Biomass dynamics associated with deforestation, fire, and, conversion to cattle pasture in a Mexican tropical dry forest

Abstract

Few studies have described losses of biomass and structure arising from the conversion of tropical dry forest (TDF) to pasture, a common land-use practice in the most widespread forest type in the Neotropics. We sampled total aboveground biomass (TAGB) at five distinct periods during the process of forest conversion to pasture. TAGB was measured after the primary forests were cut, after the initial forest slash fires, and 1 year after pasture establishment. After 2 years, TAGB was quantified before and after the first pasture fire conducted on site. To capture the variability in biomass burning, two treatments (Baja and Alta) based upon degree of biomass consumed were established in a randomized block design with three treatment blocks. Mean TAGB of the slashed primary forest was 118 Mg ha −1 in the Baja plots and 135 Mg ha −1 in the Alta plots. The slash fires resulted in biomass loss of 62% of the TAGB (75 Mg ha −1) in the Baja treatment and 80% (108 Mg ha −1) in the Alta treatment. The greatest treatment differences in consumption occurred in the large wood (>7.6 cm diameter). Fires in the Baja treatment consumed ∼31% of the large wood, but only 7% in the Alta treatment. Prior to the first pasture fire, TAGB was 40.3 Mg ha −1 in the Baja treatment, and 20.9 Mg ha −1 in the Alta treatment. The pasture fires consumed 63% of the TAGB in the Baja sites and 75% of the TAGB in the Alta treatment. Following the pasture fires, the TAGB was 14.8 Mg ha −1 in the Baja treatment and 7.6 Mg ha −1 in the Alta treatment. In the first 2 years of land cover change from dry forest to pasture there was a dramatic decline in TAGB (and hence aboveground C pools) totaling 87 and 94% of that of pre-disturbance forests. The total biomass lost via fire and decomposition was 113 and 132 Mg ha −1; fire accounted for 89–92% of the loss while decomposition/disappearance comprised 2–11% of the total biomass loss. The widespread distribution of tropical dry forests, their high rates of deforestation, and their high rates of biomass consumption during fires suggest that they are significant anthropogenic sources of atmospheric C. The dramatic loss of biomass and associated high degree of fire severity may also affect future site productivity and the capacity for these sites to function as C pools in the future.