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24

Tundra

Boreal forest

Temperate forest

Temperate grasslands, savannas and shrublands

Desert and dry shrublands

Tropical and subtropical forests

Tropical and subtropical grasslands, savannas and shrublands

Source: adapted from Olson

et al

., 2001.

DESERT AND DRY SHRUBLANDS

The large surface area of drylands gives dryland carbon sequestration a global signifi-

cance, despite their relatively low carbon density. The fact that many dryland soils have

been degraded means that they are currently far from saturated with carbon and their

potential to sequester carbon can be high.

Deserts and dry shrublands occupy regions of very low or

highly seasonal precipitation and can be found in numerous

regions including many parts of Africa, southern USA and

Mexico, parts of Asia and over large areas of Australia. The slow

growing vegetation consists mainly of woody shrubs and short

plants and is highly adapted to minimise water loss. Like plant

diversity, animal diversity is generally low.

The lack of moisture determines the way in which these ecosys-

tems process carbon. Plant growth tends to be highly sporadic

and plants invest heavily in protecting themselves against water

loss and herbivores by making their tissues tough and resistant

to decomposition. Lack of water also slows decomposition rates,

leading to the accumulation of carbon-rich dead plant material

in the soil. Amundson (2001) estimates carbon content of desert

soils as between 14 and 100 tonnes per ha, while estimates for dry

shrublands are as much as 270 tonnes per ha (Grace 2004). The

carbon stored in the vegetation is considerably lower, with typical

quantities being around 2–30 tonnes of carbon per ha in total.

Some recent studies have suggested that carbon uptake by des-

erts is much higher than previously thought and that it con-

tributes significantly to the terrestrial carbon sink (Wohlfahrt

et al.

2008). However, considerable uncertainties remain and

there is need for further research to verify these results, for ex-

ample by quantifying above- and below-ground carbon pools

over time (Schlesinger

et al.

2009).

HUMAN IMPACTS AND IMPLICATIONS FOR

CARBON MANAGEMENT

As these ecosystems are generally nutrient poor, they tend to

make poor farmland and food production on these lands is

often at a subsistence level. Land degradation, resulting from

inappropriate land uses, leads to carbon loss from the soil.