Fire in the Brazilian Amazon : 3. Dynamics of biomass, C, and nutrient pools in regenerating forests

Regenerating forests have become a common land-cover type throughout the Brazilian Amazon. However, the potential for these systems to accumulate and store C and nutrients, and the fluxes resulting from them when they are cut, burned, and converted back to croplands and pastures have not been well quantified. In this study, we quantified pre- and post-fire pools of biomass, C, and nutrients, as well as the emissions of those elements, at a series of second- and third-growth forests located in the states of Pará and Rondônia, Brazil. Total aboveground biomass (TAGB) of second- and third-growth forests averaged 134 and 91 Mg ha^-1, respectively. Rates of aboveground biomass accumulation were rapid in these systems, but were not significantly different between second- and third-growth forests, ranging from 9 to 16 Mg ha^-1 year^-1. Residual pools of biomass originating from primary forest vegetation accounted for large portions of TAGB in both forest types and were primarily responsible for TAGB differences between the two forest types. In second-growth forests this pool (82 Mg ha^-1) represented 58% of TAGB, and in third-growth forests (40 Mg ha^-1) it represented 40% of TAGB. Amounts of TAGB consumed by burning of second- and third-growth forests averaged 70 and 53 Mg ha^-1, respectively. Aboveground pre-fire pools in second- and third-growth forests averaged 67 and 45 Mg C ha^-1, 821 and 707 kg N ha^-1, 441 and 341 kg P ha^-1, and 46 and 27 kg Ca ha^-1, respectively. While pre-fire pools of C, N, S and K were not significantly different between second- and third-growth forests, pools of both P and Ca were significantly higher in second-growth forests. This suggests that increasing land use has a negative impact on these elemental pools. Site losses of elements resulting from slashing and burning these sites were highly variable: losses of C ranged from 20 to 47 Mg ha^-1; N losses ranged from 306 to 709 kg ha^-1; Ca losses ranged from 10 to 145 kg ha^-1; and P losses ranged from 2 to 20 kg ha^-1. Elemental losses were controlled to a large extent by the relative distribution of elemental mass within biomass components of varying susceptibilities to combustion and the temperatures of volatilization of each element. Due to a relatively low temperature of volatilization and its concentration in highly combustible biomass pools, site losses of N averaged 70% of total pre-fire pools. In contrast, site losses of P and Ca resulting from burning were 33 and 20% of total pre-fire pools, respectively, as much of the mass of those elements was deposited on site as ash. Pre- and post-fire biomass and elemental pools of second- and third-growth forests, as well as the emissions from those systems, were intermediate between those of primary forests and pastures in the Brazilian Amazon. Overall, regenerating forests have the capacity to act as either large terrestrial sinks or sources of C and nutrients, depending on the course of land-use patterns within the Brazilian Amazon. Combining remote sensing techniques with field measures of aboveground C accumulation in regenerating forests and C fluxes from those forests when they are cut and burned, we estimate that during 1990-1991 roughly 104 Tg of C was accumulated by regenerating forests across the Brazilian Amazon. Further, we estimate that approximately 103 Tg of C was lost via the cutting and burning of regenerating forests across the Brazilian Amazon during this same period. Since average C accumulations (5.5 Mg ha^-1 year^-1) in regenerating forests were 19% of the C lost when such forests are cut and burned (29.3 Mg ha^-1), our results suggest that when less than 19% of the total area accounted for by secondary forests is cut and burned in a given year, those forests will be net accumulators of C during that year. Conversely, when more than 19% of regenerating forests are burned, those forests will be a net source of C to the atmosphere.