Conservation and Sustainable Use of the Biosphere - WBGU

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Conservation and Sustainable Use of the Biosphere - WBGU

The terrestrial biosphere in global transition F 3.2

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est soils and their organic cover alone (WBGU,

1998b). Half of worldwide forest carbon is located in

the boreal forests of Russia, Canada and Alaska, and

84 per cent of that is in the soils. But grasslands and

wetlands are also important carbon stores. Wetlands

cover only around 3–6 per cent of the Earth’s surface,

but contain 10–30 per cent of global terrestrial carbon

(figures vary depending on the definition of wetlands

used; WBGU, 1998b).

Through photosynthesis the terrestrial biosphere

absorbs approx 120Gt of carbon from the atmosphere

every year (Gross Primary Production –

GPP). And yet only half of that amount is stored

briefly in the biomass through plant growth (Net Primary

Production – NPP), the other half is immediately

breathed out by the energy metabolism of the

plant. The majority of NPP is broken down by soil

organisms with the fixed carbon re-entering the

atmosphere. The remaining 5 per cent of GPP is

attributable to the net absorption of carbon by the

ecosystem (Net Ecosystem Productivity – NEP). An

assessment of global NEP assisted by a biome model

(Box F 2.1-1) resulted in a seasonal fluctuation

between – 0.6Gt C (October) and 1.5Gt C (July)

(Cao and Woodward, 1998). Natural disruptions such

as fire and also human intervention (eg harvesting

wood) take biomass from the ecosystems which

immediately or with some delay (for example, when

the wood products rot) leads again to the release of

CO 2

into the atmosphere. For time scales of decades

to centuries and at biome level, the net biome productivity

(NBP; Schulze and Heimann, 1998) is the

suitable measure for carbon fixing (WBGU, 1998b).

It accounts for just 0.5 per cent of GPP: This is the

carbon that over several centuries is stored as charcoal

and in stable humus. An ecosystem can have a

positive NEP over several years even though the

biome in which the ecosystem is embedded records a

negative NBP over several decades, for instance

when fires become more frequent as a result of the

warming (Schulze and Heimann, 1998). For example,

photosynthesis increased in the boreal forests of

Canada between 1981 and 1991 (because of longer

growth periods) and the NEP was positive in the

growth period.As a result of clear-cutting, deforested

areas convert from C sinks to C neutral systems –

NBP decreases. By contrast, swampy forest areas and

older forest stands have high C absorption rates and

so merit particular protection in global terms and for

the preservation of the regulatory functions in the

Earth System. The boreal primary forests in Canada

and Siberia with their moors and older coniferous

stocks are included in that category (Section F 5).

The biosphere responds to the human-induced

changes in the carbon cycle and the climate system in

positive and negative feedback effects, about which

too little is known as yet. Numerous studies and

research programmes (eg IGBP) are currently investigating

the interaction between the global carbon

cycle, climate change and the biosphere. The

increased concentration of carbon dioxide leads to

increased photosynthesis (‘CO 2

fertilization’) and

also to improved water availability. This represents a

negative feedback and an attenuation of the warming

of the Earth, though the sink effect of terrestrial biosphere

is increased. The extended vegetation period

triggered by climate change can also increase NPP.

This does not however necessarily lead to an

increased carbon uptake (Houghton et al, 1998). The

deposition of nitrogen in nitrogen-limited ecosystems

increases NPP further (‘N fertilization’), in particular

in forests of the mid- and higher latitudes of

the northern hemisphere. On the other hand as the

Earth warms respiration increases and so carbon

dioxide is again released into the atmosphere.

Positive and negative feedbacks and interactions

with other cycles impact on various time scales and

are not linear (Houghton et al, 1998; Woodwell et al,

1998). Whereas CO 2

fertilization takes immediate

effect, heterotrophic respiration (by soil organisms)

responds to higher temperature, humidity and litter

quality with some time lag. CO 2

and N fertilization

also demonstrate a clear saturation with increased

input of carbon dioxide and nitrogen, whereas the

increased respiration may begin with some delay in

comparison with photosynthesis, but then increases

exponentially with the rise in temperature (Scholes,

1999; Fig. F 3.2-1).

The latest IGBP findings show that CO 2

fertilization

has possibly been overestimated so far and that

therefore also in models calculating the sources and

CO 2 -concentration [ppmv]

600

500

400

1.5

0.5

0

300

-0.5

1800 2000 2200 2400 2600

Year

Figure F 3.2-1

Long-term development of atmospheric CO 2

concentration

(dashed line) and biospheric sink capacity (solid line). The

Scholes model indicates a stabilization of CO 2

concentration

only after several hundred years at a level of 600ppm. It

should be noted that the reduction of the carbon sink sets in

long before the atmospheric CO 2

equilibrium concentration

is reached.

Source: Scholes, 1999

2

1

C sink [Pg C year -1 ]

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