The Natural Fix?

TEMPERATE GRASSLANDS

Much of the original area of temperate grassland has been cleared for agriculture. Where natural vegetation remains, minimising human disturbance can prevent fur- ther carbon loss.

Grasslands are found across much of the world as an early succes- sional ecosystem in forested regions. They also form the natural vegetation in climates where precipitation levels are inadequate to support trees but higher than those of deserts (Woodward et al. 2004). Extensive areas of natural temperate grassland occur in South America, the USA and Central Asia. Plant growth in these grasslands is water and nutrient limited and plants allocate much of their biomass below ground, where they produce slowly decom- posing roots. Grazing animals typically play an important role in maintaining grasslands in that they accelerate carbon cycling by consuming and respiring large quantities of leaf biomass and re- turning some of this to the soil as dung. This is a form of organic carbon that is more decomposable than the leaf and root litter of grasses. In many areas this role is now performed by livestock.

Overall, temperate grasslands have low levels of plant biomass compared with forest or shrubland ecosystems (e.g. 0.68 and 7.3 t C per ha respectively in the temperate steppe of China, Fan et al. 2008). However, their soil organic carbon stocks tend to be higher than those of temperate forests (133 t C per ha, Amundson 2001). HUMAN IMPACTS AND IMPLICATIONS FOR CARBON MANAGEMENT Despite only having intermediate productivity some temperate grasslands are well suited to crop production and can produce excellent agricultural soils. In much of their natural range, e.g. the prairies of America, these have been cleared to make way for intensive agriculture.

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.

23