ECOSYSTEM-SCALE IMPACTS OF DEFORESTATION AND LAND USE IN A HUMID TROPICAL REGION OF MEXICO

Deforestation of tropical evergreen forests is a major contributor to increasing levels of greenhouse gases in the atmosphere. However, large uncertainties currently exist concerning the quantities of C and other elements lost to the atmosphere due to the conversion of primary forests to pastures and agricultural lands. Elemental losses associated with land conversion in the heavily deforested Los Tuxtlas Region of Mexico were quantified. Total aboveground biomass (TAGB) as well as carbon and nutrient pools in aboveground vegetation and soils were measured along a land-use gradient that included primary forests as well as pastures and cornfields, which represent the dominant land-use types in the region. TAGB of primary forests in the Los Tuxtlas Region averaged 403 Mg/ha; pasture and cornfield sites averaged 24 and 23 Mg/ha, respectively. Approximately 80% of TAGB of forests was composed of trees >30 cm in diameter at breast height (dbh), while trees >70 cm dbh accounted for 44% of TAGB. Conversion of forests to pastures or cornfields resulted in declines of 95% of aboveground C pools, 91% of aboveground N pools, 83% of aboveground P pools, and 89–95% of aboveground S pools in sites ranging in age from 3 to 45 years since deforestation. In contrast to aboveground pools, soil pools of C, N, and S to a 1 m depth were highly variable and did not show detectable declines in pasture and cornfield sites compared to forest sites, nor did they decline with increasing periods of land use. Average C mass in soils of forest, pasture, and cornfield sites ranged between 166 and 210 Mg/ha; mass of N and S in soils ranged from 16 to 20 and from ∼3 to 4 Mg/ha, respectively. Approximately 50% of the combined aboveground and soil pools of C were lost as a result of deforestation and land use. Because the vast majority (>90%) of N and S pools were present in the relatively stable pools of these young volcanic soils, less than 10% of combined aboveground and soil N and S pools were lost due to land-use change in the Los Tuxtlas Region.