SOIL AND BIOMASS CARBON IN A COSTA RICAN CLOUD FOREST: CARBON SEQUESTRATION DYNAMICS

Deforestation in the tropics, through loss of carbon stocks in both biomass and soil carbon, is generally considered a significant factor in the anthropogenic increase in atmosphere CO₂ and consequent radiative forcing of climate. Therefore, understanding the rates and effectiveness of carbon sequestration through reforestation is essential for developing strategies of remediation. An estimated 80% of the tropical forest area in Costa Rica has been lost since WWII, but recent decades have witnessed government policies to encourage reforestation of ranch and farm lands, as well as protect remaining forests. To assess the effectiveness of reforestation in sequestering biomass and soil carbon we measured carbon stocks at two forest reserves in the cloud forest region of Monteverde, comparing cleared land, experimental secondary forest plots and mature disturbed forest at each location. The forest reserve site at the Monteverde Institute (MVI) is at approximately 300 m higher elevation than the reserve at La Calandria (LC). Consequently, MVI experiences a slightly cooler and moister climate than LC. Measurements were made of height and diameter of all vegetation with stem diameter > 1 cm in 100 m² quadrats. Five 30 cm soil cores were extracted from each quadrat and the C content of each 10 cm interval determined by combustion analysis. At the MVI site, mature forest holds an average 447 Mg ha^-1 C in biomass and 210 Mg ha^-1in soil, most of which is contained in the upper 10 cm. At the LC site, mature forest stores an average 179 Mg ha^-1 C in mature forest and 165 Mg ha^-1 C in the soil. The secondary forest at MVI was six years old when measured and held 2.6 Mg ha^-1 C in biomass and 144 Mg ha^-1 C in the soil. At LC, the secondary forest was 13 years old when sampled and held 21.4 Mg ha^-1 C in biomass and 126 Mg ha^-1 C in soil. At both sites, the biomass on cleared land is negligible and the soil stores 126 Mg ha^-1 C. These results demonstrate that the clearing of mature forest contributes significant amounts of carbon to the atmosphere, mostly from biomass loss. Restoration of the lost biomass carbon requires many decades of undisturbed growth. The differences in carbon stocks between the two sites reflect increased productivity with greater moisture availability at higher elevation.