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Growth, biomass and carbon sequestration of fast-growing tree species under high-density plantation in Prayagraj, Uttar Pradesh, India

We studied the growth performance of fast-growing trees, viz. Eucalyptus spp., Casuarina equisetifolia, Gmelina arborea and Melia dubia in high-density plantation in Prayagraj, Uttar Pradesh, India, with different spacings, viz. 1 × 1 m, 1.2 × 1.2 m and 1.5 × 1.5 m. The experiment was established in July 2019 and data were recorded for the first and second year. In the second year maximum height increment was found in T1 (Eucalyptus 1 × 1 m; 3.42 m) followed by T5 (Eucalyptus 1.2 × 1.2 m; 3.40 m) and minimum in T2 (Casuarina 1 × 1 m; 1.39 m), whereas maximum girth increment was found in T1 (Eucalyptus 1 × 1 m; 12.43 cm) followed by T9 (Eucalyptus 1.5 × 1.5 m; 10.66 cm) and minimum in T6 (Casuarina 1.2 × 1.2 m; 6.46 cm). Maximum biomass in the first year was found in T6 (Casuarina 1.2 × 1.2 m; 15.51 t ha), followed by T1 (Eucalyptus 1 × 1 m; 14.71 t ha) and minimum in T12 (Melia 1.5 × 1.5 m; 0.66 t ha), whereas in the second year maximum biomass was found in T1 (Eucalyptus 1 × 1 m; 202.72 t ha), followed by T5 (Eucalyptus 1.2 × 1.2 m; 98.81 t ha) and minimum in T12 (Melia 1.5 × 1.5 m; 17.34 t ha). Carbon stock and carbon sequestration were maximum in the first year in T6 (Casuarina 1.2 × 1.2 m) followed by T1 (Eucalyptus 1 × 1 m) and minimum in Melia (1.5 × 1.5 m), with values of 7.75, 7.35, 0.33 t ha, and 28.42, 26.96, 1.21 t ha respectively. Whereas in the second year maximum was found in T1 (Eucalyptus 1 × 1 m) followed by T5 (Eucalyptus 1.2 × 1.2 m) and minimum in T12 (Melia 1.5 × 1.5 m), with values of 101.36, 49.41, 8.67 t ha, and 371.59, 181.12 and 31.78 t ha respectively. Maximum productivity was found in T1 (Eucalyptus 1 × 1 m; 188.01 t ha) followed by T5 (Eucalyptus 1.2 × 1.2 m; 89.88 t ha) and minimum in T12 (Melia 1.5 × 1.5 m; 16.68 t ha).

INDIA is under tremendous pressure to meet the growing demand for wood and wood products, such as pulp and paper. The projected demand by 2020 is a staggering 13.2 million tonnes. The use of paper is directly related to the economic growth of a country 1 . India has large tracts of unused land/wastelands, both in the forest and non-forest areas which could be used for energy-tree plantations.
Approximately 68.35 million hectares (M ha) land in the country is under wastelands 2 .
High-density plantation (HDP) is defined as planting at a density in excess of that which gives maximum crop yield at maturity if the individual tree grows to its full natural size. Fast-growing wood species have been widely used for plantation forest and community forest, so that sustainability of the wood is more promising 3 . Tree plantations are artificial forests that differ from natural forests in terms of their structure and functions 4 . Afforestation combined with revegetation is the dominant focus for landscape planning designed to promote the recovery of goods and services provided by these ecosystems. Such efforts improve the livelihoods of people in local communities and yield other environmental benefits 5 . Recently, fast-growing species and plantations have been considered to play an important role in overall strategies aimed at mitigating climate change, favouring the progressive shift from a fossil fuel-based economy to a biobased economy 6,7 .
Woody biomass offers major advantages over biomass from pastures and agricultural crops as it has better energy properties and less carbon dioxide (CO 2 ) emissions 8 . Trees play a vital role in mitigating the diverse effects of environmental degradation and increasing CO 2 concentration in the atmosphere, and also climate change 9 . Planting high-value agricultural crops is not feasible on degraded community and private lands due to soil moisture and fertility constraints. Establishing high-density woody plantations followed by intensive management serve as the key to utilizing such lands productively.
The experimental design adopted was randomized block design (RBD) with 12 treatments and three replications, viz. T 1 : Gmelina (1.5 × 1.5 m) and T 12 : Melia (1.5 × 1.5 m). The growth performance data were collected for the first and second years.
Girth at breast height (GBH) of standing trees was recorded with a measuring tape at 1.37 m above ground level. The height of standing trees was recorded using a clinometer. Volume of the trees was calculated using the quarter girth formula as follows. 2 , 4 where V is the volume (m 3 ), g the GBH (m) and h is the height of the tree (m). Aboveground biomass (AGB): To estimate biomass, volume was multiplied by wood density (WD) of the tree and biomass expansion factor (BEF). The stem wood biomass was then 'expanded' to total AGB of the tree, including leaves, twigs, branches, bole and bark using BEF 10 . For this study BFF value of 1.5 was used 11 .

Productivity (P) was estimated as follows
where B 1 is the biomass in the first year and B 2 is the biomass in the second year.
The carbon stock of tree was computed by multiplying biomass value with carbon concentration conversion factor value 13 (generally taken as 0.50 t ha -1 ) and the same method adopted for calculation 10,14,15 .
Carbon stock (t ha -1 ) = TB (t ha -1 ) × CF, where CF is conversion factor. The carbon stock was then multiplied by 44/12 to estimate CO 2 sequestration 15 .
Five-year-old fuelwood species with planting at the closest spacing (1 × 1 m) gave green biomass yield (81.0 t ha -1 for Acacia auriculiformis and 68.9 t ha -1 for Casuarina equisetifolia 28 . Table 2 shows carbon stock and carbon sequestration under HDP of fast-growing tree species. The data indicated that the first year maximum carbon stock was found in T 6 (7.75 t ha -1 ; Casuarina 1.2 × 1.2 m) followed by T 1  Table 3 shows the productivity of fast-growing tree species under HDP. The data indicate that maximum productivity was found in T 1 (188.01 t ha -1 ; Eucalyptus 1 × 1 m) followed by T 5 (89.88 t ha -1 ; Eucalyptus 1.2 × 1.2 m) and minimum in T 12 (16.68 t ha -1 ; Melia 1.5 × 1.5 m). A four-year-old Leucaena leucocephala plantation with plant spacing of 0.6 × 0.6 m, showed high net primary productivity (33 t ha -1 yr -1 ), closely followed by E. tereticornis (29 t ha -1 yr -1 ) 33 . The net primary productivity of Eucalyptus plantation (23.4 t ha -1 yr -1 ) was similar to P. deltoides plantation (25 t ha -1 yr -1 ) and the natural sal forest (22 t ha -1 yr -1 ) 34 .
The correlation between height increment and girth increment showed significantly positive correlation (r = 0.821) (Figure 1). Table 4 shows the correlation matrix between volume, biomass, productivity, carbon stock and carbon sequestration in the second year for fast-growing species. Biomass was significantly correlated with volume (0.935), while productivity was significantly correlated with volume (0.950) and biomass (0.998). Carbon stock was significantly correlated with volume (0.935), biomass (1) and productivity (0.998), whereas carbon sequestration was significantly correlated with volume (0.935), biomass (1), productivity (0.997) and carbon stock (1).
Fast-growing tree species under HDP showed that in the first year, among four species, viz. Eucalyptus, Casuarina, Gmelina and Melia maximum growth performance (height, girth and biomass) was observed in Casuarina at 1.2 × 1.2 m spacing, followed by Eucalyptus (1 × 1 m) and minimum in Melia (1.5 × 1.5 m). In the second year maximum growth performance (height, girth and biomass) was recorded for Eucalyptus for three spacings, viz. 1 × 1 m, 1.2 × 1.2 m and 1.5 × 1.5 m followed by Casuarina and minimum in Melia. The maximum productivity was found in T 1 (Eucalyptus 1 × 1 m) followed by T 5 (Eucalyptus 1.2 × 1.2 m) and minimum in T 12 (Melia 1.5 × 1.5 m). The finding also suggests that performance of Casuarina in the first year is best, whereas in the second year Eucalyptus performs best among all species.