Biometric parameters of the seven bamboo species
In bamboo culm has surprising biomass productivity due to its habit of producing sprouting from horizontal rhizome systems annually that allows annual harvesting without exposing the soil surface. Bamboo plays an important role in global carbon cycle, by sequestering atmospheric carbon through its biomass accumulation process39–42. In seven bamboo species studied, among three major biomass component (culm, branch and foliage), culm is the most important with highest share (69.56 to 78.71%) to above ground biomass which is more than double to combine biomass share of branch and foliage. Moreover, bamboo culm has high commercial value due its multi-purpose utility43. Distribution of percentage (%) share of culm, branch and foliage to above ground fresh weight varies significantly between different bamboo species (Figure 2).
Analysis of variance (ANOVA) analysis revealed significant (p<0.05) differential productivity of above ground biometric components (culm diameter, culm height, culm fresh weight, foliage fresh weight and above ground fresh weight) for the seven bamboo species. Tukey’s HSD post-hoc test, revealed that on an average D. hamiltonii has highest biometric productivity for culm diameter D (5.99 cm), culm height H (11.91m), culm fresh weight (16.92kg), foliage fresh weight (1.53kg) and above ground fresh weight (20.39kg). Forbranch fresh weight, though D. hamiltonii was found to be have the highest productivity but not significantly higher than B. balcooa, B. bambos, B. nutans and B. vulgaris (Table 3). Differential growth rate of the seven bamboo species, evidenced need of species specific relationship rather general relationship study for establishing model for biomass prediction.
Ratio of dry to fresh weight of seven bamboo species were estimated for culm, branch, foliage and above ground biomass to convert fresh weight to dry weight using the formula: dry weight (kg) = Ratio × fresh weight (kg). The ratio in different biomass components within species found significantly different. The highest dry to fresh weight ratio was found for culm (0.63), branch (0.62) and above ground biomass (0.61) in B. nutans, where as, in foliage highest ratio (0.53) was achieved for in D. strictus. The relationship equation between dry weight and fresh weight indicated that variability of fresh weigh directly proportional to dry weight (biomass) (Table 4).
Development and validation of allometric models
From the analysis of above ground and culm biomass, it was found that biomass production in the seven bamboo species has a high positive correlation with diameter at breast height (D) and culm height (H) but did not have significant correlation with age. This was similar to findings point out by Yen et al.42 for bamboo species S. dullooa, P. polymorphum and M. baccifera. So, D, combination of D and H was used for prediction of biomass using combine data of more than three years oldbamboo plantation. Many authors already observed D and D2H as good predictor variable for bamboo allometric modelling 27–29, 34–35. The results revealed that D was a valid independent variable but using volume (D2H) as compound independent variable or incorporation of H as separate independent variable, further improved prediction accuracy for both above ground biomass and culm biomass except bamboo species B. bambos and B. nutans for fresh above ground biomass and B. bambos for fresh culm weight prediction (Table 5-8). Here, use of H as additional independent variable did not improve prediction accuracy but increase prediction error may be due to measurement error in H data or incorporation of H may have masking effect on D during model fitting. A1 with D as independent variable alone, performed very well and usually very close to A2 and A3 model. Further biomass equation using D as independent variable has practical advantage of easy measurement45–46. A1 model was also reported as reliable model for biomass prediction by Yen et al.25, Kaushal et al.21.
General power form allometric equation A2 with volume (D2H) as an independent variable, found to be most reliable for above ground fresh weight prediction in bamboo species D. stocksii and B. vulgaris, whereas for fresh culm weight prediction A2 to be outperformer forfive bamboo species viz., B. balcooa, B. nutans, D. hamiltonii, D. stocksii and B. vulgaris. This finding was consistent with allometric models established for biomass prediction in thin-walled bamboo species by Singnar et al.47 and general findings of Chave et al.37 in woody species for above ground biomass prediction using bole volume (D2H).
Moreover, three parameter allometric equation A3 with D and H as independent variables, found to be most reliable in bamboo species B. balcooa, D. hamiltonii and D. strictus for above ground fresh weight prediction, whereas for fresh culm weight prediction A3was most appropriate model in D. strictus. For A3 use of D and H as independent variable provided more flexibility to establish relationship with biomass response.
The fitting and prediction accuracy of best fitted allometric models for fresh above weight prediction were visually investigated through line plots. The residual plot of prediction error values evidenced that allometric models using weighted maximum likelihood nonlinear fixed effects modelling method capture heteroscedastic relationship between the biomass response and the predictor variables well and produced much lower residuals. Further, residuals found to be random (lack of any pattern) visually which indicate high fitting accuracy (Fig. 3-4).
Generate synthetic values as a proxy height using H-D model
It was very often that best fitted allometric models were not possible to apply due to missing observation of H48. To get H value, synthetic H values were generated as proxy H using H-D allometric model. H-D allometric equation developed for the seven bamboo species revelaed high fitting accuracy with high Adj.R2 (>0.7) except B. nutans (0.69). Highest fitting accuracy was observed in B. bambos, whereas lowest value observedin D. hamiltoni i(Table 9-10). Similar, result was also pointed out by Kempes et al.9 and Sileshi10. H-D relationship was developed using >3 year old bamboo plantation where H reached to plateau i.e., there is no growth in diameter and height49. As per our knowledge, for the first time this study has established allometric modelling for quantifying H-D relationship between the seven bamboo species viz., B. bambos, B. nutans, D. stocksii, B. vulgaris, B. balcooa, D. hamiltonii, and D. strictus.