Spatial pattern of above-ground biomass (AGB) and carbon stock (Cp)
Field-based AGB estimates, though time and cost ineffective, are vital to characterizing forest ecosystems and to assess the potential production of tropical forests (Behera et al., 2017). The total AGB and Cp of all studied Sal forests were 410.69 Mg ha–1, and 193.06 Mg C ha–1 respectively, which is higher than the AGB of Sal plantation forest of Meghalaya, Northeast India (406 Mg ha–1) (Baishya et al., 2009) as well as Sal forests of the Himalayas (78 to 378 Mg ha–1) (Gautam et al.,, 2011). The present record of AGB is within the reported range (150 to 698 Mg ha–1) of various tropical forests of the world (Brown and Lugo, 1982; Shrestha et al.,, 2000; Terakunpisut et al.,, 2007) as well as comparable with the reported range (28.1 to 330.87 Mg ha–1) ofvarious tropical deciduous forests of India (Ranawat and Vyas, 1975; Singh and Singh, 1981; Negi et al.,, 1995; Salunkhe et al.,, 2016) (Table 2).The forest wise AGB and Cp of tree species in different studied Sal forests were ranged from 0.19 to 24.75 Mg ha–1 (mean 4.46 ± 0.45 SE) and 0.09 to 11.63 Mg C ha–1 (mean 2.10 ± 0.21 SE) respectively. AGB (24.75 Mg ha–1) and Cp (11.63 Mg C ha–1) were maximum at Chirua, Kanke (JH019) and minimum (0.19 Mg ha–1 and 0.09 Mg C ha–1 respectively) at Agartoli, Angara (JH016) (Table 3). The spatial pattern of AGB showed that maximum studied forests (65 %) had very low AGB (<5.00 Mg ha–1) followed by low AGB (5.01 to 10.00 Mg ha–1) forests (25 %), moderate AGB (10.01 to 15.00 Mg ha–1) forests (7 %), high AGB (15.01 to 20.00 Mg ha–1)forests (2 %) and only one forests (1 %) had very high AGB (>20 Mg ha–1) located in the northwest corner of Ranchi. The carbon sequestration potential of forests depends on forest type, age of forest, and age of trees, as well as its basal cover and density (Terakunpisut et al., 2007). Species wiseAGB ranged from 0.001 to 7074.94 Mg ha–1 (mean 106 ± 71 SE) and Cp ranged from 0.0005 to 3325.22 Mg C ha–1 (mean 49.76 ± 33.54 SE) in Sal forests of Ranchi. Out of 103 recorded tree species, Shorea robusta had the highest AGB, and Cp (7074.94 Mg ha–1, and 3325.22 Mg C ha–1) followed by Madhuca longifolia (1875.67 Mg ha–1 and 881.5649 Mg C ha–1), Diospyros melanoxylon (1014.12 Mg ha–1 and 476.63 Mg C ha–1), Semecarpus anacardium (221.81 Mg ha–1 and 104.25 Mg C ha–1), Syzygyum cumini (99.80 Mg ha–1 and 46.90 Mg C ha–1), and Buchanania cochinchinensis (84.43 Mg ha–1 and 39.68 Mg C ha–1) (Table 4).. More diverse plant communities have a higher chance of including highly productive species that dominates the community, as S. robusta dominated the whole AGB in Sal mixed tropical moist deciduous forest in the upper Gangetic plains adjoining Himalayan foothills in Uttar Pradesh, India (Behera et al., 2017). Among all the tree species, S. robusta individually shared 64.87 % of the total AGB and only three species (S. robusta, M. longifolia and D. melanoxylon) had AGB >1000 Mg ha–1 and shared 93.40 % of the total AGB. Most of the tree species (65 tree species) (63.11%) had AGB <1 Mg ha–1, and 34 species (33.01 %) had AGB between 1 to 100 Mg ha–1.
On the other hand, AGB in different regions of Ranchi ranged from 0.90 to 111.74 Mg ha–1 (mean 6.21 ± 0.45 SE) and the highest AGB recorded in Namkum with 111.74 Mg ha–1 (mean 6.21 ± 0.45) followed by Kanke with 50.56 Mg ha–1 (mean 5.62 ± 0.45 SE), Burmu with 42.85 Mg ha–1 (mean 4.46 ± 0.45 SE), Tamar 40.21 Mg ha–1 (3.09±0.41 SE), and the lowest AGB was recorded in Ormanjhi (0.90 Mg ha–1). Tree biomass in forests vary with forest type, species composition, stand age, size class of trees, site conditions, rainfall pattern, edaphic factors and altitude (Sharma et al.,, 2011; Zhao et al.,, 2014; Cao et al.,, 2018). Similarly, highest carbon stock was recorded in Namkum with 52.52 Mg C ha–12.26 ± 0.26 SE) followed by Kanke with 23.76 Mg C ha–1 (2.09 ± 0.21SE), Burmu with 20.14 Mg C ha–1 (2.53 ± 0.58 SE), Tamar with 18.90 Mg C ha–1 (2.53 ± 0.58 SE), Bero 18.50 Mg C ha–1 (2.61 ± 0.47 SE), and the lowest Cp was recorded in Rahe (0.21 Mg C ha–1) (Table 5). Maximum AGB and Cp in Namkum mainly due to greater basal area 3344.54 m2 ha–1 (185.80 ± 21.74 SE) and maximum numbers of sampled transects (18) as compared to other regions of Ranchi.
The total basal area as well as density of trees in all studied Sal forests were 144.63 m2 ha–1 and 515 ind. ha–1 respectively. Species wise basal area of trees ranged from 0.0002 to 120.81 m2 ha–1 (mean 1.40 ± 1.17 SE) and density ranged from 0.02 to 416 ind. ha–1 (mean 5 ± 4.04 SE). S. robusta had maximum basal cover (120.81 m2 ha−1) as well as density (416 ind. ha–1) among all tree species and is the reason for its maximum AGB and Cp. However, no similar trends were observed in all other trees. B. cochinchinensis had the second largest basal area (4.63 m2 ha−1) followed by Mangifera indica (3.06 m2 ha−1), D. melanoxylon (1.64 m2 ha−1) and Terminalia arjuna (1.48 m2 ha−1). While, D. melanoxylonhad the second highest density (23 ind. ha−1) followed by B. cochinchinensis (15 ind. ha−1) (Table 4).. On the other hand, forest wise stand basal area and stand density of trees ranged from 30.72 to 367.84 m2 ha−1 (mean 135.28 ± 7.24 SE), and 136 to 1712 ind. ha–1 (mean 515 ± 34.16 SE) respectively. Maximum tree density (1712 ind. ha–1) was recorded in Paina Pahar, Lapung (JH059) and minimum (136 ind. ha–1) in Silway, Namkum (JH01), while, basal area of trees was highest (367.84 m2 ha–1) in Harbul, Namkum (JH061), and minimum (30.72 m2 ha–1) in Koijam, Burmu (JH029) (Table 2).. Maximum studied forests (30 %) had moderate basal cover (100.1 to 150 m2 ha–1) followed by high basal cover (150.1 to 200 m2 ha–1) forests (24 %), while maximum studied forests (47 %) had less tree density (150 to 300 ind. ha–1) followed by very less tree density (<150 ind. ha–1) forests (27 %), moderate tree density (301 to 450) forests (13 %) and rest 14 % forests had high and very high tree density (>450 ind. ha–1).
Spatial pattern of plant diversity attributes
A total of 103 tree species belonging to 81 genera and 33 families are recorded in 46 ha study plots, which is lower than the earlier records from moist Sal forests of northern West Bengal (134 trees) (Kushwaha and Nandy, 2012), and tropical deciduous forest in Mudumalai Wildlife Sanctuary (124 trees in 6.1 ha sampled plots) (Reddy and Ugle, 2008), but quite higher than Western Terai Sal forests of Nepal (28 trees) (Timilsina et al., 2007). Fabaceae (20 spp.) is the most species rich family followed by Moraceae (08 spp.), Euphorbiaceae (07 spp.) and 14 families are monotypic. On the other hand, Ficus (06 spp) is the most species rich genera followed by Terminalia (05 spp.) and 68 genera are monotypic. Tree diversity ranged from 01 to 27 species (mean 12 ± 0.60 SE) in all 92 studied Sal forests of Ranchi. Tree diversity in terms of Shannon-Weiner diversity index (H’) was ranged from 0.45 to 2.76 (mean 1.65 ± 0.48 SE), which is falls within the range (0.62–3.96) reported by earlier workers for various tropical deciduous forests of India (Tripathi and Singh, 2009; Naidu and Kumar, 2016; Sahoo et al.,, 2020). Maximum studied forests (42 %) had moderate tree diversity (H’: 1.51 to 2.00) followed by less diversity (H’: 1.00 to 1.50) forests (28 %), high diversity (H’: 2.01 to 2.50) forests (17 %), very less diversity (H’: <1.00) forests (9), and very high diversity (H’>2.50) forests (3 %). The possible reason of lower H’ in the studied Sal forests may possibly be due to anthropogenic disturbances resulting in habitat destruction, which leads to the survival of less number of tree species and their individuals. The highest tree diversity in terms of Shannon H’ (2.76) was recorded in Sal forest located at Koijam, Burmu (JH029) and the highest E (0.87) was recorded in Siram, Burmu (JH030). The values of H’, CD, E, Dmg, Dmn, and ENS do not follow a definite pattern in all studied Sal forests of Ranchi (Table 2).. Evenness index also known as species equitability ranged from 0.25 to 0.87 (mean 0.68 ± 0.11 SE), while ENS ranged from 2 to 16 (mean 6 ± 0.30). ENS denotes the amount of diversity directly compared with the within-community, and among community components, provides more interpretable, and comparable assessments of biodiversity as compared to species richness, H’, and CD (Jost, 2007). It is the true diversity of community used to assess species diversity on the basis of H’, and responds to either known alteration in assemblage or environmental variables (Cao and Hawkins, 2019). On the other hand, CD ranged from 0.10 to 0.72 (mean 0.34 ± 0.01), which was within the reported range of CD (0.19 to 0.99) for forest vegetation (Whittaker, 1965). Lower CD (0.10) in few studied Sal forests (JH029, and JH030) indicate that dominance is shared by more than one species, and values of CD were lower in contrast with high species diversity (H’) (2.76 and 2.74) as species diversity behaves inversely to the index of dominance (CD) (Odum, 1971). Dmg ranged from 0.17 to 11.68 (mean 2.06 ± 0.15) and Dmn ranged from 0.11 to 2.03 (mean 0.78 ± 0.04) in various studied Sal forests of Ranchi. The species richness in terms of Dmg and Dmn were 10.23 and 0.68 respectively for all studied Sal forests of Ranchi.
On the basis of H’ and E, Sal forests were classified into highly diverse (HD) with H’> 2.00, moderately diverse (MD) (H’ = 1.6–2.0), low diversity (LD) (H’<1.6) forests and highly even (HE) with E > 0.75, moderately even (ME), E = 0.6–0.75, and poorly even (LE) E<0.06 forests. Further, unique combination of H’ and E (HD-LE, HD-ME, HD-HE, MD-LE, MD-ME, MD-HE, LD-LE, LD-ME, LD-HE) was used to classify all the studied Sal forests of Ranchi (Table 3).. Grouping of H’-E at different nine combinations illustrates that only 03 (3.26%) studied forests (JH003, JH070, and JH077) showed extreme tree diversity with high H’ and low E (ETDFs). Similarly, very high tree diversity forests with high H’ and moderate E (VHTDFs) was recorded in 11 (11.96 %) studied forests and high tree diversity forests with high H’ and high E (HTDFs) was recorded in 04 (4.35 %) studied forests, while only one studied forests (JH002) classified as tree diversity forests with moderate H’ and low E (TDFs) (Figure 2)..
Relationship between plant diversity attributes and AGB
The relationships between tree basal cover, density, plant diversity attributes (Dmg, Dmn, H’, CD, ENS, and E), and AGB was documented in Sal forests of Ranchi. Correlation of basal area with AGB was positive, and statistically significant (r = 0.71, p<0.05) indicating that basal area is a major indicator of AGB. Strong relationships between AGB and basal area have also been reported by several workers in various types of forests (Cannell, 1984; Rai and Proctor, 1986). Likewise, high AGB indicate high Cp, so correlation between AGB and Cp was positive and highly significant (r = 1.00, p<0.01). AGB showed insignificant, negative correlation with tree density (r = - 0.17) indicating that forests with higher tree density have reduced AGB. Generally, in dense forests, the availability of soil nutrients and water, due to high intraspecific competition cannot properly availed by plants for their growth and development may be the reason of lower AGB in higher tree density forests. Inverse relationship of total tree density with AGB was also observed in Terai Shorea forest and Shorea-Terminalia forest of south-western part of Nepal (Giril et al.,, 1999), while a strong positive relationship of density and AGB was observed in Katerniaghat Wildlife Sanctuary, a tropical moist deciduous forest in the upper Gangetic plains adjoining Himalayan foothills in Uttar Pradesh, India (Behera et al., 2017). Again, basal area had insignificant negative relationship with H’ (r = -.13), ENS (r = -.11) and Dmg (r = -.004), while it has significant negative correlation with E (r = -.27, p<0.01) and Dmn (r = -.26, p<0.05). On the other hand, a positive linear relationship had been observed between diversity and total tree basal area (a substitute for biomass) in tropical deciduous forests in India (Sagar and Singh, 2006). The highest AGB (>20 Mg ha–1) was recorded at high basal area (>160 m2 ha–1) as AGB increases with the increase in basal area, but no similar trends had been followed by all studied Sal forests (Figure 3a).. Similarly, very low density forests (<150 ind. ha–1) (Figure 3b) had highest AGB (>20 Mg ha–1), as well as highest species diversity (>2.0) (Figure 3c). A highly significant positive correlations of AGB with H’ (r =.58, p<0.01), Dmg (r =.31, p<0.01), Dmn (r =.49, p<0.01), and ENS (r =.57, p<0.01) were observed in the present study, while AGB-E relationship was statistically significant and positive (r =.26, p<0.05) (Table 7, Figure 4a-f),, possibly due to continual disturbance does not allow biomass to concentrate in only the strongest competitors. AGB increased with increase in tree diversity, as high species richness helps in increased nutrient use efficiency (Ruijven and Berendse, 2005). On the other hand, species richness is highly correlated with other plant diversity attributes and a high correlation exists between H’ and species richness across tropical forests (Gentry, 1988). The present study also recorded highly significant positive correlation of H’ with Dmg (r =.49, p<0.01), and Dmn (r =.84, p<0.01). The tropical deciduous forests in India experience frequent, large-scale human disturbances from mining, power generation, grazing, tree felling, and extraction of forest resources (Kumar and Saikia, 2020). Disturbance regimes differ greatly among tropical forests as they experience more frequent or smaller scale disturbances and may account for differences in species richness-AGB relationships in tropical forests (Phillips et al., 1994). In conformity with the present study, positive relationships have been observed between plant diversity/richness and AGB in various forests (Caspersen and Pacala, 2001; Erskine et al., 2006; Houle, 2007). However, negative (Lugo, 1992, Wardle et al., 1997), and no relationships (Vila et al., 2003) between plant diversity/richness and AGB have also been documented in forests. Although results are not consistent regarding the species richness-AGB relationship in forests, trees grow faster and attain greater biomass in forests (Erskine et al., 2006; Potvin and Gotelli, 2008). Therefore, positive relationships may be the most obvious expectation for the species richness-AGB relationship in forests. High and positive correlation of AGB with H’ and ENS signifies the diversity of tree species in studied forests did have major effects on growth and development by intraspecific competition among the species. However, AGB was negatively correlated with CD (r = -.57, p<0.01)may be because of CD inversely related to H’. The negative CD-AGB relationships suggest that a few species become more dominant at high biomass, instead of the biomass being distributed evenly among all species (Vance-Chalcraft et al., 2010).