GROWTHS AND CARBON STOCKS IN RUBBER PLAN- TATIONS ON CHAKKARAT SOIL SERIES, NORTHEASTERN THAILAND

Growths and carbon stocks in a series of 1, 5, 10, 15, and 20-year-old para rubber plantations, together with a natural forest, on Chakkarat soil series in northeastern Thailand were investigated. In total, 15 40×40 m sampling plots were used for studying rubber growths, 3 plots per each age class plantation and 1 plot for the natural forest. In each plot, stem girth at 1.3 m above ground, crown width, and tree height were measured. One rubber tree having the mean growth in each age class plantation was cut and separated to stem, branch, leaf, and root biomass for estimating carbon amounts. Three soil pits were made in each plot, and soil samples were collected along with the soil profile. Soil physical and chemical properties were analyzed in the laboratory. Rubber tree densities varied between 469-500 trees/ha (75-80 trees/rai). Stem girth and height growths increased with the plantation age. The growth was very rapid for rubber trees having ages between 1 and 15 years old and became slow for the older trees. The biomass amounts of the 1, 5, 10, 15, and 20-year-old plantations were in the order of 20.17, 52.47, 92.59, 123.50, and 147.83 Mg/ha, respectively. The ecosystem carbon stocks in these plantations increased with tree ages at 25.53, 46.70, 71.06, 86.18, and 97.11 Mg/ha, respectively. Two compartments were involved; (1) biomass carbon: 11.42, 29.87, 52.54, 70.13, and 83.74 Mg/ha; and (2) soil carbon: 14.26, 16.83, 18.52, 16.05, and 13.37 Mg/ha. The total carbon storage in the natural forest was 134.62 Mg/ha; 124.20 Mg/ha in the biomass and 10.42 Mg/ha in the soil. The young plantations had high carbon percentages in the soil and low in the biomass whereas carbon allocation in the older plantations was higher in the biomass and lower in the soil system.