Carbon accumulation and distribution in Pinus massoniana and Cunninghamia lanceolata mixed forest ecosystem in Daqingshan, Guangxi, China

ACTA ECOLOGICA SINICA Volume 26, Issue 5, May 2006 Online English edition of the Chinese language journal Cite this article as: Acta Ecologica Sinica, 2006, 26(5), 1320−1329.

RESEARCH PAPER

Carbon accumulation and distribution in Pinus massoniana and Cunninghamia lanceolata mixed forest ecosystem in Daqingshan, Guangxi, China Kang Bing1, 2, Liu Shirong2, *, Zhang Guangjun1, Chang Jianguo2, 3, Wen Yuanguang4, Ma Jiangming2, Hao Wenfeng1 1 College of Life Sciences, Northwest Sci-Tech University of Agriculture and Forestry, Yangling 712100, China 2 Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, China 3 Shanxi Academy of Forestry, Taiyuan 030012, China 4 Forestry College, Guangxi University, Nanning 530001, China

Abstract: Carbon accumulation and distribution were studied at three sampling plots in a 13-year-old mixed planatation of Pinus massoniana and Cunninghamia lanceolata in Daqingshan, Guangxi, China. The results showed that carbon content varied with tissues and tree species, but the total carbon content of Pinus massoniana was higher than that of Cunninghamia lanceolata. The average tissue carbon contents of Pinus massoniana were: wood (58.6%) > root (56.3%) > branch (51.2%) > bark (49.8%) > leaf (46.8%), while those of Cunninghamia lanceolata were: bark (52.2%) > leaf (51.8%) > wood (50.2%) > root (47.5%) > branch (46.7%). The carbon contents of the soil (at a depth of 60cm) ranged from 1.45% to 1.84% with an average of 1.70%. Carbon contents were higher in the surface soil (0–20cm) than in the deep layer (below 20cm). The average carbon contents were the highest for trees (51.1%), followed by litter (48.3%), shrubs (44.1%), and herbs (33.0%). The biomass of the trees in the three plots ranged from －2

85.35 t hm

－2

with an average of 93.83 t hm 2, in which 75.7%–82.6% was Pinus massoniana. The biomass of the －

to 101.35 t hm

－2

understory was 2.10–3.95 t hm

with an average of 2.72 t hm 2, while the standing stock of ground litter was 5.49–7.91 t hm

－2

－

with

－2

an average of 6.75 t hm . The carbon storage in the mixed plantation reached the maximum in the soil layer (69.02%), followed by vegetation (29.03%), and standing litter (1.82%). The carbon storage in the tree layer occupied 23.90% of the total ecosystem and 97.7% of the vegetation layer. Pinus massoniana accounted for 65.39% of the total carbon storage in the tree layer. Tissue carbon storage was directly related to the corresponding amount of biomass. Trunks had the highest carbon storage, accounting for 53.23% of the trees in Pinus massoniana and 55.57% in Cunninghamia lanceolata, respectively. Roots accounted for about 19.22% of the total tree carbon. The annual net productivity of the mixed plantation was 11.46 t hm 2 a 1, and that of sequestered carbon was 5.96 －

－2

t hm

－1

a

－2

, which was equivalent to fixing CO2 of 21.88 t hm

－

－1

a . The plantation was found to be an important sink of atmospheric

CO2. Key Words: carbon distribution; carbon storage; Pinus massoniana; Cunninghamia lanceolata; plantation

1

Introduction

The concentration of CO2 in the atmosphere has increased by 31% over the past 150 years[1, 2], due to the burning of fossil fuels and changes in land use. Studying the carbon cycle is the best way to estimate the contents of CO2 and other greenhouse gases, and their impact on the biosphere[3]. CO2 is considered the most important greenhouse gas, and its source and pool are global hotspots. People utilize the carbon sinks in

vegetation and soil in the land ecosystem to increase carbon － storage and slow down global changes[4 6]. Being the main type of vegetation, forests account for almost 46.3% of total carbon storage, while soil occupies 73% of the total soil carbon storage[7]. Forests assimilate plentiful CO2 from the atmosphere by growth. Their storage ability depends on the type of forests, species, age, and their relationship to human activities[8]. Overseas research has focused on the carbon sinks, which are brought about by forest management and foresta-

Received date: 2005-10-18; Accepted date: 2006-03-17 *Corresponding author. E-mail: [email protected] Copyright © 2006, Ecological Society of China. Published by Elsevier BV. All rights reserved.

KANG Bing et al. / Acta Ecologica Sinica, 2006, 26(5): 1320–1329

tion[4 6]. However, Chinese research is lagging in this field[9 12]. All studies conducted in this country have concentrated on pure plantations and original forests. Owing to their diversity and instability, coniferous mixed forests have been rarely studied[13]. Similar to other regions of the world, Chinese subtropical areas also suffer from human disturbance, such as changes in land use, and forests are almost subject to deforestation, which make the forest ecosystems very fragile[14,15]. There are representative vegetation types at the study site dependent on the state of vegetation carbon storage. The mixed forest ecosystems of Pinus massoniana and Cunninghamia lanceolata occupy a considerable proportion at this site. No reports have so far been published about the carbon storage and distribution of mixed forests in this region. This study is aimed to provide academic proof of the role played by forests in the carbon balance, its impact on the research of global changes, the productivity of the forest ecosystems and the human effect on the carbon cycle. －

2

－

Materials and methods

2.1 Study site The research area is situated on the southeastern border of the Guangxi Zhuang Autonomous Region, China, located at a latitude of 21°57′–22°19′N and a longitude of 106°39′– 106°59′E. This region belongs to the southeastern edge of the subtropical monsoon climate area, and is adjacent to the humid and semi-humid area of the northern tropical zone. Sunlight, heat and water are abundant in this area. The mean annual air temperature is 20.5–21.7℃, while its highest temperature is 40.3℃, and its lowest temperature is −1.5 ℃. The active accumulated temperature (≥10℃) is 6000–7600℃. The average annual rainfall is 1200–1500mm, and the annual evapotranspiration is 1261–1388mm. The relative humidity is 80%–84%. The main geomorphic types are lower mountains and hills, while the zonal soil type is latosol with a depth of 1 m. Three plot areas were selected according to the slope position (upside, middle part, underside) in the 13-year-old mixed forest of Pinus massoniana and Cunninghamia lanceolata. Furthermore, four permanent quadrats with a size of 20 m×30 m were established in every plot area. The main shrub species

are Quercus glauca, Maesa japonica, Evodia lepta, Ficus cunia, Mussaenda pubuscens, etc. The main herb species are Miscanthus floridulus, Cyrtococcum patens, Lophatherum gracile, Microstegium vegans, etc. 2.2 Measurement of tree biomass and net productivity The tree (＞4 m high) height and the diameter at breast height (DBH) were measured in nested 10 m×10 m subplots within the 20m×30m quadrats (Table 1). Seven standard trees in one subplot were chosen and logged. The aboveground biomass was measured using the segmenting method (1m long). Standard trees were divided into five parts — trunk, branch, root, leaf, and bark. The root biomass was estimated using the layer digging method and the roots were divided into different segments according to their depth (0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm). All of the samples were dried to a stable weight in an oven at 80℃. Forest biomass can be estimated using the relative growth equation. The annual net average of the biomass is an index obtained to measure the net productivity[16]. The biomass equation of the Chinese fir: WWood=0.009068D2H+1.9250 r=0.998 WBark=0.000939D2H+0.9114 r=0.983 WBranch=0.001202D2H+0.5201 r=0.979 － WLeaf= − 1.2158+0.001571D2H − 6.7863×10 8×(D2H)2 r=0.994 － WRoot= − 2.1921+0.006678D2H − 1.1687×10 6×(D2H)2+ －11 2 3 8.2639×10 ×(D H) r=0.997 The biomass equation of the Masson pine: WWood=0.01231D2H+0.8642 r=0.995 － WBark= − 0.2092+0.002404D2H − 5.2407×10 7×(D2H)2+ -11 2 3 4.8697×10 ×(D H) r=0.999 2 WBrunch=0.005886D H − 1.5623 r=0.999 － WLeaf=0.2104-0.000159D2H+8.4844×10 7×(D2H)2 − －11 2 3 7.1947×10 ×(D H) r=0.999 WRoot=0.004719D2H − 0.8714 r=0.987 The understory vegetation (＜4m high) biomass was measured in five plots measuring 1 m×1 m within one quadrat using the whole plant harvest method. Five contiguous plots measuring 1 m2 were located around the center of the two diagonals. Shrub samples were divided into leaf, stem, and root, while herb samples were divided into two categories—

Table 1 Parameters structure of tree layers in the mixed forest of Pinus massoniana and Cunninghamia lanceolata in Daqingshan Plot 1 2 3

DBH (cm)

Density (Individuals hm 2) －

Height(m)

Species

Age(a)

Range

Mean

SE

Range

Mean

SE

Mean

P

13

6.5–12.5

15.1

1.95

8.4–21.5

10.5

0.85

1079

C

13

4.5–8.5

11.8

1.05

6.2–11.5

9.0

0.53

892 1379

P

13

6.5–14.8

14.7

1.23

6.5–22.6

10.6

0.72

C

13

5.2–9.6

11.1

0.94

5.2–11.4

8.9

0.49

963

P

13

7.5–8.6

14.1

1.03

6.5–13.4

11.2

0.92

1375

C

13

6.8–11.2

11.0

0.85

5.5–10.3

10.0

0.67

754

P: Pinus massoniana, C: Cunninghamia lanceolata

KANG Bing et al. / Acta Ecologica Sinica, 2006, 26(5): 1320–1329

aboveground and belowground. About 20 boxes measuring 1 m2 were set up in order to determine the annual amount of litter. The method for drying samples was similar to that used in the treatment of tree samples. 2.3 Biomass of plants and standing ground litter The biomass of the tree layers was calculated (Table 2). It was evident that the biomass varied in different plots. In the three plot areas, Masson pine accounted for 75.66%, 77.89%, and 82.64% of the total biomass, respectively. The biomass components of the parts of Masson pine or

Chinese fir are different. The biomass proportion of the parts of Masson pine is: wood (50.7%)＞branch (20.21%)＞root (19.21%)＞bark (5.73%)＞leaf (3.98%), while that of Chinese fir is: wood (57.19%) ＞ root (22.47%) ＞ bark (9.65%) ＞ branch (8.93%)＞leaf (1.76%). The biomass of wood is the greatest of all the parts with an average of 51.87%. The mean biomass of shrubs, grasses, and ground litter is － － － 2.51 t hm 2, 0.21 t hm 2, and 6.75 t hm 2, respectively. 2.4 Sample collection and chemical analysis The tree samples were collected according to the different

Table 2 Biomass (t hm 2) of trees in the mixed forest of Pinus massoniana and Cunninghamia lanceolata in Daqingshan －

Plot area

Species Pinus massoniana

1

Cunninghamia lanceolata Subtotal

Pinus massoniana 2

Cunninghamia lanceolata Subtotal

Pinus massoniana

3

Cunninghamia lanceolata

Subtotal

Leaf

Branch

Trunk

Bark

Root

3.55

13.09

32.51

3.64

11.79

(5.49)

(20.27)

(50.34)

(5.64)

(18.26)

0.46

1.91

12.33

(2.21)

(9.21)

(59.36)

2.09

Total 64.58 (100)

3.98

(10.06)

(19.16)

4.01

15.00

44.84

5.73

15.77

(4.70)

(17.57)

(52.54)

(6.71)

(18.48)

4.21

15.89

39.80

4.53

14.34

(5.35)

(20.17)

(50.53)

(5.75)

(18.20)

0.28

1.84

11.68

2.00

6.56

(1.25)

(8.23)

(52.24)

(8.94)

(29.34)

4.49

17.73

51.48

6.53

20.9

(4.44)

(17.53)

(50.90)

(6.46)

(20.67)

20.77 (100) 85.35 (100) 78.77 (100) 22.36 (100) 101.13 (100)

4.18

15.84

39.67

4.53

14.29

(5.32)

(20.18)

(50.53)

(5.77)

(18.20)

78.51

0.30

1.54

9.89

1.64

3.12

(1.81)

(9.34)

(59.98)

(9.95)

(18.92)

4.48

17.38

49.56

6.17

17.41

95.00

(4.72)

(18.29)

(52.17)

(6.49)

(18.33)

(100)

(100) 16.49 (100)

Data in the brackets represent percentage

Table 3 Biomass of understory plants and standing ground litter (t hm 2) in the mixed forest of Pinus massoniana and －

Cunninghamia lanceolata in Daqingshan Biomass (t hm 2) －

Component

Plot 1

Plot 2

Plot 3

Mean

Understory plant Shrub layer Aboveground

2.66 (1.24)

1.22 (0.57)

1.28 (0.60)

Underground

0.97 (0.37)

0.72 (0.28)

0.68 (0.26)

1.72 (0.80) 0.79 (0.30)

Subtotal

3.63 (1.61)

1.94 (0.85)

1.96 (0.86)

2.51 (1.10)

Aboveground

0.22 (0.09)

0.09 (0.03)

0.09 (0.03)

0.13 (0.05)

Underground

0.10 (0.04)

0.07 (0.03)

0.07 (0.03)

0.08 (0.03)

Subtotal

0.32 (0.13)

0.16 (0.06)

0.16 (0.06)

0.21 (0.08)

Fresh

3.32 (1.64)

3.98 (1.96)

4.30 (2.12)

3.87 (1.91)

Decomposed

2.17 (1.01)

2.85 (1.33)

3.61 (1.68)

2.88 (1.34)

Subtotal

5.49 (2.65)

6.83 (3.29)

7.91 (3.80)

6.75 (3.25)

Herb layer

Ground litter

Values in parentheses are standard errors

KANG Bing et al. / Acta Ecologica Sinica, 2006, 26(5): 1320–1329

Table 4 Carbon concentration in different parts of Pinus massoniana and Cunninghamia lanceolata in Daqingshan (%) Species

Leaf

Pinus massoniana

Cunninghamia lanceolata

Branch

Wood

Bark

Root

Mean

46.8

51.2

58.6

49.8

56.3

(2.12)

(3.21)

(5.21)

(5.97)

(1.25)

52.5 (4.21)

51.8

46.7

50.2

52.2

47.5

49.7

(2.32)

(2.54)

(4.81)

(2.04)

(1.78)

(3.25)

Values in parentheses are variance coefficients

parts. The samples of standing ground litters were divided into two categories—fresh and decomposed. The soil samples were gathered from three soil layers (0–20 cm, 20–40 cm, 40–60 cm), and the soil bulk density was used to estimate the soil mass. Living samples were oven-dried at 80℃, and dead samples were air-dried. Chemical analysis was taken using the ] potassium dichromate heating method[17 . SPSS11.0 software was used to analyze the significance of the variation. 2.5 Method of calculating carbon storage On the basis of the investigated biomass, the vegetation carbon storage was measured using the product of the biomass and the carbon content. The soil carbon storage is the product of the carbon content, the soil bulk density, and the soil depth.

3

Results

3.1 Carbon content of components in the mixed forest ecosystem 3.1.1 Carbon content of tree parts The mean carbon content of the two tree species is different (Table 4); Pinus massoniana has a higher carbon content than Cunninghamia lanceolata. The average tissue carbon contents of Pinus massoniana are: wood (58.6%) > root (56.3%) > branch (51.2%) > bark (49.8%) > leaf (46.8%), while those of Cunninghamia lanceolata are: bark (52.2%) > leaf (51.8%) > wood (50.2%) > root (47.5%) > branch (46.7%). Of all the tree parts in the two forests, the carbon content was highest in wood. But these variations in carbon contents for the different parts are insignificant with the variance coefficient (%) ranging from 1.25 to 5.97. Table 5 Carbon content in understory plants Layer

Component

Shrub layer

Above ground

46.7

Underground

38.5

Mean

44.1

Aboveground

41.2

Herb layer

Standing litter

Carbon content (%)

Underground

32.7

Mean

33.0

Fresh

49.3

Decomposed

46.5

Mean

48.3

3.1.2 Content of understory vegetation and soil A chemical analysis of the samples was conducted (Table 5). The results show that the average carbon content percentage of understory vegetation is the greatest for litter (48.3%), followed by shrubs (44.1%), and then herbs (33.0%). Whether shrubs or herbs, the aboveground carbon content is higher than that of the belowground[18]. The variations of the carbon content for the different layers are insignificant (P﹥0.05). The soil bulk density of the tree layers (0–20 cm, 20–40 － － cm, 40–60 cm) was 1.125ｇcm 3, 1.234ｇcm 3 and 1.357ｇ －3 cm , respectively. Carbon content decreases with soil depth increasing (Table 6). The carbon content of the soil (at a depth of 60 cm) ranges from 1.45% to 1.84% with an average of 1.70%. The carbon content was higher in the surface soil (0–20 cm) than in the deep layer (below 20 cm). Table 6 Carbon content in soil Carbon content (%)

Soil depth (cm)

Plot 1

C.V (%)

Plot 2

Plot 3

Mean

0–20

2.25

2.82

3.03

2.70

8.63

20–40

1.28

1.94

1.65

1.62

11.77

40–60

0.82

0.75

0.78

0.78

2.60

1.45

1.84

1.82

1.70

7.44

29.07

32.66

35.99

Mean of different depth C.V (%)

On the whole, the carbon content of plants follows obvious rules. Aboveground, the range of the carbon content is: tree layer＞standing litter＞shrub layer＞herb layer, while that of the root is: tree layer＞shrub layer＞herb layer. The soil carbon content is the lowest of all components in the plantation. 3.2 Carbon storage and distribution in mixed forest ecosystem The carbon storage and distribution of two tree species are accordant in three plot areas, and the biomass has a positive relationship with the carbon storage. The carbon storage of Masson pine accounts for 65.3% of the total tree carbon storage, and that of Chinese fir covers 34.61%. Of all parts, trunk carbon storage is the greatest, occupying 53.23% of the total carbon storage of Masson pine and 55.57% of Chinese fir. Furthermore, the root carbon storage takes up 19.22% of the total carbon storage of trees. The carbon storage distribution

KANG Bing et al. / Acta Ecologica Sinica, 2006, 26(5): 1320–1329

Table 7 Carbon storage distribution in different parts of tree layers in the mixed forest of Pinus massoniana and Cunninghamia lanceolata in Daqingshan (t hm 2) －

Plot

Species Pinus massoniana

1

Cunninghamia lanceolata Subtotal

Pinus massoniana 2

Cunninghamia lanceolata Subtotal

Pinus massoniana 3

Cunninghamia lanceolata Subtotal

Leaf

Bark

Root

1.66

Branch 6.70

19.05

Wood

1.81

6.64

Total

(4.63)

(18.68)

(53.12)

(5.05)

(18.52)

35.86

0.24

0.89

6.19

1.09

1.89

(2.33)

(8.64)

(60.10)

(10.58)

(18.35)

(100) 10.30

1.90

7.59

25.24

2.90

8.53

(4.12)

(16.44)

(54.68)

(628)

(18.48)

(100) 46.16

1.97

8.14

23.32

2.26

8.07

(4.50)

(18.60)

(53.29)

(5.16)

(18.44)

(100) 43.76

0.15

0.86

5.98

1.04

3.12

(1.35)

(7.71)

(53.63)

(9.33)

(27.98)

(100) 11.15

2.12

9.00

29.30

3.30

11.19

(3.86)

(16.39)

(53.36)

(6.01)

(20.38)

(100) 54.91

1.96

8.11

23.25

2.26

8.05

(4.49)

(18.59)

(53.29)

(5.18)

(18.45)

(100) 43.63

0.16

0.72

4.96

0.86

1.48

(1.96)

(8.80)

(57.33)

(10.51)

(18.09)

(100) 8.18

2.12

8.83

28.21

3.12

9.53

(4.09)

(17.04)

(54.45)

(6.49)

(18.39)

(100) 51.81 (100)

Data in parentheses represent percentage

of the branch and the bark of the two tree species was different; the proportion of the branch was higher than that of the bark in Masson pine, but is lower than that in Chinese fir. The vegetation carbon storage in tree plot areas was 47.90 t － － － hm 2, 55.82 t hm 2, 52.73 t hm 2 and the proportion of trees was 96.37%, 98.44% and 98.26%, respectively. Trees are the major carbon pool in the vegetation layer. Standing litter contributes less, accounting for only 5.5%–7.2% of the total vegetation carbon storage. Carbon storage for the whole soil (at a depth of 60 cm) in － － the three plot areas was 104.47 t hm 2, 48.18% t hm 2 and － 134.13 t hm 2, respectively, contributing 47.86%, 48.18% and 50.83% in the upper layer (0–20 cm). Therefore, carbon storage in the surface layer contributes much more than that in the lower layer. Forest soil in subtropical areas is very fragile. The carbon storage of a mixed forest ecosystem can be divided into three parts (vegetation, standing litter and soil). The － total carbon storage in the three plot areas is 155.02 t hm 2, －2 －2 190.80 t hm and 190.66 t hm , respectively. Productivity plays an important role in maintaining the ecosystem. The range of carbon pool distribution was the greatest for soil, followed by the vegetation and litter. The mean value of the － vegetation carbon pool was 51.91 t hm 2, accounting for 29.03% of the total carbon storage. The average value of the － litter carbon pool was 3.25 t hm 2, accounting for 29.03% of the total carbon storage, and the litter carbon pool was 3.25 t － hm 2, accounting for only 1.82%. Although the litter contrib-

utes less, it is the coupling storeroom of the carbon cycle between the soil and the plant. The soil is an important carbon Table 8 Carbon storage in each component of the forest ecosystem of the mixed forest of Pinus massoniana and Cunninghamia lanceolata in Daqingshan Carbon storage （t hm 2） －

Component

Plot 1

Plot 2

Plot 3

Mean

Tree Leave

1.90

2.12

2.12

2.04

Branche

7.59

9.00

8.83

8.47

Wood

25.24

29.30

28.21

27.58

Bark

2.90

3.30

3.12

3.11

Root

8.53

11.19

9.53

9.75

46.16

54.91

51.81

50.72

Aboveground

1.33

0.60

0.63

0.85

Underground

0.41

0.31

0.29

0.34

Subtotal Understory plant

Subtotal

1.74

0.91

0.92

1.19

47.90

55.82

52.73

51.91

Fresh

1.64

1.96

2.12

1.91

Decomposed

1.01

1.33

1.68

1.34

Subtotal

2.65

3.29

3.80

3.25

0–20cm

50.63

63.45

68.18

60.75

20–40cm

31.59

47.88

44.78

41.42

40–60cm

22.25

20.36

21.17

21.26

Total for soil

104.47

131.69

134.13

123.43

Total

155.02

190.80

190.66

178.83

Total for vegetation Standing litter

Soil depth

KANG Bing et al. / Acta Ecologica Sinica, 2006, 26(5): 1320–1329

pool of the mixed forest ecosystem, accounting for 69.02% of the ecosystem’s total carbon storage. Belowground carbon storage (including soil and roots) accounts for 74.47% of the total carbon storage of the ecosystem. In other words, the proportion between the aboveground and belowground is 1:2.92, approximating to that of the evergreen broad-leaved forest dominated by Cyclobalanopsis glauca[19], but higher than that of forests in general. 3.3 Annual net productivity of mixed plantation The annual carbon storage in mixed forest plantations can be estimated by the annual net productivity of all components and the corresponding carbon content[12] (Table 9). The annual － － net productivity of mixed plantations is 11.46 t hm 2 a 1, and －2 －1 the annual net carbon storage is 5.96 t hm a , which was － － equivalent to fixing CO2 of 21.88 t hm 2 a 1. The annual net productivity of Masson pine (age in 14 a) in the central moun－ － tainous region of Guangxi, China is 17.63 t hm 2 a 1, and the －2 －1[16] annual carbon storage is 9.08 t hm a . The carbon pool － － of the tropical mountainous rainforest is 3.818 t hm 2 a 1[20]. The carbon pool of the Chinese fir (age in 27a) forest ecosystem in southern area of Jiangsu, China[21]. The ability of sinking carbon of the mixed pine forest shows an improvement when compared with that of the Chinese fir forest, while it is reduced slightly when compared with that of Masson pine. The forest is not only an important gene pool but also a sink of atmospheric CO2. The destruction of forests provides the atmosphere with a source of CO2. When trees are cut down, apart from some parts of the wood being used, other parts such as leaves, branches, barks, and roots can be decomposed or used as fuel for burning, and so CO2 is released into the atmosphere. Thus, it can be observed that forests have a close relationship with fluctuations in CO2 concentrations in the atmosphere. Ascertaining the ability of assimilating CO2 in the forest ecosystem is the main content of study on the ecosystem productivity. Table 9 The annual carbon storage in mixed forest plantations Component

Annual net productivity － － (t hm 2 a 1 )

Trunk

2.12

Conversion into CO2 － － (t hm 2 a 1 ) 7.78

Root

1.39

0.75

2.75

Leaf

0.33

0.16

0.59

Bark

0.47

0.24

0.88

Branch

1.28

0.65

2.39

Annual litter Total

4

3.74

Annual net carbon storage － － (t hm 2 a 1 )

4.25

2.04

7.49

11.46

5.96

21.88

Conclusion

The average tissue carbon contents of Pinus massoniana are: wood (58.6%) > root (56.3%) > branch (51.2%) > bark

(49.8%) > leaf (46.8%), while those of Cunninghamia lanceolata are: bark (52.2%) > leaf (51.8%) > wood (50.2%) > root (47.5%) > branch (46.7%). The content orders of various parts of Chinese fir and Masson pine in the 13-year-old mixed plantation are similar to those of an 11-year-old Chinese fir in Huitong, Hunan Province, China[22] and a 14-year-old Masson pine in Guangxi, China[16], respectively. Contributing to the averages of carbon content, Masson pine has a higher carbon content, a result relative to the coronal structure and photosynthetic characteristics of the two tree species. Masson pine has an upper position in the coronal and can utilize sufficient sunlight energy to synthesize organic substances. The carbon content of the understory vegetation is lower than that of trees, whereas the carbon content of shrubs is higher than that of herbs. The carbon content of vegetation is relative not only to the species but also to the understory environment. Trees can absorb more sunlight for photosynthesis and accumulate more organic substances, while understory vegetation is shaded by foliage, which is a disadvantage in terms of photosynthesis. In terms of the decomposed partial organic matter, the carbon content of the decomposed litter is lower than that of the fresh litter. The carbon content of standing litter in this mixed plantation is higher than that of camphor[23], meaning that the litter decomposes gradually in the broad-leaved forest. Soil carbon storage in the upper layer (0–20 cm) is different in the three plot areas: plot area 3＞plot area 2＞plot area 1, which does not accord with the biomass distribution of trees in the three plot areas: plot area 2＞plot area 3＞plot area 1 for carbon eluviating from the higher to the lower slopes. The carbon pool for the soil in the surface layer makes a far greater contribution than that in the lower layer. With an increasing soil depth, carbon content becomes lower gradually. With the growth of vegetation, dead roots and leaf litters will return to the surface, and the organic matter and carbon contents are higher than those in the lower layer. Carbon storage distribution accords with biomass distribution, while vegetation productivity is important to maintain the carbon pool of the ecosystem. The average carbon storage in － this forest is 178.83 t hm 2, which is higher than that of the － 11-year-old Chinese fir (144.22 t hm 2) [22], and lower than [16] that of the 14-year-old Masson pine . Tree carbon storage in this forest is greater than that of the 11-year-old Chinese fir[22], － but close to that of the 14-year-old Masson pine (53.78 t hm 2)[16]. The carbon storage of vegetation in the mixed plantation is － 51.91 t hm 2, occupying 29.03% of the forest ecosystem’s total carbon storage, and approximating to the mean carbon storage － (57.07 t hm 2) of the vegetation in China, but larger than － that of the warm temperate coniferous forests (43.26t hm 2) －2 [24] and the temperate coniferous forests (44.97t hm ) , and lower than the average level of the Masson pine in China － (62.44 t hm 2)[9] and the tropical or subtropical coniferous

KANG Bing et al. / Acta Ecologica Sinica, 2006, 26(5): 1320–1329

forests (63.7 t hm 2)[25]. The tree’s carbon storage accounts for 23.90% of the total carbon storage and 97.7% of the vegetation carbon storage. The Masson pine contributes largely to the carbon storage in the mixed forest, accounting for 65.39% of the tree carbon storage, while the Chinese fir accounts for 34.61%. The trunk contributes largely to the carbon storage of all parts, accounting for 65.39% of the trees’ carbon storage. After the trees being felled, the trunk carbon is used in wooden furniture and other products; therefore, it became the cushion carbon pool for the significance of the carbon velocity and turnover during environmental monitoring[26]. Furthermore, the root has quite a proportionate relationship to the total carbon storage with the Chinese fir accounting for 18.47% of the total carbon storage, and the Masson pine accounting for 18.94%. Root carbon storage accounts for 19.22% of the total, and the roots are kept in the soil to reduce carbon release after the felling of the trees. The carbon storage of the branches, barks, and leaves occupy 26.87% of the total. CO2 is released because of some human activities, such as collecting firewood and burning vegetation on mountains[23]. Still furthermore, forest management such as uprooting and picking up resin in forests also makes the vegetation lose carbon. Hence, conservation of understory vegetation is useful for the carbon pool in all forest ecosystems. － Litter carbon storage is very low, as much as 3.25 t hm 2. Heat and zonal features directly influence on the litter carbon storage. In general, with the latitude increasing, more carbon accumulates due to the inferior decomposing conditions. Meanwhile, carbon storage is lower where there is rapid decomposition[23]. － Total soil carbon storage (0–60 cm) is 123.43 t hm 2, occupying 69.02% of the total carbon storage in the forest ecosystem, which is 2.38 times that of the vegetation carbon storage. Soil is a very important carbon pool. The mean soil － carbon storage of Chinese forests is 193.55 t hm 2, which is about 3.4 times that of vegetation[24]. The proportion between the soil organic carbon content and the vegetation carbon storage is lower than the mean value in China. On the one hand, abundant water and heat resources are available for the biomass accumulation in the subtropical area. On the other hand, soil carbon storage is relatively lower because of the intensive soil respiration and the soil nutrient assimilated by vegetation[24]. Soil carbon storage on the surface (0–20 cm) is － 60.75 t hm 2, accounting for 49.22% of the total soil carbon storage. Soil and water conservation can effectively maintain the carbon sink in the soil. Understory vegetation plays an important role in maintaining the land power, water and soil[27,29] . The annual net productivity of the mixed plantation is pri－ － marily estimated as 11.46 t hm 2 a 1 C , and converted into －2 －1 5.96 t hm a , which is equivalent to fixing CO2 of 21.88 t － － hm 2 a 1. This indicates that plantation is an important sink of －

atmospheric CO2. The forest has a close relationship with carbon concentration fluctuations.

Acknowledgements This study was mainly supported by the National Key Project for the Tenth Five Year Plan, China (No.2001BA510B06), and SFA 948 program, China (No.2001-14, 2004- 4-66). We thank WEN Yuanguang, and CAI Daoxiong for their help in initiation of this study, and LIANG Hongwen, ZHU Hongguang, LU Lihua, and GUO Wenfu for their field investigations.

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