BIOMASS, CARBON, AND NUTRIENT DYNAMICS OF SECONDARY FORESTS IN A HUMID TROPICAL REGION OF MÉXICO

Tropical secondary forests have the capacity to function as large carbon and nutrient sinks and may offset losses resulting from deforestation and land use. In the heavily deforested Los Tuxtlas Region of México, aboveground biomass as well as aboveground and mineral soil C, N, S, and P pools were quantified in 11 secondary forest sites. These sites ranged in age from 6 mo to 50 yr following abandonment and had experienced between 1 and 30 yr of land use prior to abandonment. Total aboveground biomass (TAGB) increased with increasing site age and ranged from 4.8 Mg/ha in a recently abandoned site to 287 Mg/ha in the 50-yr-old secondary forest site. Results indicate that secondary forests would reach TAGB levels equivalent to those of primary forests in the Los Tuxtlas Region after 73 yr. Furthermore, mean annual aboveground biomass accumulation (ABA) of secondary forests was strongly and inversely related to the duration of prior land use. Aboveground pools of C, N, S, and P were also positively correlated with secondary forest age. For forests between 6 mo and 50 yr of age, C pools increased from 2 to 136 Mg/ha, N increased from 72 to 1167 kg/ha, S increased from 9 to 147 kg/ha, and P increased from 5 to 147 kg/ha. In contrast, C, N, and S pools in mineral soil to a 1-m depth remained relatively stable throughout the successional chronosequence and averaged 207, 20, and 3.4 Mg/ha, respectively. Mineral soil pools did not differ with respect to forest age or prior land use history and did not differ from soil pools of primary forest, cornfield, and pasture sites in the Los Tuxtlas Region. Dynamics of the combined aboveground and mineral soil C pools (i.e., excluding C in root biomass) were characterized by increasing contributions from aboveground pools with increasing forest age; aboveground C pools accounted for 9% and 42% of the combined pool in the youngest and oldest forests, respectively. In contrast, changes in combined aboveground and mineral soil pools of N and S during secondary succession were relatively small because >90% of N and S mass was located in mineral soil pools.