Belowground carbon stocking potential in some tropical plantations

Eucalypts (Eucalyptus tereticornis) and teak (Tectona grandis) are the most important forest plantation species in tropical India occupying thousands of hectares. Besides, Acacia mangium has been recently used in the different agroforestry systems. Eucalypts, mainly used as pulpwood, are coppiced every seven years for three consecutive rotations. Hence, the root system of these trees prevails in the soil for more than 21 years. Teak, on the other hand, is a long rotation timber species, yielding intermediate yield through a few thinning operations and the final yield when harvested at 50 years. The estimation of the aboveground biomass has been successfully done for most plantation species. However, the belowground biomass has always presented problems in estimation of the carbon stock. This paper presents the results of some excavation studies done in Kerala, in the southwestern part of peninsular India. In E. tereticornis, the root system (20 years old) penetrated as deep as 9.5 m, giving a total root biomass of 53.0 kg per tree (dry wt.). In four-year-old T. grandis, the root system penetrated as deep as 3 m of soil and the average root biomass was 1.9 kg. In 10-year-old A. mangium, the root system was 1.08 m deep, and the average total root biomass per tree was 57.3 kg. From the above determination of the root biomass in three important plantation species, the following conclusions can be drawn. A eucalypt plantation can sequester 62.3 tons of carbon/ha in the root system, thereby giving a CO2 reduction of 228.9 tons/ha (2500 stems/ha). In a four-year-old teak plantation, the carbon sequestered is 2.37 tons/ha, giving a reduction of 8.7 tons of CO2/ha (2500 stems/ha). In A. mangium, the carbon sequestered by the root system is 72 tons/ha, giving a reduction of 263 tons of CO2/ha (2500 stems/ha). Although strict comparison is not possible with limited data from trees of different ages, it is apparent from the above data that fast growing plantation tree species are efficient in sequestering atmospheric carbon dioxide