 2001) Carbon storage in grasslands of China was estimated by the carbon density method and based on a nationwide grassland resource survey finished by 1991. The grasslands in China were classified into 18 types, which are distributed mostly in the temperate region and on the Tibetan Plateau, and scattered in the warm-temperate and tropical regions. Based on the median estimate, vegetation, soil and total carbon storage of grasslands in China were 3?06, 41?03 and 44?09 Pg C, respectively. Vegetation had low carbon storage and most carbon was stored in soils. Of the four types of regions that have grasslands, alpine region (54?5%) and temperate region (31?6%) hold more than 85% of the total grassland carbon (in both vegetation and soils) in China. Considering specific types within these two regions, three grassland types, alpine meadow (25?6%), alpine steppe (14?5%) and temperate steppe (11%) constituted more than half of all carbon stored in China’s grasslands. In general and regardless of regional vegetation types, steppes (38?6%) and meadows (38?2%) made up more than 2/3 of total grassland carbon. The carbon storage in alpine grasslands may have a significant and long-lived effect on global C cycles. This study estimated more carbon storage in vegetation and less in soils than previous studies. The differences of grassland carbon between this study and two previous studies were due probably to four reasons, i.e. different estimation methods, different classification systems of grasslands, different areas of grasslands, and different carbon densities. China’s grasslands cover only 6–8% of total world grassland area and have 9– 16% of total carbon in the world grasslands. They make a big contribution to the world carbon storage and may have significant effects on carbon cycles, both in global and in arid lands. # 2002 Elsevier Science Ltd. Keywords: carbon storage; carbon density; vegetation area; soil carbon; alpine grasslands; temperate grasslands; China; arid lands
Introduction and literature Grasslands are one of the most widespread vegetation types worldwide, covering 15 million km2 in the tropics and a further 9 million km2 in temperate regions, together z Corresponding author. Tel: +49 3641 64 3743; Fax: +49 3641 64 3789; E-mail: [email protected] 0140-1963/02/020205 + 14 $35.00/0
# 2002 Elsevier Science Ltd.
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nearly one-fifth of the world’s land surface (Scurlock & Hall, 1998). Natural grasslands both in tropical and in temperate regions play a significant role in the regional climate and global carbon cycle (Hall & Scurlock, 1991; Hall et al., 1995; Sala et al., 1996). Carbon storage and carbon sinks in the vegetation and soils of grasslands in temperate and tropical regions were previously studied based upon different data and methods at a regional to global scale, particularly within the context of global change (e.g. Lieth, 1978; Hall & Scurlock, 1991; Long & Hutchin, 1991; Thornley et al., 1991; Parton et al., 1993, 1995; Fisher et al., 1994, 1995; Hall et al., 1995; Tate et al., 1995; Scurlock & Hall, 1998). However, carbon storage may have been underestimated, especially in the soils and in the tropics (Scurlock & Hall, 1998). Recent studies of grassland carbon suggested that natural grass ecosystems might be responsible for a substantial proportion (as much as 20% or more) of total terrestrial production, and cautiously proposed that these biomes might already constitute an annual sink of about 0?5 Pg carbon (Scurlock & Hall, 1998). Grasslands in China cover vast, continuous areas of the northern temperate regions with a continental arid climate, the Tibetan Plateau at very high elevations, and small, disjunct areas of the warm temperate and tropical regions controlled by the monsoon climate (Editorial Committee for Vegetation of China, 1980; Hou et al., 1982). Several estimates of grassland distribution are now available at the national level, i.e. 355 million ha (DAHV & CISNR, 1994) and 399 including 354 million ha of major 17 grassland types (DAHV and GSAHV, 1996) according to the national grassland resource survey, 360 million ha based on the Map of Grassland Resources in China at 1:4M scale (CISNR, 1996), 349 million ha by the Map of Land Use in China at 1:4M scale (Wu, 1991), and 406 million ha by the Vegetation Map of China at 1:4M scale (Hou et al., 1982). The areas of grasslands in China are similar regardless of the data source, about 40% of the total territory of China (Chen & Fischer, 1998). The vast area and wide distribution of Chinese grasslands implies that they would have significant effects on regional climate and regional to continental carbon cycles. There is relatively little research on carbon storage and carbon cycle in grasslands of China (Fang et al., 1996; Feng et al., 2001; Ni, 2001). Some studies present carbon estimates from all vegetation types in all of China, but they only included the carbon storage in grasslands at the national level (Fang et al., 1996; Ni, 2001). Total biomass (above- and below-ground) estimates of Chinese grasslands were calculated by Fang et al. (1996) based on areal extent, the range of forage yield and above-below-ground biomass ratio. However, carbon of grasslands estimated by the biomass method is not completely accurate because of the lack of information on the biomass component of grassland vegetation, especially below-ground biomass that was estimated only by the ratio of above- to below-ground biomass (Fang et al., 1996). Ni (2001) also calculated the carbon storage of Chinese ecosystems including grasslands using the carbon density method, but the classification of grasslands derived from 1960 to 1970s investigations is very simple. On the other hand, strong human activities in the last two decades have had an impact on Chinese grasslands (distributions and areas). These may influence the accuracy of the carbon estimation (Ni, 2001). The areal extent of vegetation is a critical factor influencing the estimate of carbon in terrestrial ecosystems, regardless of the method used. Ni (2001) has demonstrated that different spatial resolutions of vegetation, and subsequently different estimated vegetation areas could impact on the calculations of carbon storage within ecosystems. Differences in vegetation classification also affect the calculation of area and determination of carbon density (Ni, 2001). After the two phases of long-term investigation of grassland resources in China from 1949 to 1978 and from 1979 to 1990s (Chen & Fischer, 1998), more detailed classification and more accurate estimates of the areal coverage of grasslands in China are now available (DAHV & CISNR, 1994; DAHV & GSAHV, 1996). Data on Grassland Resources in China provided detailed descriptions and data on grasslands based on a nationwide grassland
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resource survey in 1991 (DAHV & CISNR, 1994). Therefore, it is now possible to use the carbon density method in combination with the improved classification scheme and new estimates of grassland area to more accurately calculate carbon storage in Chinese grasslands. Then, the ‘new and better’ carbon storage estimate of China’s grasslands will be compared to previous studies, with a specific focus on temperate and alpine grasslands.
Material and methods Grasslands in China Data on grassland type, total area, utilizable area, forage yield, dominant species and distributions (Table 1) were obtained from a specialized statistical book Data on Grassland Resources of China (DAHV & CISNR, 1994). These data were collected and sorted systematically from all Chinese counties and provinces (autonomous regions and municipalities) excluding Taiwan, based on a 10-year nationwide grassland resource survey completed in 1991. There are four grassland classification schemes in China, including (a) 18 vegetation types in the national grassland survey (DAHV & CISNR, 1994), (b) more than 50 types in the Map of Grassland Resources in China (CISNR, 1996), (c) three types in the Map of Land Use in China (Wu, 1991), and (d) more than 40 types in the Vegetation Map of China (Hou et al., 1982). Chen & Fischer (1998) digitized and simplified the Map of Grassland Resources in China at 1 : 4M scale (CISNR, 1996). They used a 17-type grassland classification scheme, excluding the tropical dry shrubtussock with savanna, which covered only a very small area (Chen & Fischer, 1998). In the current work, the DAHV & CISNR classification scheme of grasslands (18 types) was used (Table 1) for the following reasons. First, the 18 grassland types have enough information to characterize Chinese grasslands. Second, they match the grassland areas and other characteristics in the national grassland survey (DAHV & CISNR, 1994). Finally, they are simplified from the more comprehensive Map of Grassland Resources in China (CISNR, 1996). The grasslands of China are generally and mainly distributed in the temperate and alpine regions in north and west China, respectively. The temperate meadow–steppe, steppe, desert–steppe, as well as part of lowland and mountain meadow are distributed in the northern region (Table 1). Some extend to north-eastern (temperate meadow) and north-western (temperate steppe–desert and desert) regions. The Tibetan Plateau has unique alpine meadow–steppe, steppe and desert–steppe types, which are quite different from the temperate meadow, steppe and desert (Table 1). In east and south parts of China, the warm-temperate and tropical tussocks, shrubs and savannas are sparsely distributed, and deciduous and evergreen forests dominate (Table 1). Carbon density Biomass carbon densities from Olson et al. (1983) and soil carbon densities from Zinke et al. (1984) were used in this paper to give a range of low, median and high carbon densities for every grassland type (Table 2). These values were multiplied by the utilizable areas to obtain the carbon storage of grasslands in China (Table 2). Areas of sediment, road, bare land and river have been excluded from the estimate of utilizable area (DAHV & CISNR, 1994), and therefore more accurately reflect the total utilizable grassland areas in China.
Utilizable area (ha)
Forage yield (kg1 ha)
Ecosystem complexes (Olson et al., 1983)
Dominants
Temperate meadow– steppe
14,519,331
12,827,411
1465
Cool grassland
Temperate steppe
41,096,571
36,367,633
889
Cool grassland
Temperate desert–steppe
18,921,607
17,052,421
455
Cool grassland
6,865,734
6,011,528
307
41,623,171
35,439,220
284
Very cold Tibet meadows and parklands Cool grassland
Leymus chinense, Stipa baicalensis, Filifolium sibiricum, Carex, Festuca, Artemisia, Lespedeza Leymus chinense, Stipa grandis, Stipa Krylovii, Stipa bungeana, Agropyron cristatum, Cleistogenes squarrosa, Artemisia frigida, Artemisia, Caragana, Ephedra Stipa klemenzii, Stipa brevifolia, Stipa glareosa, Stipa goboca, Artemisia, Caragana Stipa purpurea, Stipa capillacea, Festuca, Carex
9,566,006
7,752,078
195
Alpine desert
Alpine meadow– steppe Alpine steppe Alpine desert–steppe
Stipa purpurea, Festuca subsessiliflora var. basiplumosa, Poa, Artemisia, Carex Stipa glareosa, Stipa purpurea, Caragana, Artemisia, Ceratoides compacta
Distribution North-east and northwest China North China
North and north-west China West China West China West China
J. NI
Area (ha)
Grassland
208
Table 1. Information on grasslands in China (from DAHV & CISNR, 1994). Grassland was defined as coverage of steppe community greater than or equal to 5%; canopy index of tree community less than or equal to 0?3; and canopy index of shrub community less than or equal to 0?4. The grassland area was summarized by areas measured from the grassland map at a 1:50,000 or 1:100,000 scale, differing from region to region. The utilizable area equals total grassland area minus areas of sediment, road, bare land and river. The forage yield is a mean value of the 10-year production, estimated by measuring above-ground standing crops in fresh weight in the field during the highest yield period in summer and autumn. The fresh weight was converted to dry weight based on grassland type-based dry/fresh weight ratio (DAHV & CISNR, 1994). Every grassland classification was assigned to an ecosystem complex (from Olson et al., 1983) in order to obtain their carbon densities in vegetation and soils (Olson et al., 1983; Zinke et al., 1984). More details can be found in Data on Grassland Resources of China (DAHV & CISNR, 1994)
10,673,418
9,140,926
465
Cool semi-desert
Temperate desert
45,060,811
3,060,4131
329
Desert and semidesert
Alpine desert
7,527,763
5,592,765
117
Warm-temperate tussock
6,657,148
5,853,667
1643
Sand desert: scrub/ herb or barren Warm/hot grassland
Warmtemperate shrub-tussock
11,615,910
9,773,518
1769
Warm/hot grassland with relatively more shrub, trees
Tropical tussock
14,237,195
11,419,999
2643
Warm/hot grassland
Stipa glareosa, Salsola abrotanoides, Ceratoides latens, Reaumuria soongarica, Caragana, Artemisia Sympegma regelii, Iljnia regelii, Ephedra, Nitraria sibirica, Haloxylon, Artemisia, Caragana, Zygophyllum, Calligonum, Halocnemum strobilaceum Ceratoides compacta, Ajania, Artemisia Spodiopogon sibiricus, Bothriochloa ischaemum, Themeda triandra var. japonica, Eulalia pallens, Carex, Artemisia Bothriochloa ischaemum, Themeda triandra var. japonica, Miscanthus sinensis, Vitex negundo var. heterophylla, Ziziphus jujuba, Lespedeza, Carex, Artemisia Miscanthus floridulus, Miscanthus sinensis, Heteropogen contortus, Cymbopogon distans, Imperata cylindrica var. major, Arundinella hirta, Ischaemum ciliare, Dicranopteris dichotoma
North-west China North-west China
West China Central east and central south China Central east and central south China
Central south and south China
CARBON STORAGE IN GRASSLANDS OF CHINA
Temperate steppe–desert
209
210
Table 1Fcontinued.
Grassland
Area (ha)
Utilizable area (ha)
Forage yield (kg ha)
Tropical 17,551,276 shrub-tussock
13,447,569
2527
Tropical dry 863,144 shrub-tussock with savanna Lowland 25,219,621 meadow
639,429
2719
21,038,409
1730
16,718,926
14,923,439
1648
Alpine meadow
63,720,549
58,834,182
882
Swamp
2,873,812
2,253,714
2183
Warm/hot grassland with relatively more shrub, trees
Dominants
Miscanthus floridulus, Miscanthus sinensis, Heteropogen contortus, Imperata cylindrica var. major, Arundinella hirta, Ischaemum ciliare, Rhododendron, Melastoma Warm/hot grassland Heteropogen contortus, Pinus with relatively more yunnanensis, Acacia shrub, trees farnesiana, Flacourtia indica Heath and moorland, Calamagrostis angustifolia, maritime scrub with Achnatherum splendens, meadows Sanguisorba officinalis, Carex, Deyeuxia angustifolia, Aeluropus littoralis var. sinensis Maritime/montane grass Festuca ovina, Deyeuxia arundinacea, Arundinella chenii, Stipa aliena, Carex, Poa Kobresia pygmaea, Kobresia Very cold Tibet meadows and parklands humilis, Kobresia capillifolia, Kobresia schoenoides, Kobresia littledalei, Festuca rubra, Polygonum macrophyllum, Polygonum viviparum, Carex, Rhododendron Carex muliensis, Phragmites Swamps/shrub/herb marshes australis, Scirpus triqueter, Kobresia
Distribution Central south and south China
South China Whole China J. NI
Mountain meadow
Ecosystem complexes (Olson et al., 1983)
North and west China West and south-west China
North, west and central China
Table 2. Carbon density (from Olson et al., 1983; Zinke et al., 1984) and carbon storage (low: L, median: M and high: H) in vegetation and soils of grasslands in China. Every grassland type was assigned to an ecosystem complex (see Table 1) described in Olson et al. (1983), and then the carbon density in vegetation and soils of each ecosystem complex at three levels (low, median and high) was obtained from Olson et al. (1983) and Zinke et al. (1984). The carbon densities multiplied by the utilizable area (see Table 1) give the carbon storage of grasslands in China
Grassland
M
H
Vegetation carbon storage (Pg C)
L
M
H
9?5
11?2
13?3
L
M
H
Soil carbon storage (Pg C)
Total carbon storage (Pg C)
L
M
H
L
M
H
0?13 0?19 0?26
1?22
1?44
1?71
1?35
1?63 1?96
1
1?5
2
0?5 0?5 0?5 0?5 0?5 0?3 0?2 0?02 0?5 1
1 1 1 1 0?8 0?6 0?4 0?05 1 1?6
2 2 4 2 1?5 1 1 0?2 2 3
11?6 7?2 15?7 14 14 7 4?1 14 10 10
12?3 8?7 18?2 17 17 8 6?2 17 13 13
13 10?2 20?7 19 19 9 8?3 19 14 14
0?18 0?09 0?03 0?18 0?04 0?03 0?06 0?00 0?03 0?10
0?73 0?34 0?24 0?71 0?12 0?09 0?31 0?01 0?12 0?29
4?22 1?23 0?94 4?96 1?09 0?64 1?25 0?78 0?59 0?98
4?47 1?48 1?09 6?02 1?32 0?73 1?90 0?95 0?76 1?27
4?73 1?74 1?24 6?73 1?47 0?82 2?54 1?06 0?82 1?37
4?40 1?31 0?97 5?14 1?12 0?67 1?32 0?78 0?61 1?08
4?84 1?65 1?15 6?38 1?38 0?79 2?02 0?95 0?82 1?43
0?5 1 1
1 1?6 1?6
2 3 3
11 11 6?7
14 14 7?3
15 15 7?9
0?06 0?11 0?23 0?13 0?22 0?40 0?01 0?01 0?02
1?26 1?48 0?04
1?60 1?88 0?05
1?71 2?02 0?05
1?31 1?61 0?05
1?71 1?94 2?10 2?42 0?06 0?07
1 0?5 0?5 1?5 0?64
1?5 1 1 3 1?15
2 2 4 6 2?37
9?5 15?7 15?7 11?6 11?02
11?2 18?2 18?2 12?3 13?16
13?3 20?7 20?7 13 14?73
0?21 0?07 0?29 0?03 1?67
0?36 0?17 0?06 0?35 0?06 0?05 0?12 0?00 0?06 0?16
0?32 0?15 0?59 0?07 3?05
5?46 2?08 1?48 7?44 1?59 0?91 2?85 1?07 0?94 1?66
CARBON STORAGE IN GRASSLANDS OF CHINA
L
0?42 2?00 2?36 2?80 2?21 2?67 3?22 0?30 2?34 2?72 3?09 2?42 2?87 3?39 2?35 9?24 10?71 12?18 9?53 11?30 14?53 0?14 0?26 0?28 0?29 0?30 0?34 0?43 7?08 34?52 41?03 46?37 36?18 44?09 53?44 211
Temperate meadow–steppe Temperate steppe Temperate desert–steppe Alpine meadow–steppe Alpine steppe Alpine desert–steppe Temperate steppe-desert Temperate desert Alpine desert Warm-temperate tussock Warm-temperate shrub-tussock Tropical tussock Tropical shrub-tussock Tropical dry shrub-tussock with savanna Lowland meadow Mountain meadow Alpine meadow Swamp Mean carbon density and total carbon
Soil carbon density (kg m2)
Vegetation carbon density (kg m2)
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Results According to the carbon densities in vegetation and soils (Table 2) and the utilizable areas (Table1), the low, median and high values of carbon in vegetation, soils and total carbon storage for each grassland type are given in Table 2. Based on the median estimate, vegetation, soil and total carbon of grasslands in China were 3?06, 41?03 and 44?09 Pg C, respectively. Vegetation has low carbon storage and most carbon is stored in soils (Table 2). The alpine meadow has the highest carbon storage both in vegetation and in soils, covering 25?6% of total carbon in grasslands of China (Table 2). The alpine steppe (14?5%) and temperate steppe (11%) also have higher carbon storage (Table 2). Together, these three grassland types make up more than half of all of the carbon being stored in China’s grasslands. This is due to these three grasslands having the highest utilizable areas and higher carbon densities (Table 1). The warm temperate and tropical tussocks as well as swamp, however, have the lowest carbon because of their small areas and lower carbon densities (Table 2). In terms of the grassland type, steppe has the highest carbon storage in vegetation and soils, totaling 17?03 Pg C (38?6%), with meadows storing 16?83 Pg C (38?2%). These two types constitute more than 2/3 of the total carbon stored in grasslands of China (Fig. 1). Other three grassland types, desert, tussock and swamp have a carbon storage of only 10?22 Pg C, less than 1/3 of the total carbon of grasslands in China (Fig. 1). Based on regional grassland distribution, the alpine region has the highest carbon storage, 24?03 Pg C, making up 54?5% of the total carbon (Fig. 2; Table 2). The temperate region also has a large area and therefore a high carbon storage of 13?94 Pg C (31?6% of total carbon) (Fig. 2). Together, these two regions constitute more than 85% of all the carbon stored in grasslands of China (Fig. 2). In warmtemperate and tropical regions, however, the grasslands have very low carbon storage (Fig. 2). Discussion Grasslands play a very important role in modeling the biospheric feedbacks of atmospheric CO2 increase and climate changes (Hall et al., 1995; Thornley & Cannell, 1997). It has long been suggested that the majority of the ‘missing sink’ for carbon in the global CO2 balance may be located in temperate and tropical regions (e.g. Tans et al., 1990). Therefore, the role of grasslands, with large area both in tropics and in temperate regions, should not be overlooked in global biogeochemical cycles (Hall & Scurlock, 1991; Thornley et al., 1991; Parton et al., 1993, 1995; Fisher et al., 1994, 1995; Hall et al., 1995; Tate et al., 1995; Scurlock & Hall, 1998). In China, grasslands are mostly distributed in temperate regions (the semi-arid and arid north China) and in alpine regions (the high and cold west China) (Hou et al., 1982; DAHV & CISNR, 1994). The largest proportion of carbon is stored in alpine and temperate grasslands (Table 2; Fig. 2). The importance of temperate grassland carbon storage is well known (e.g. Thronley et al., 1991; Hall et al., 1995; Tate et al., 1995; Sala et al., 1996), but the importance of alpine carbon storage is less appreciated. In grassland ecosystems carbon is stored mostly in soils, where C turnover times of the bulk soil carbon are relatively long (Hall et al., 1995; Tate et al., 1995; Matthews, 1997). 95% of the carbon stored in alpine grasslands of China are stored in soils (Table 2), which is 55?6% of the total carbon storage in China’s grassland soils (Fig. 2). Similar to high-latitude tundra ecosystems (Hobbie et al., 2000), the high elevation of alpine regions results in low temperature and low evapotranspiration in alpine region. Therefore, soil carbon is difficult to decompose and mostly remains in soils for a long time. If climate change and CO2 increase were to would impact on soil carbon, especially in cooler regions (Hall et al., 1995), changes of
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Figure 1. Utilizable areas and median carbon stored in grasslands of China by grassland type. Steppe includes temperate meadow–steppe, temperate steppe, temperate desert–steppe, alpine meadow–steppe, alpine steppe and alpine desert–steppe. Desert includes temperate steppe–desert, temperate desert and alpine desert. Tussock includes warm-temperate tussock, warm-temperate shrub-tussock, tropical tussock, tropical shrub-tussock and tropical dry shrub-tussock with savanna. Meadow includes lowland meadow, mountain meadow and alpine meadow. Swamp includes swamp grassland.
alpine grassland carbon storage may have a significant and long-lived effect on global C cycles. Fang et al. (1996) and Ni (2001) calculated the grassland carbon of China based on the biomass and organic carbon matter method and the carbon density method, respectively, when they estimated carbon storage of terrestrial ecosystems in China (Table 3). Compared to the carbon storage of grasslands in this paper, considering the meadow and tussock as steppe ecosystems (Table 3), the sort for vegetation carbon is Fang o this paper o Ni and that for soil carbon is this paper oNi oFang. In total, the carbon storage in grasslands of China estimated by this study was lower than those calculated by Ni (2001) and Fang et al. (1996). The differences of grassland carbon between this study and two previous studies are probably due to the following four reasons. First, the classification of grasslands was quite different. Fang et al. (1996) used a very coarse classification of eight types (Table 3). Ni (2001) classified the grasslands
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Figure 2. Utilizable areas and median carbon stored in grasslands of China by region. Temperate region includes temperate meadow–steppe, temperate steppe, temperate desert– steppe, temperate steppe–desert, temperate desert, lowland meadow and swamp. Warmtemperate region includes warm-temperate tussock and warm-temperate shrub-tussock? Tropical region includes tropical tussock, tropical shrub-tussock and tropical dry shrub-tussock with savanna. Alpine region includes alpine meadow–steppe, alpine steppe, alpine desert– steppe, alpine desert, mountain meadow and alpine meadow.
into 11 types (Table 3) based on the vegetation map of China (Hou et al., 1982). In this study, however, 18 grassland types were used based on the nationwide grassland survey (DAHV & CISNR, 1994). The differences in classification result in the differences in area and carbon density, subsequently carbon stored in grasslands of China. Second, authors used the different areas of grasslands, i.e. 569?9 million ha (Fang et al., 1996), 405?87 million ha (Ni, 2001) and 355?3 million ha (298?97 for the utilizable area) in this study. This is a very important factor that influences the estimation of carbon (Ni, 2001). The live vegetation area of grasslands of Fang et al. (1996) was derived from statistics of land use resources and an agricultural atlas of China from the 1980s. The soil area was calculated from all soils in which grasslands are able to grow (Fang et al., 1996). Clearly, the live grassland area was not accurate enough and soil area was overestimated. Thus, Fang’s grassland area is much higher than those of other calculations (Table 3). The grassland area from Ni (2001) was calculated from The Vegetation Map of China
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Table 3. Areas and carbon storage in Pg C of grasslands in China estimated by different authors
Grassland
Total Area Vegetation Soils carbon carbon (106 ha) carbon
Steppe*
430?66
1?02
63?44
Desert Swamp Subtotal
128?24 11?00 569?90
0?01 0?20 1?23
3?52 7?78 74?74
Steppe z
220?11
2?66
32?78
Desert} 152?57 Swamp11 33?19 Subtotal 405?87
1?12 0?87 4?66
16?86 4?08 53?72
Steppe
115?45
1?20
15?83
Meadow Tussock Subtotal
94?80 41?13 251?38
1?05 0?55 2?81
Desert Swamp Subtotal
45?34 2?25 298?97
0?18 0?07 3?06
Estimated method
Reference
64?46 Biomass and organic Fang et al., carbon matterw (1996) 3?53 7?98 75?97 35?45 Carbon density, median estimatez 17?98 4?95 58?38
Ni (2001)
This study
15?78 5?56 37?17
17?03 Carbon density, median estimate 16?83 6?11 39?98
3?58 0?28 41?03
3?76 0?34 44?09
*Meadow–steppe, typical steppe, desert–steppe, alpine steppe, tussock, savannas and grasslands in desert area. zArid shrublands/steppe, temperate savannas (grass-scrub), temperate steppe, temperate deserted steppe, alpine steppe and alpine meadows. }Temperate desert, tundra and alpine desert. 8Bogs/mires of cool or cold climate, wetlands, swamps and marshes. wFang et al. (1996) classified the grasslands in China into eight types: Meadow-steppe, typical steppe, desert–steppe, alpine steppe, tussock, savannas and grasslands in desert regions, desert, and swamp. They used the biomass multiply areas to product vegetation carbon, and the soil organic carbon content, areas, mean depth and mean voluminal weight to product the soils carbon. zNi (2001) used the same method as this study (carbon densities multiply vegetation areas) to obtain the vegetation and soils carbon in grasslands of China, but the grassland classification and carbon density of every type were quite different.
(Hou et al., 1982). This value was derived from the 1970s grassland distributions and therefore potentially overestimates the actual grassland area compared with today because of long-term human disturbances. The utilizable area used in the current study excluded areas of sediment, road, bare land and river from the total grassland area (DAHV & CISNR 1994). It is more accurate than other areal calculations such as the statistical data from provinces (Fang et al., 1996) and the potential area of grasslands from the vegetation map of China (Ni, 2001). If the relatively accurate grassland area of this study (355?3 million ha) was used with methods of Fang et al. (1996) and Ni (2001) to recalculate the carbon storage in grasslands of China, the new estimates would both be lower than those estimated by the two previous studies. In other words, Fang et al. (1996) and Ni (2001) overestimated the carbon storage in China’s grasslands. Third, the biomass field measurements had many uncertainties and were not accurate because they were averaged from only a few samples in grasslands (Fang et al., 1996). Specifically, the measurements of below-ground biomass were very few and were calculated by the ratio of above- to below-ground biomass (Fang et al.,
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1996). This approximation may lead to very low values of vegetation carbon in grasslands of China estimated by Fang et al. (1996), compared with the other two estimates (Tables 2 and 3). The fourth cause of differences in carbon storage estimates relates to carbon density. In this paper and also in Ni (2001), carbon densities in vegetation and soils were estimated using common values from global ecosystems (Olson et al., 1983; Zinke et al., 1984). Although these values have general global averages and are used largely by ecologists studying carbon of terrestrial ecosystems, they are not perfectly suited to Chinese grasslands. However, more accurate carbon densities in vegetation and soils of Chinese grasslands have not been available till now. Only soil organic carbon of some grassland types in some places was reported (Fang et al., 1996; Feng et al., 2001). The common carbon densities worldwide, therefore, are still the useful values in estimating grassland carbon at present. In summary, Fang et al. (1996) underestimated the vegetation carbon and overestimated the soil carbon of grasslands in China. Ni (2001) also overestimated both the vegetation and soil carbon. The two previous studies also overestimated the total carbon storage in grasslands of China. The estimation of carbon in this study might be more credible because of more accurate grassland areas and more reasonable vegetation classification scheme than the two previous studies. What proportion do China’s grasslands contribute to the world grassland carbon storage? It is not easy to answer this question because estimates of the amount of grassland carbon worldwide vary, specifically with respect to soil carbon (Hall et al., 1995; Scurlock & Hall, 1998). The grassland ecosystem complexes (cold and warm grass/shrub, desert and semidesert, swamps, marshes and bogs/mires, alpine ecosystems) have 50?4 Pg C in vegetation in the area of 5155 million ha, based on the carbon densities (Olson et al., 1983). The temperate thorn steppe, cool temperate steppe, tropical desert bush, warm desert, cool desert and wetlands have 435?7 Pg C in soils in the area of 3510 ha, based on soil surveys (Post et al., 1982). From these estimates, China’s grassland carbon is 6?1% of the world grassland live carbon, 9?4% of the global grassland soil carbon, and 9?1% of total grassland carbon. By comparison, China’s grasslands constitute 5?8–8?5% of world grassland area. Prentice et al. (1993) also estimated the present world grassland carbon when they modeled global terrestrial carbon storage using the carbon density method from Olson et al. (1983) and Zinke et al. (1984). The warm grass/shrub, cool grass/shrub, hot desert and semidesert combined have 27?9 Pg C in vegetation, 250?5 Pg C in soils and 279 Pg C in total carbon, with a total area of 4160 million ha (Prentice et al., 1993). Using these estimates, carbon storage in China’s grasslands is 10?9% of the world grassland live carbon, 16?4% of the world grassland soil carbon and 15?8% of the total grassland carbon in ca. 7?2% of the total world grassland area. Therefore, China’s grasslands cover only 6–8% of the total world grassland area and have 9–16% of the total carbon in the world grasslands. They make a big contribution to the world carbon storage and may have significant effects on carbon cycles, both in global and in arid lands within a global context. The research was funded by the National Natural Science Foundation of China (NSFC No. 39970154), the China Ministry of Science and Technology (G1999043507), the Alfried Krupp von Bohlen und Halbach-Stiftung, the Max-Planck Society (MPS) and the Max-PlanckInstitute for Biogeochemistry (MPI-BGC), Germany. The author would like to thank two anonymous reviewers for their thoughtful comments and useful information on an earlier version of the manuscript. Thanks are also due to Dr Karen E. Kohfeld for her improvement of English language of this paper.
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