Global Features

The carbon cycle of terrestrial ecosystems from 1901 to 2000 was simulated with Sim-CYCLE (Fig. 2). In the 1980s and 1990s, the annual average GPP and NPP were estimated to be 130.7 and 62.5 Pg C y-1, respectively. Plant biomass and soil organic carbon were estimated as 481 and 1429 Pg C, respectively. However, during the 20th century, GPP, NPP, and plant biomass were estimated to have increased gradually, as atmospheric CO2 concentration increased (Fig. 2). The temporal change in soil-carbon storage was slightly complicated, in that it decreased from the 1930s to the 1950s and then increased again from the 1960s to the 1970s. The magnitude of the soil-carbon change (about 10 Pg C) was one-third of the variation of plant biomass, and was mostly determined by the climatic variation, especially in temperature (e.g., the global mean temperature was higher in the 1930s and 1940s than in the 1920s and 1960s). On average, terrestrial vegetation received 330.4 x 1021 J of PAR, of which 56.2% (185.6 x 1021 J) was absorbed by canopy. Accordingly, the average LUEs for GPP and NPP were readily calculated as 0.70 and 0.34 gCMJ-1, respectively. Fig. 2 shows that the estimated mean LUEs have gradually increased, with a small magnitude of interannual variability, although the total APAR has also increased. This implies that the elevated atmospheric CO2 concentration (and to some extent climatic change) resulted in higher photosynthetic light-use efficiencies, through the CO2 fertilization effect.

Fig. 3 shows the estimated distributions of annual APAR, GPP, and LUE(GPP), averaged over the 1980s and 1990s. Apparently, more PAR was absorbed by tropical and subtropical forests, except for some cloudy ones, leading to higher photosynthetic productivity. In these forests, the humid climate allows them to sustain a higher leaf area index, which absorbs a large fraction of incident solar radiation. Table 2 summarizes biome-specific aspects with respect to light-use efficiency. For example, on average, tropical rain forest absorbed 2600 MJ m-2 of PAR and assimilated 2107 g C m-2 of

90 88 86 84 82 80

0.36

0.34

0.30

LUE

Soil C

130 1

125 u

120 g

115 W

1435

eg 1425 Oh,

W 1420 1415

1900 1920 1940 1960 1980 2000

Year

Figure 2: Interannual changes in the estimated global terrestrial processes, from 1901 to 2000: absorbed photosynthetically active radiation (APAR), gross primary production (GPP), net primary production (NPP), light-use efficiency (LUE) of GPP and NPP, and plant and soil-carbon storage.

atmospheric CO2, annually. Because of optimal temperature and moisture conditions, the tropical rain forest showed the highest LUE (1.02 g C MJ 1 for GPP) among the biomes. On the other hand, cooler biomes (e.g., tundra and cool semi-desert scrub) showed lower LUEs (0.35 gCMJ-1) because of insufficient warmth. In spite of dryness, tropical savanna showed higher LUE

Figure 3: Estimated global distributions of (a) APAR, (b) GPP, and (c) LUE(GPP), averaged from 1981 to 2000 (For colour version, see Colour Plate Section).

1200

Figure 4: Latitudinal distributions of absorbed PAR (APAR), productivities (GPP and NPP), and light-use efficiencies (LUE(GPP) and LUE(NPP)), averaged from 1981 to 2000 (For colour version, see Colour Plate Section).

1200

Figure 4: Latitudinal distributions of absorbed PAR (APAR), productivities (GPP and NPP), and light-use efficiencies (LUE(GPP) and LUE(NPP)), averaged from 1981 to 2000 (For colour version, see Colour Plate Section).

(0.96 gCMJ 1 for GPP) because of the dominance of C4 grass species. Fig. 4 shows the estimated latitudinal distributions of APAR, GPP/NPP, and LUE. The land area between 60°N and 20°S absorbed a comparable amount of PAR, although there are differences in land area and incident irradiance. The latitudinal distribution of GPP and NPP shows two peaks: one around 50°N and the other around the equator. Lower productivity in northern middle latitudes (around 30°N) was attributable to a broad area of arid and infertile ecosystems. The latitudinal distribution of LUE was mostly similar to that of productivity, except in the northernmost and southernmost areas. Specifically, warm and humid ecosystems around the equator show higher LUE, especially for GPP. However, smaller differences among latitudes were found for the LUE of NPP, because larger respiratory consumption of tropical forests offsets a part of the higher photosynthetic efficiency. Table 3 summarizes the latitudinal features for every 30° latitudinal zone. Apparently, the southern low (0-30°S), northern low (0-30°N), and northern-middle (30-60°N) zones absorbed comparable amounts of PAR, about 53 x 1021 J y_1, although the incident PAR was different among the three zones. The southern and northern low zones accounted for nearly 70% of the photosynthetic CO2 assimilation by the terrestrial biosphere, with a higher LUE than other zones. In contrast, the northern-middle and high zones accounted for nearly 60% of the total carbon storage (1080 Pg C out of 1910 Pg C), and there was a clear difference in the carbon cycle regime between the low- and high-latitudinal zones.

It is seen from Fig. 5 that APAR, GPP, and LUE changed roughly in parallel seasonally because, in general, higher incident radiation leads to

CO 00

Table 3:

Summary of the simulation result for each of the 30°-zones

averaged over 1981-2000

Land area

PAR (1021

APAR (1021

GPP (Pg C)

LUE (GPP)

NPP (Pg C)

LUE(NPP)

Plant C

Soil C

(106 km2)

J yr"1)

J yr"1)

(g C MJ"1)

(g C MJ"1)

(Pg C)

(Pg C)

90-60°N

15.2

22.2

15.9

7.9

0.50

4.8

0.30

35

382

60-30°N

46.0

103.7

54.7

28.5

0.52

15.4

0.28

104

559

30-0°N

36.6

109.3

54.4

44.0

0.81

19.4

0.36

156

213

0-30°S

29.0

80.7

52.3

45.4

0.87

20.5

0.39

174

227

30-60°S

5.5

14.4

8.3

4.9

0.59

2.4

0.29

12

48

Total

132.3

330.4

185.6

130.7

0.70

62.5

0.34

481

APAR GPP LUE (GPP)

APAR GPP LUE (GPP)

Figure 5: Seasonal change in the estimated global distributions of APAR, GPP, and LUE(GPP), averaged from 1981 to 2000: DJF (December, January, and February), MAM (March, April, and May), JJA (June, July, and August), and SON (September, October, and November), averaged from 1981 to 2000 (For colour version, see Colour Plate Section).

Figure 5: Seasonal change in the estimated global distributions of APAR, GPP, and LUE(GPP), averaged from 1981 to 2000: DJF (December, January, and February), MAM (March, April, and May), JJA (June, July, and August), and SON (September, October, and November), averaged from 1981 to 2000 (For colour version, see Colour Plate Section).

Month

Figure 6: Seasonal change in the global total APAR, GPP, and LUE(GPP), averaged from 1981 to 2000.

Month

Figure 6: Seasonal change in the global total APAR, GPP, and LUE(GPP), averaged from 1981 to 2000.

warmer conditions that enable plants to photosynthesize with a higher light-use efficiency. Higher LUEs occurred in tropical rain forests all year round and in temperate and boreal forests in their growing season (e.g., June, July, and August [JJA in Fig. 5] in northern taiga). In tropical, temperate, and boreal deciduous forests, APAR and LUE differed drastically between the dormancy and growing period, reflecting their clear leaf phenology. In arid regions, however, water limitation inhibits plants from expanding leaf area and utilizing solar energy for photosynthesis all year. Fig. 6 indicates that globally, APAR and GPP (and also NPP, not shown) changed almost in parallel seasonally, but the global average LUE showed a considerable degree of seasonal change (0.575-0.807 gCMJ-1 for GPP and 0.260-0.420 gCMJ-1 for NPP).

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