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Biomass and carbon stock in Acacia catechu (Khair) plantation in tropical environment

The present study on “Biomass and carbon stock in Acacia catechu plantation in tropical environment” was carried out at State Forest Research and Training Institute Raipur (Chhattisgarh), during the year 2020-2021. The total tree density in A. catechu plantation site was 575 stems ha -1 . The total basal area of tree layer in A. catechu plantation site was 18.6 m 2 ha -1 . Total sapling and seedling density in A. catechu plantation site was 50 stems ha -1 and 200 stems ha -1 respectively. Total abundance for sapling and seedling layer in A. catechu plantation site was 4.83 and 12.22 respectively. The Shannon index in A. catechu plantation site for the tree, sapling and seedling layer was 0.43, 1.46 and 0.93, respectively. The Simpson’s index for tree, sapling and seedling layer were 0.13, 0.62 and 0.42, respectively. The evenness in A. catechu plantation site for tree, sapling and seedling were 0.39, 1.33 and 1.34, respectively. Species richness for tree, sapling and seedling layer was 0.31, 0.51 and 0.18, respectively. Beta diversity in A. catechu plantation site for tree, sapling and seedling layer was 1.76, 2.5 and 2.3, respectively. Total biomass, litter mass and carbon stock in A. catechu plantation site was 177.42, 8.49 t ha -1 , 77.38 t ha -1 respectively. Soil pH, EC, total available nitrogen, available phosphorous, total available potassium and organic carbon in upper soil layer (0-10 cm) of A. catechu plantation site was 6.02, 114.7 ds/m, 288.51 kg/ha, 6.68 kg/ha, 123.42 kg/ha and 0.76%, respectively. Soil pH, EC, total available nitrogen, available phosphorous, total available potassium and organic carbon in lower soil layer (10-20 cm) of A catechu plantation site was 6.5, 115.8 ds/m, 163.07 kg/ha, 6.38 kg/ha, 122.3 kg/ha and 0.70%, respectively. Study revealed the potential of A. catechu in biomass production and carbon storage and may contribute to climate mitigation process.


Introduction
Tropical deciduous forests grow in a variety of climates, mostly with alternate wet and dry periods. The structure, content, and functioning of deciduous forests, on the other hand, fluctuate with the duration of the wet season, quantity of rainfall, latitude and altitude (Shankar, 2001), and the effects of human and animal activities. Tropical forests are disappearing at an alarming rate of 0.8-2.0% per year (May and stumpf, 2000) [18] as a result of excessive cutting of timber and other forest produce (Raghubanshi and Tripathi, 2009), and it has been declared that continuous biomass extraction activities may prevent the goal of conserving biodiversity from being achieved (Schaik et al., 1997). Habitat degradation, over exploitation, pollution, and species introduction has all been cited as important causes of biodiversity loss in India (UNEP, 2001). The tropical forest is one of the most diverse and complex terrestrial ecosystems, accounting for almost half of the world's live terrestrial carbon sinks. (Acacia catechu) is a deciduous tree with a light feathery crown and dark brown, glabrous, slender, thorny, shining branchlets, usually crooked. Bark dark brown or dark grey, brown or red inside, nearly 12-15 mm in thickness, rough, exfoliating in long narrow rectangular flakes which often remain hanging. Biomass is a key for understanding a variety of biological processes in forest ecosystems, including energy flow, water movement, and nutrient cycling (Chaturvedi and Singh, 1987;Tiwari, 1994) [5] . One of the most pressing global carbon concerns now is the fast rising quantity of CO 2 in the atmosphere at (2 ppm yr -1 ) and its potential to alter global climate. CO 2 and other greenhouse gases in the atmosphere have elevated the global average surface temperature by 0.6 to 0.2 degrees Celsius (IPCC, 1999). To address this issue, the IPCC (1996) [15] called for expanding the C pool by extensive afforestation and reforestation, as well as conserving the present C pool in the terrestrial environment. Rapid land use and land cover change has resulted in large-scale carbon degradation in tropical ecosystems during the last several decades.

Method
For vegetation characterization, a stratified random sample technique was used. The plantation site was studied for vegetation analysis by randomly placing 20m × 20m quadrates. Tree vegetation was studied by placing ten quadrats of 20m x 20m randomly. A 2 m x 2 m quadrates was placed in the middle of each 20 m x 20 m quadrates for enumeration of saplings and seedlings. At 1.37 m above ground level, the adult individual's girth was measured. As a result, all individuals were counted by species and their girths were measured. The vegetational data were used to calculate density, frequency and dominance (Curtis and Mclntosh, 1950) [8] . The IVI was measured as the sum of relative density, relative frequency and relative dominance (Phillips, 1959). Species diversity were calculated using density values from Shannon-Weiner information function (Shannon and Weaver, 1963). Concentration of dominance was calculated using Simpson's index (Simpson, 1949). Species richness following Margalef (1958) [17] , equitability following pielou (1966) and beta diversity following Whittaker (1972).

Biomass estimation
For the measurement of biomass, allometric equations relating tree circumference to biomass developed earlier by Singh and Mishra (1979) for dry deciduous forest species were used (Apendix). Computation protocol as described by Chaturvedi and Singh (1989) and Singh and Singh (1991) were followed.

Litter mass
By using 50 cm x 50 cm randomly placed quadrates, forest floor litter was collected and then categorized into different viz., fresh leaf litter, wood litter and partially decayed litter. The collected litter was brought to the laboratory and oven dry weights were determined.

Carbon estimation
For the estimation of carbon stock, carbon concentrations reported by Singh (2010) were used. The carbon storage for the vegetation components were computed as the sum of the products obtained by multiplying dry weights of components with their mean carbon concentrations. The values for carbon stock in different components were summed to get the total carbon stock by the vegetations.

Soil analysis
Soil samples were collected from 0-10 cm and 10-20 cm soil depth and were analyzed for pH, EC, available nitrogen, available phosphorous, available potassium following Jackson, (1958) [16] .% organic carbon of soil was determined by the Walkley and Black method following Jackson, (1958) [16] . All the results were expressed on oven dry weight basis.

Species diversity of A catechu plantation site
The Shannon index values calculated in A catechu plantation site for the tree, sapling and seedling layer was 0.43, 1.46 and 0.93, respectively. The simpson's index values calculated for tree, sapling and seedling layer was 0.13, 0.62 and 0.42, respectively. The evenness measured in A catechu plantation site for tree, sapling and seedling was 0.39, 1.33 and 1.34, respectively. Species richness measured for tree, sapling and seedling layer was 0.31, 0.51 and 0.18, respectively. Beta diversity calculated in A catechu plantation site for tree, sapling and seedling layer was 1.76, 2.5 and 2.3, respectively

Biomass (t ha -1 ) in A. catechu plantation site
The total biomass in A. catechu plantation site was 177.42 t ha -1 of which 155.88 t ha -1 was above ground and 21.54 t ha -1 in below ground parts. Acacia catechu had the highest biomass 171.93 t ha -1 followed by T. grandis 3.77 t ha -1 and L.

Litter mass (t ha -1 ) in A. catechu plantation site:
The total litter mass was 8.49 t ha -1 in A catechu plantation site. Of the total litter mass 3.06 t ha -1 was leaf litter, 2.29 t ha -1 was wood litter and 3.14 t ha -1 was partially decomposed litter. The leaf litter, wood litter and partially decomposed litter constituted 36.04%, 26.97%, 36.98% of the total litter mass.

Physico-chemical properties for upper and lower soil layer in A. catechu plantation site
Soil pH, EC, total available nitrogen, available phosphorous, total available potassium and organic carbon in A catechu plantation was 6.02, 114.7 ds/m, 288.51 kg/ha, 6.68 kg/ha, 123.42 kg/ha and 0.76%, respectively. Soil pH, EC, total available nitrogen, available phosphorous, total available potassium and organic carbon in A catechu plantation was 6.5, 115.8 ds/m, 163.07 kg/ha, 6.38 kg/ha, 122.3 kg/ha and 0.70%, respectively.