Relationships of Fire, Biomass and Nutrient Dynamics along a Vegetation Gradient in the Brazilian Cerrado

Abstract

1 The variable structure and mass of vegetation, nutrient pools, and responses to fire were quantified along a common vegetation gradient from grassland (campo limpo) to open evergreen woodland (cerrado sensu stricto) in the Brazilian Cerrado. 2 The biomass of the fuel loads (i.e. that proportion of the above-ground biomass susceptible to combustion) ranged from 7128 kg ha-' in campo limpo to 10031 kg ha-' in cerrado sensu stricto. Grasses comprised 91-94% of the total above-ground biomass in the campo limpo and campo sujo grasslands. In the campo cerrado 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 297% 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 (uncombusted fuels and ash combined) was < 8% of the prefire mass in campo limpo and campo sujo but was 2 36% in the campo cerrado and cerrado sensu stricto. A circular relationship existed between vegetation composition, fuel composition, and fire behaviour. For example, fire-line intensity was significantly greater in grasslands, which would facilitate grass dominance by increasing above-ground tissue damage of trees and shrubs. (4) Within the fuel load, a consistently increasing pool size of N, P, C, and S was measured along the gradient from campo limpo to cerrado sensu stricto. Total mass of N increased from 24 to 55 kg ha-', total mass of P increased from 1.7 to 3.5 kg ha-' , and total mass of C increased from 3389 to 4657 kg ha-'. This trend was reversed for K (i.e. 18.5 kg ha-' in Campo limpo to 13.8 kg ha-' 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 cerrado sensu stricto. For example, 2 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-') of all nutrients was not significantly different among the vegetation types. However, significantly greater quarltities of nutrients were lost as particulates in Cerrado sensu stricto while significantly greater quantities of nutrients were volatilized during fire in Campo limpo. 7 Nitrogen pools located in soils to a 10 cm depth ranged from 656 to 1670 kg ha-'. 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 cerrado sensu stricto. Fires resulted in ecosystem losses of < 3.8% of this pool. Ecosystem losses of this magnitude are likely to be compensated for by natural nutrient inputs in one to three years.