MSc

A field study was conducted with acacia species on an 18-year-old stand with 3m×3m spacing at the arboretum of College of Forestry, Thrissur, Kerala to evaluate the growth, biomass production, carbon sequestration and nutrient accumulation in four acacia species viz. Acacia auriculiformis, Acacia mangium, Acacia crassicarpa and Acacia aulacocarpa. The objective of the study included quantifying the biomass production potential, harvest related nutrient export from the site, characterising the root distribution pattern of these trees and to develop allometric equations for aboveground biomass, aboveground C sequestration, volume and bole volume.
The above ground biomass was estimated from 20 destructively sampled trees from each species and the belowground biomass was estimated following root excavation of average sized trees of each species. Significant differences were observed for the tree growth parameters except DBH. Acacia aulacocarpa recorded the highest growth rates in terms of height closely followed by Acacia auriculiformis. Among the species, Acacia auriculiformis recorded the highest stand total biomass (432.08 Mg ha-1) and the lowest by Acacia mangium (367.76 Mg ha-1). The most important component of total biomass undoubtedly, was the bole while foliage contributed least to biomass yield. Maximum aboveground and belowground biomass was recorded for Acacia auriculiformis (336.29 Mg ha-1and 95.79 Mg ha-
1respectively).
Carbon sequestration potential was estimated for both aboveground and belowground biomass. Maximum mean tree C sequestration was recorded for Acacia auriculiformis (176.38 kg C tree-1) followed by Acacia aulacocarpa (165.54 kg C tree-1). The bole portion sequester major portion of C (63.61% to 71.28%) followed by root portion (19.1% to 23.78%). MAI in total stand C sequestration was maximum for Acacia auriculiformis (10.89 Mg C ha-1yr-1) closely followed by Acacia aulacocarpa (10.22 Mg C ha-1yr-1). Stand level biomass C sequestration in the leaf and twig portion varied significantly among the acacias. Soil C sequestration
under each species was estimated upto one meter depth. Maximum soil organic carbon (SOC) was accumulated in the surface soil (0-20 cm) for all the species. Acacia auriculiformis (77.96 Mg C ha-1) recorded the highest total SOC followed by Acacia mangium (74.75 Mg C ha-1). The treeless plots consistently recorded the lowest value of SOC in all the depth zones.
Nutrient concentrations (N, P and K) in the biomass components were recorded highest for the leaf portion and the highest stand nutrient accumulation was recorded for the bole portion. The order of nutrients in the plant were N> K> P. The nutrient accumulation in the stand level was also recorded highest for Acacia auriculiformis. The order of nutrient accumulation in the soil was N> P> K. No significant variation was observed in root distribution pattern of different acacia species. However, the maximum root spread was recorded for Acacia mangium (5.23 m) and root length for Acacia crassicarpa (1.49 m