Salinity based allometric equations for biomass estimation of Sundarban mangroves

Highlights

• Total biomass was found to be higher in Sonneratia apetala in western Sundarbans.

• AGB and BGB of Avicennia alba and Excoecaria agallocha were higher in central Sundarbans.

• The two sectors are drastically different with respect to salinity.

• Significant impact of salinity was observed on total biomass of selected species.

• Allometric equations for biomass are species-specific and not site-specific.

Abstract

Biomass estimation was carried out for three even-aged dominant mangrove species (Avicennia alba, Excoecaria agallocha and Sonneratia apetala) in two regions of Indian Sundarbans with two distinct salinity regimes for three consecutive years (2008–2010) and the results were expressed in tons per hectare (t ha⁻¹). In the western region, the total mean biomass of the mangrove species varied as per the order A. alba (41.65 t ha⁻¹ in 2008, 55.79 t ha⁻¹ in 2009, 60.86 t ha⁻¹ in 2010) > S. apetala (31.76 t ha⁻¹ in 2008, 32.81 t ha⁻¹ in 2009, 39.10 t ha⁻¹ in 2010) > E. agallocha (13.89 t ha⁻¹ in 2008, 15.54 t ha⁻¹ in 2009, 18.28 t ha⁻¹ in 2010). In the central region, the order was A. alba (42.06 t ha⁻¹ in 2008, 57.09 t ha⁻¹ in 2009, 64.57 t ha⁻¹ in 2010) > E. agallocha (15.30 t ha⁻¹ in 2008, 20.02 t ha⁻¹ in 2009, 24.24 t ha⁻¹ in 2010) > S. apetala (6.77 t ha⁻¹ in 2008, 9.46 t ha⁻¹ in 2009, 11.42 t ha⁻¹ in 2010). Significant negative correlation was observed between biomass of S. apetala and salinity (p < 0.01), whereas in case of A. alba and E. agallocha positive correlations were observed (p < 0.01). Species-wise linear allometric regression equations for biomass prediction were developed for each salinity zone as a function of diameter at breast height (DBH) based on high coefficient of determination (R² value). The allometric models are species-specific, but not site-specific.

Introduction

Mangroves are a taxonomically diverse group of salt-tolerant, mainly arboreal, flowering, plants that grow primarily in tropical and subtropical regions [1]. Estimates of mangrove area vary from several million hectares (ha) to 150,000 km² worldwide [2]. The most recent estimates suggest that mangroves presently occupy about 14,653,000 ha of tropical and subtropical coastline [3]. The field survey of mangrove biomass and productivity is rather difficult due to muddy soil conditions and the heavy weight of the wood. The peculiar tree form of mangroves, especially their unusual roots, has attracted the attention of botanists and ecologists [4]. Allometric equations for mangroves have been developed for several decades to estimate biomass and subsequent growth. Most studies have used allometric equations for single stemmed trees, but mangroves sometimes have multi-stemmed tree forms, as often seen in Rhizophora (Garjan), Avicennia (Baen), and Excoecaria (Gewan) species [5], [6] that often create difficulty in developing allometric equations with accuracy. Clough et al. [5] showed that the allometric relationship can be used for trunks in a multi-stemmed tree. Moreover, for dwarf mangrove trees, allometric relationships have been used to estimate the biomass [7]. Basically the dwarfness of mangroves is caused due to high salinity. Presence of salt is a critical factor for the development of mangrove ecosystems. At lower intensities it favors the development of mangroves eliminating more vigorous terrestrial plants which other wise could compete with. On the contrary at increased level it might cause overall degradation of mangroves. Salinity is also a controlling factor for mangrove seedling recruitment and the relation is negatively proportional. Siddiqi [8] noted reduced recruitment of Heritiera fomes (Sundari) and Excoecaria agallocha seedling in the Sundarbans mangrove forest with increased salinity. Ball and Pidsley [9] observed adverse impact of increased salinity on canopy development, leaf initiation, and leaf area expansion in Sonneratia alba (Sada Keora) and Sonneratia lanceolata (Keora).

In the maritime state of West Bengal, situated in the northeast coast of India, the adverse impact of salinity on the growth of mangrove species has been documented [10], [11]. Salinity, therefore, greatly influences the overall growth and productivity of the mangroves [12]. The Indian Sundarbans exhibits two significantly different salinity regimes due to siltation that prevent the flow of Ganga–Bhagirathi–Hooghly water to the central region. This has made the ecosystem a unique test bed to observe the impact of salinity on the biomass and allometric trait of the mangrove species.