ECOSYSTEM STRUCTURE IN THE BRAZILIAN CERRADO - A VEGETATION GRADIENT OF ABOVEGROUND BIOMASS, ROOT MASS AND CONSUMPTION BY FIRE

Conversion to permanent agriculture is rapidly occurring over vast are as of the 1.8 million km(2) Brazilian Cerrado; a region that is natura lly a mosaic of grasslands, savannas and evergreen tropical woodlands. Yet, few studies have quantified total biomass of plant communities i n this ecosystem, particularly the belowground component; a C pool of potential global significance. Total biomass (aboveground and belowgro und), and the quantity of biomass consumed by fires were measured in f our plant communities comprising a vegetation gradient from pure grass land (campo limpo) to a woodland with a closed canopy of tall shrubs a nd scattered trees (cerrado dense) near Brasilia, DF, Brazil. Total ab oveground biomass (TAGB) increased along this gradient from 5.5 Mg ha( -1) in campo limpo to 29.4 Mg ha(-1) in cerrado dense. Vegetation stru cture varied among communities; trees were nonexistent in campo limpo, but were at a density of 1000 ha(-1) and a biomass of 12.9 Mg ha(-1) in cerrado dense. Fires consumed 92 and 84% of the TAGB in campo limpo (pure grassland) and campo sujo (savanna), respectively. In cerrado a berto and cerrado dense, trees and tall shrubs were little affected by fire. Combustion factors of the TAGB in these communities was 54 and 33%, respectively. The total biomass consumed by fire ranged from 5.0 Mg ha(-1) in campo limpo to 13.5 Mg ha(-1) in cerrado aberto. Compared to other widespread Brazilian ecosystems (tropical dry forest and eve rgreen forest), the Cerrado has a lower aboveground biomass. The TAGB of cerrado dense is < 9% of that of Amazonian tropical evergreen fores t. The total quantity of biomass consumed by fire, and hence emissions td the atmosphere is lower in intact Cerrado communities compared to fires in slashed tropical forest. Total belowground biomass (TBGB) inc reased from 16.3 Mg ha(-1) in campo limpo, to 30.1 Mg ha(-1) in campo sujo, to 46.5 Mg ha(-1) in cerrado aberto, and to 52.9 Mg ha(-1) in ce rrado dense. This quantity of belowground biomass is similar to, or ex ceeds that reported for many tropical dry and moist forests. More than 80% of the TBGB occurred in the upper 30 cm of the soil, except for c errado dense (71%) where a greater proportion of tree roots were prese nt at deeper levels. Root:shoot ratios were very high in all sites ran ging from 2.9 in cerrado dense to 7.7 in campo sujo. Total ecosystem p lant biomass (the total aboveground biomass and TBGB combined) ranged from 21.9 Mg ha(-1) in campo limpo to 77.9 Mg ha(-1) in cerrado dense. More than 71% of the live phytomass (aboveground biomass + root bioma ss) is belowground in the Brazilian Cerrado. At current rates of land cover change in the Brazilian Cerrado, these ecosystem pools are likel y significant sources of increasing atmospheric C and other greenhouse gasses.