Savanna Biomass and Productivity

Global NPP, the net production of plant biomass, is approximately 67.6GtCyr~ of which almost 30% occurs in savanna ecosystems (19.9 Gt yr~ ). This production occurs on 18% of the global land surface, demonstrating that savannas are relatively productive ecosystems. Mean savanna NPP has been estimated at 7.2 tCha~ yr~ (Table 2), lower than typical values for

Table 2 Savanna biomass, soil carbon stocks and productivity

Parameter

Mean (sd)

Range

Biomass and soil stocks (tCha-1)

Aboveground biomass

10.6 (9.0)

1.8-34

Belowground biomass

19.5 (14.9)

4.9-52

Total biomass

33.0 (22.9)

9.4-84

Root : shoot ratio

2.1 (2.0)

0.6-7.6

Soil organic carbon

174.2 (126.0)

18-373

Savanna area (M km-2)

27.6

Total carbon pool (Gt C)

326

Productivity (tCha-1 y-1)

NPP

7.2 (5.1)

1.4-22.8

NEP

0.14

Data from Grace J, San JJ, Meir P, Miranda HS, and Montes RA (2006) Productivity and carbon fluxes of tropical savannas. Journal of Biogeography. 33: 387-400.

Data from Grace J, San JJ, Meir P, Miranda HS, and Montes RA (2006) Productivity and carbon fluxes of tropical savannas. Journal of Biogeography. 33: 387-400.

the other major tropical ecosystem, rainforest, which ranges from 10 to 15 tC ha-1. Savanna NPP and biomass varies by an order of magnitude (Table 2), as would be expected given their geographic range and structural variation. The relative production of trees versus grasses is also highly variable, but in general, NPP of the C4 grass layer is 2-3 times that of tree NPP. Biomass stored in above- and belowground pools determines the root:shoot ratio and these data from a range of savanna sites around the world give a global mean of approximately 2 (Table 2). This reflects the investment in root systems and belowground storage organs, such as lignotubers, to maintain uptake of moisture and nutrient from sandy, nutrient poor savanna soils and to survive disturbance.

Savanna photosynthesis and growth is highly seasonal and interannual variability high. Mesic savanna may receive annual rainfall associated with rainforest ecosystems, yet productivity is significantly lower, due largely to annual drought, poor soils, and impacts of disturbance. Long-term (as opposed to annual) estimates of savanna productivity need to include loss of biomass due to fire and herbivory. Including fire and herbivory impacts on productivity estimates gives the carbon sequestration rate, which represents the net gain (sink) or loss of carbon from the ecosystem to the atmosphere. While wet season productivity can be very high in savannas, much of a wet-season's herbaceous productivity can be lost via fire or grazing. Woody biomass tends to be a less dynamic, longer-term carbon-storage pool than the herbaceous components of savanna. Savanna fire results in a significant release of greenhouse gases, including CO2, CO, methane, nonmethane hydrocarbons, nitrous oxide, particulate matter and aerosols, equivalent to 0.5— 4.2 Gt C yr~ . Fire reduces net savanna sequestration rate by about 50% and protection of savannas from fire and grazing results in an increase in woody biomass which can result in a long-term increase in stored soil carbon. Savanna sink strength in mesic Orinoco savannas in South America (~1500 mm annual rainfall) has been measured at 1tCha_1yr~\ with this sink maintained over a 25-year period in plots with fire and grazing excluded. Similarly, the carbon sink strength of north Australian, Eucalyptus-dominated savannas receiving approximately the same rainfall has also been estimated at approximately 1tC ha~ yr~ , with this sink measured at sites burnt but not grazed. This carbon is likely being stored in woody biomass and soil organic carbon pools, with a small fraction being stored as black carbon (charcoal), a resilient carbon pool. Savanna soil carbon storage is by far the largest pool of carbon (Table 2) and soil carbon represents a longer-term storage of carbon when compared to the more dynamic vegetation components. Burning also influences nutrient dynamics via losses due to volatilization (vaporization) of lighter elements such as nitrogen and sulfur. At a global scale, savannas and tropical seasonally dry forests represent a significant source of N2O to the atmosphere (4.4TgN2Oyr_1). Shifts to a more frequent fire regime may result in a significant net loss of nitrogen, as savannas are in general nitrogen-poor. Many grass species are able to recover quickly after fire, with re-growth attractive to grazing animals, due to the relatively high nutrient content of the foliage.

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