The Kaptai National Park was established in 1999 in Rangamati Hill Tracts District and lies between 22°27´ to 22°32´ N latitudes and 92°30´ to 92°16´ E longitudes. It encompasses an area of 5464 hectares within two forest ranges, namely Kaptai Range and Karnaphuly Range under the management of Rangamati Hill Tracts South Forest Division . Many scientists reported that there are about 412 species of plants in these forests. It was also reported that the dominant tree species in these forests included Dipterocarpus turbinatus Gaertn, Protium serratum (Wall. ex Coelbr.) Engl., Terminalia bellirica (Gaertn.) Roxb., Trewianudi flora L., Aphanamixis polystachya (Wall.) R. Parker and Lagerstroemia speciosa L. etc. About 90% land of the area was hilly; settlers occupy 4% area, and 6% land was arable (Khisa 1997).
The mean annual temperature and rainfall were 20.500C and 2531 mm, respectively. The mean annual humidity was around 80% (Meteorological Station, Kaptai Power Development Board Records of 2016).
Biomass and carbon stocks of trees were estimated by allometric equations (non-destructive) method and the data are analysed in the Bangladesh Forest Research Institute. Seventy seven tracks or reference points, each being 500 m apart from the other, and all tracks were selected over the total park area using the Global Positioning System (GPS). Four circular plots of 10 m radius (314 m2 area) were demarked in north-south and east-west directions at a distance of 100 m from the center of each track.
Thus, there were 308 (4 × 77) sampling plots and sampling was done between January 2014 and December 2016. The number of trees species and the number of individuals of tree species in each plot were recorded. Diameter at breast height (DBH; 1.3 m above the ground level) and height (H) of each tree were measured by Hega- altimeters. Aboveground biomass (it include main stem, branches and leaves), below ground biomass and total biomass and carbon storage for each tree were estimated using the allometric equations (Alves at al. 1997, Brown 1997, Schroeder et al. 1997). The equations are shown below:
AGB = exp. {-2.4090 + 0.9522 ln (D2HS)}
where, AGB is the aboveground biomass (kg), H is the height of the trees (m), D is the diameter at breast height (cm) and S is the specific gravity of timber (Mg m-3 ). There were six timbers species in the study and their specific gravities (S) were 0.57, 0.59, 0.75, 0.65, 0.42 and 0.71 Mg m-3 in L. speciosa, Tectona grandis, Swietenia macrophylla, Acacia auriculiformis, Gmelina arborea and Dipterocarpus turbinatus, respectively (Sattar et al. 1999). Belowground biomass (BGB) was calculated considering 15% of the aboveground biomass (Mac Dicken 1997).
BGB= (AGB×15/100)
TCS= (AGB+BGB) ×0.50
The above and below ground biomasses were added to get the total biomass of a tree. The total carbon stock (TCS) is calculated taking 50% of the total biomass as carbon (Brown et al.1989). Biomass and carbon per tree values were multiplied by the number of trees per circular plot and then values were converted to hectare to obtain biomass and carbon stock per hectare. ANOVA and DMRT for carbon stock per tree was performed by using SPSS 20 version.