Scalingup calculations and global CH4 budgets

Scaling-up calculations by Martius et al (1996), Sanderson (1996), Bignell et al (1997) and Sugimoto et al (2000) used differing assumptions and principles, but all four agree that CO2 fluxes by termites are in the range 2-5 per cent of the total from all terrestrial sources. 2 per cent is a large figure for a single insect order with about 0.01 per cent of the global terrestrial species richness, but is nevertheless only a minor source in the context of the whole C budget. With CH4 emission, it now appears that the net contribution by termites is very small, despite the fact that some trophic groups produce this potent greenhouse gas in large amounts at the point scale. Independent studies using, respectively, static chambers and natural stable isotope ratios showed that mound walls and undisturbed soil between mounds have a sink capacity exceeding production at the landscape level, presumably due to the presence of methylothrophic bacteria (Sugimoto et al, 1998b; MacDonald et al, 1999). Sanderson (1996) estimates the global emission of CH4 by termites as 19.7 ± 1.5Tg yr-1 (approximately 4 per cent of global totals), while Bignell et al (1997)

Figure 5.1 Locality CH4 budgets for near primary and primary forest in Cameroon and Borneo (kg ha-1 yr1)

Note: For each forest system the left-hand block illustrates the most commonly encountered epigeal termite mounds and arboreal nests. The right-hand block illustrates termite galleries in the soil (in some cases associated with mound-building species). Upward arrows indicate measured CH4 effluxes (from mounds) and estimated gross CH4 effluxes (from soil termite populations). Downward arrows indicate the net CH4 fluxes measured in soils (both sites are net sinks). The illustrated mounds/nests are a) Astalotermes quietus, b) Procubitermes arboricola, c) Cephalotermes rectangularis, d) Cubitermes spp, e) Cubitermes fungifaber, f) Thoracotermes macrothorax, g) Nasutitermes spp, h) Bulbitermes spp, j) Dicuspiditermes spp, k) Prohamitermes mirabilis, l) Procapritermes spp, m) Macrotermes gilvus (strictly not a forest species, but associated with tracks and paths), n) Hospitalitermes hospitalis, and o) Lacessititermes sp. Effluxes from mounds g, m and n have not been determined and are not in the budgets. Also note, some mounds may be partly or wholly occupied by other species as secondary colonizers. *The gross CH4 production by soil termites in Cameroon is the lower estimate of Bignell et al (1997). Higher fluxes are possible, reflecting large inter-annual variations in population size. Not to scale.

Source: Data from Bignell et al (1997), Macdonald et al (1999) and Jeeva (1998). Figure from Sugimoto et al (2000), by kind permission of Springer Sciences and Business Media give estimates in the range 17-96Tg yr-1. Neither Sanderson (1996) nor Bignell et al (1997) take account of local CH4 oxidation in their calculations. The arguments given above concerning the sink capacity of tropical forest soils clearly suggest that actual termite-mediated fluxes to the atmosphere in these ecosystems may be small or non-existent. Sugimoto et al (1998b) have made the only calculation of global production of CH4 that uses the field observations of actual (rather than potential) oxidation rates, which suggested that only 17-47 per cent (30 per cent on average) of the CH4 produced in epigeal termite mounds is released to the atmosphere. Although mounds can be large point sources, they may nevertheless be small contributors to the gross efflux from all termite sources at landscape level (Sugimoto et al, 1998b).

Comparing the four most comprehensive scaling-up exercises (Martius et al, 1996; Sanderson, 1996; Bignell et al, 1997; Sugimoto et al, 1998b), it is apparent that the main differences in protocols are the factoring-in of CH4 oxidation and the areas assigned to regional vegetation/biomes types (which then feed into estimations of termite biomass). This is important in the case African rainforests, which have the highest recorded abundance and biomass of termites on earth and are in general (perhaps one should say in principle) dominated by soil-feeding forms with higher rates of CH4 production. However, estimates of net global production that do not consider oxidation are unrealistic, whatever the accuracy of other assumptions they contain, so the range of 1.47-7.41Tg yr-1 given by Sugimoto et al (1998b; see Table 5.2) should be considered the best available. This is much less than the approximately 20Tg yr-1 still quoted in some chapters of the most recent report of the IPCC (IPCC, 2007).

Table 5.2 Global estimation of CH4 emissions by termites

Region and

Area

Biomass (g m-2)

Emissions (Tg yr-1)

land use

(106 km2)

Min

Max

Min

Max

Africa

rainforest

2.219

24

130

0.38

2.20

savanna

6.836

5

20

0.23

0.93

woodland

7.401

5

20

0.08

0.30

cultivated land

3.247

0

10

0

0.07

Indian subcontinent + SE Asia

rainforest

1.773

5

20

0.06

0.22

savanna

0.476

5

20

0.01

0.03

Woodland

1.277

5

20

0.02

0.09

cultivated land

4.330

0

10

0

0.16

South America

rainforest

5.318

10

60

0.27

1.59

savanna

2.259

5

20

0.05

0.22

woodland

6.271

5

20

0.15

0.60

cultivated land

3.471

0

10

0

0.17

Australasia + Oceania

rainforest

0.051

5

20

<0.01

0.01

savanna

0.881

5

20

0.03

0.11

woodland

3.884

5

20

0.12

0.48

cultivated land

1.398

0

10

0

0.09

Middle East

savanna

0.125

5

20

<0.01

0.01

woodland

2.879

5

20

0.06

0.22

cultivated land

0.707

0

10

0

0.03

Temperate forest

3.863

1

3

0.04

0.07

Total

58.7

1.47

7.41

Source: Table from Sugimoto et al (2000) by kind permission of Springer Sciences and Business Media

Source: Table from Sugimoto et al (2000) by kind permission of Springer Sciences and Business Media

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