Fine root production and its contribution to the total biomass of the Sundarbans mangrove forest

Mangrove forests have high net primary productivity (NPP) which is widely recognized that those trees allocate excessive amounts of biomass to their root systems. Fine root production, in particular, accounts for a significant proportion of the NPP. However, there is a scarcity of information on fine root production of mangrove ecosystems especially Sundarbans mangrove forest. Quantification of Fine Root Biomass (FRB) (≤2mm) and Fine Root Production (FRP) were studied in two different saline zones (Oligohaline and Mesohaline zone) of  the Sundarbans Reserve Forest (SRF), Bangladesh. In addition, structural composition and species-wise contributions to total biomass carbon (TBC) were also analyzed. There's no previous study that looked at both the contribution of fine roots to biomass carbon and fine root production in association with species diversity and species-wise contributions to total biomass carbon in SRF. Samples were collected from 20 plots of 2000m² at two different locations to estimate the FRB and FRP (oligohaline zone and mesohaline zone). Using sequential soil cores and long-term ingrowth core methods, we studied the FRB and FRP in two separate saline areas of the Sundarbans mangrove forest. Besides, the soil sample was sorted into three segments according to the depth of the soil. After being washed, overall mean of FRB and FRP 31.9±1.9 Mgh^-1and 2.3±0.07 Mgh^-1y^-1 respectively. Of these two saline zones, FRB and FRP values in oligohaline zone were 17.73 Mgh^-1 and 2.38 Mgh^-1y^-1 respectively, which were higher than the FRB and FRP (14.2 Mgh^-1 and 2.11 Mgh^-1y^-1 respectively) values in the mesohaline zone area. Among these three diameters class, the production of 1-2 mm fine roots was advanced than 0.5-1 mm and ≤ 0.5mm diameter class respectively. On the other hand, mean above-ground, below-ground, and total biomass carbon (TBC) of the studied areas were 87.18 ± 18.3, 48.38 ± 8.0 and 135.57 ± 26.3 Mg ha^-1 respectively. Based on the species IvI, Excoecaria agallocha was the principle species in the mangrove community along the oligohaline zone of Sundarbans. In addition, the IvI value of Sonneratia apetala was also higher than any other species in the mesohaline zone. Heritiera fomes contributed the most to the TBC, while E. agallocha was the second and A. officinalis was the third highest contributor of carbon accumulation in the oligohaline zone. In contrast, S. apetala donated highest amount of biomass carbon in the mesohaline zone. The present study reveals that the value of fine roots production, total biomass carbon and the species richness of the SRF is higher than any other terrestrial forest of Bangladesh that might be indicated for higher rate of sequestering atmospheric carbon. As a result Sundarbans can play a vital role for mitigating global climate change, that commencing their ecological significance. In future decision-making process, its need to be considered that Sundarbans has indisputable tenors on mitigating the effects of climate change which cannot be ignored.

• Supervision