RELATIONSHIPS OF FIRE, BIOMASS AND NUTRIENT DYNAMICS ALONG A VEGETATION GRADIENT IN THE BRAZILIAN CERRADO

1 The variable structure and mass of vegetation, nutrient pools, and r esponses to fire were quantified along a common vegetation gradient fr om grassland (campo limpo) to open evergreen woodland (cerrado sensu s tricto) in the Brazilian Cerrado. 2 The biomass of the fuel loads (i.e . that proportion of the above-ground biomass susceptible to combustio n) ranged from 7128 kg ha(-1) in campo limpo to 10031 kg ha(-1) in cer rado sensu stricto. Grasses comprised 91-94% of the total above-ground biomass in the campo limpo and campo sujo grasslands. In the campo ce rrado and cerrado sensu stricto communities, graminoids comprised only 27% of the fuel load biomass; the remainder was composed of deadwood (18%), dicot leaf litter (36%), dicots and shrub leaves (18%). 3 Fires consumed greater than or equal to 97% of the above-ground biomass in the grasslands, but only 72 and 84% of the fuel load in campo cerrado and cerrado sensu stricto, respectively. The total residual mass (unco mbusted fuels and ash combined) was < 8% of the prefire mass in campo limpo and campo sujo but was greater than or equal to 36% in the campo cerrado and cerrado sensu stricto. A circular relationship existed be tween vegetation composition, fuel composition, and fire behaviour. Fo r example, fire-line intensity was significantly greater in grasslands , which would facilitate grass dominance by increasing above-ground ti ssue damage of trees and shrubs. (4) Within the fuel load, a consisten tly increasing pool size of N, P, C, and S was measured along the grad ient from campo limpo to cerrado sensu stricto. Total mass of N increa sed from 24 to 55 kg ha(-1), total mass of P increased from 1.7 to 3.5 kg ha(-1), and total mass of C increased from 3389 to 4657 kg ha(-1). This trend was reversed for K (i.e. 18.5 kg ha(-1) in Campo limpo to 13.8 kg ha(-1) in cerrado sensu stricto). 5 Carbon, N, and S were the nutrients that were consistently lost in the highest quantities during fire. Losses of P were intermediate, and K and Ca were negligible. 6 The percentage of the above-ground N, C, and S pools that were lost by fire decreased along the vegetation gradient from campo limpo to cerr ado sensu stricto. For example, greater than or equal to 90% of the N pool was lost by fire in the grasslands while < 56% of the N pool was lost in the tree-dominated communities. Total site loss (kg ha(-1)) of all nutrients was not significantly different among the vegetation ty pes. However, significantly greater quantities of nutrients were lost as particulates in Cerrado sensu stricto while significantly greater q uantities of nutrients were volatilized during fire in Campo limpo. 7 Nitrogen pools located in soils to a 10 cm depth ranged from 656 to 16 70 kg ha(-1). Of the total N pool consisting of fuel loads and soils, the proportion in fuels ranged from 1.4% in campo limpo to 4.0% in cer rado sensu stricto. Fires resulted in ecosystem losses of < 3.8% of th is pool. Ecosystem losses of this magnitude are likely to be compensat ed for by natural nutrient inputs in one to three years.