Introduction

The physical and environmental properties of bamboo make it an exceptional economic resource for a wide range of uses, including poverty reduction. It is one of the fastest growing plants and can be harvested annually without depletion or deterioration of the soil. Bamboos complete their height growth within 2–4 months and have quick renewal capacity (Nath et al. 2004; Yen and Lee 2011). Therefore, the biomass of mature bamboo stands can be assumed to be steady-state (Chen et al. 2009). Bamboo is widely used in construction, for making woven wares and is becoming popular as an excellent substitute for wood in producing pulp, paper, board and charcoal.

Study of culm characteristics of bamboos at different age and diameter classes helps in selection of plus clump or culm which improves the commercial utilization of a particular species (Inoue et al. 2013). Characterization of bamboo in relation to its anatomical, physical, mechanical and chemical properties helps to determine the maturation age, and this enhances processing and utilization (Hisham et al. 2006). The anatomical structure of a bamboo culm determines its properties and characterizes the culm in relation to its end product (Latif 2001). Certain attributes, such as culm size and culm wall thickness influence the range of their usage (Wong 1982). Strength, straightness, lightness combined with hardness, variation in size, ease of propagation, its short growth period to maturity and its availability for harvesting have expanded the range of uses of bamboo (Sharma 1982). Generally bamboo culm matures at two to three years, reaching its maximum strength (Espiloy 1987, 1994; Latif et al. 1990). However, bamboo properties differ by species, age, location and external factors (Hisham et al. 2006; Nath et al. 2015a). Correal and Arbelaez (2010) reported that the age of bamboo is a key factor that affects its mechanical properties. The length of internodes determines the load bearing capacity of bamboo as well as its ability to peel, which in turn influences its use (Pattanaik et al. 2004). Guan et al. (2012) suggested that muli bamboo can be processed into high-value hardwood flooring due to its high-strength. Management strategies can also affect the end use of bamboo (Yen 2015; Nath et al. 2015b).

The genus, Melocanna has two species and is one of the 10 genera of the Paleotropical woody bamboo subtribe Melocanninae under the tribe Bambuseae (BPG 2012). Melocanna baccifera, commonly known as 'muli' bamboo, is native to India, Bangladesh, Myanmar (Burma) and Nepal (Watson and Dallowitz 1992; Banik 2000). In India, it is mainly distributed in the northeastern states (Assam, Manipur, Meghalaya, Mizoram and Tripura) (Biswas et al. 1991). This species is included among the 20 priority bamboo species identified by INBAR and IPGRI (Rao et al. 1998). Of 145 bamboo species found in India, M. baccifera constitutes about 20 % of the total bamboo resource and it is the second largest after Dendrocalamus strictus (45 %) (NCDPD 2010). It is one of the most important bamboo species in terms of its distribution, high economic and ecological values particularly in northeast India. It is the dominant forest bamboo in the hilly tracts of Cachar District with some cultivated stands in the homegardens of the local peoples (Nandy et al. 2004). Knowledge of the mensurational properties of muli bamboo from this region is important for sustainable commercial utilization of this species. The physical characteristics and culm green weight relationship of muli bamboo based on different culm ages and diameter classes has not been reported. The present study was undertaken to investigate culm characteristics of M. baccifera by age class and to develop corresponding regression models. These regression models will be helpful for yield estimation of a stand without harvesting the culms. Our results can facilitate culm selection, yield estimation, and sustainable commercial utilization of the species.

Materials and methods

The Barak Valley region, the southern part of Assam, covers an area of 6922 sq. km. The region shares its border with Dima Hasao District and the state of Meghalaya to the north, the state of Manipur to the east, the state of Mizoram to the south and the state of Tripura and the Sylhet district of Bangladesh to the west (Roy and Bezbaruah 2002). Barak Valley consists of three districts (Cachar, Hailakandi and Karimganj) that support large natural stands of muli and isolated stands in homegardens (Nandy et al. 2004). Forest vegetation of the district is categorized as Cachar tropical evergreen and semi-evergreen forest (Champion and Seth 1968). The present study was conducted in a privately owned forest stand at Innerline Reserve Forest (24°40.392'N and 92°45.292'E) of Cachar District. The muli stand selected for the present study was extensively managed with culm age ranges from current-year to four-year of age. The climate of the area is subtropical warm and humid with average rainfall of 2300 mm, most of which is received during the southwest monsoon season (May to September). Stand characteristics were studied in seven randomly located 5 × 5 m quadrats. The number of culms within the quadrat was recorded according to the age of the culms and culm density was calculated per hectare. Our study area was an extensively managed, homogenous stand with culm ages ranging from current-year to four-year old in ratios of 4:2:2:1:1 and culm size (circumference) ranges from 5–6 cm to 15–16 cm.

Tagging the bamboo culms every year for age determination is a time consuming and laborious method. Moreover, it is very difficult to tag all the culms in forested areas. Culm age can be determined based on the colour of the outline, status of the culm sheath, the outward appearance of culms, and the development of branches and leaves (Banik 2000). A brief summary of age determination is as follows: (1) A currentyear (6 month to 1 year) culm has culm-sheaths attached and the culm surface is covered with a clear white powder, only a few leaves are developed; (2) A 1 year (1-2 year) culm bears culm sheaths that are beginning to rot, white powder on the surface of the culm disappears gradually, and the culm turns light green; (3) On a 2-year-old (2-3 year) culm, the sheath has begun to drop, and the culm bottom has been invaded by mold and turns dark green; (4) On a 3-year-old (3-4 year) culm, the sheath has disappeared from the surface of the culm, which is moldy and has become yellowish green in color; (5) On a 4-year-old (4-5 year) culm, the culm surface is coarse, covered with mold and moss, and turns brownish green. Pattanaik et al. (2004) suggested that the issue of diameter at breast height (DBH) measurement in bamboos needs some investigation. Therefore, the point 1.3 m above ground level where DBH is typically recorded can fall on different positions respect to internodes and on different internode numbers of the culm which can lead to false diameter measurement. To avoid this problem, culm diameter was measured at the middle point of the internode nearest or above to the height of 1.3 or 1.37 m from the ground.

The required data for the present study was gathered from freshly harvested culms. After the selection of culms by age, girth of the culms was measured at the height of 1.3 m from the ground and culms were separated into 11 size classes, viz. 5–6, 6–7, 7–8, 8–9, 9–10, 10–11, 11–12, 12–13, 13–14, 14–15, and 15–16 cm. Three culms were harvested from each available size class of each age class. Height of the culms, internode length and internode diameter from bottom to top were measured and the number of nodes in each culm was counted. Each culm was cut into three equal sections and the fresh weight was recorded separately using a digital scale. Culm wall thickness of the base and apex of each section of the culms were measured using a vernier caliper to the nearest mm. Average culm wall thickness in all four directions and average diameter were calculated. The solid volume was determined for each section.

The solid volume of each section of the culms was calculated using the formula given by Tandug and Torres (1985), which is given below:

$ V{\rm{ = }}\frac{{\left({{\rm{Ba - Bh}}} \right){\rm{ + }}\left({{\rm{bs - bh}}} \right)}}{{\rm{2}}}L $

where: V is the solid volume in cubic centimeter of the section, Ba the area in square centimeter at the large end of the section, Bh the area in square centimeter at the large end of the hollow portion, bs the area in square centimeter at the small end of the section, bh the area in square centimeter at the small end of the hollow portion, and L is the length in centimeter of the section.

The total green weight was derived by summing the values obtained for each section. The total solid culm volume and culm green weight for each culm were determined and used in the fitted regression model.

Statistical analyses were performed using Microsoft Excel (version 2010) and SPSS (version 19). Tukey's test was used to identify significant differences between means (at 5 % level of significance) of culm characteristics by culm age. Descriptive statistical analysis was performed for different parameters. Regression models were developed for each parameter. Culm circumference was converted into diameter and it was categorized into three classes, viz. small (diameter of 2–3 cm), intermediate (diameter of 3–4 cm) and large (diameter of 4–5 cm). Regression equations for internode length and internode diameter on internode number of each age class was developed according to the three diameter classes. Regression equations were also developed for the following relationships: (1) Culm height and culm DBH, (2) Culm green weight and culm DBH, (3) Culm solid volume and culm DBH, and (4) Culm green weight and culm solid volume.

Results and discussion Culm characteristics

Culm height ranged from 3.2 to 15.3 m with a mean of 9.15 m in three-year-old culms and highest culm height was recorded for four-year-old culms (Table 1). Mean culm height for the different age classes were similar. Number of internodes per culm ranged from 14 to 49 with a mean of 35.2 in 1-year-old culms. Krishnaswamy (1956) reported that bamboos vary considerably in size depending on the species, locality and vigour of the clump. Among our five culm age classes, three-year-old culms had the highest mean internode length, internode diameter, total culm height and culm green weight whereas current-year culms had the lowest means for all parameters. Certain attributes, such as culm size and culm wall thickness influence the range of potential culm usage (Wong 1982). Sattar et al. (1990) reported that M. baccifera and Bambusa balcooa attain maturity with respect to bending strength at 3 years.

Internode length and internode number

Internode length of culms in the small diameter class (DBH of 2–3 cm) increased from the basal part of the culm to the 7–10th internode (at heights of 2.30–2.91 m from the ground), and then it decreased gradually to the 18–23rd internode, after which a slight increase was noticed (Fig. 1). Internode length of culms in the intermediate diameter class (DBH of 3–4 cm) increased from the basal part of the culm to the 12–15th internode (at heights of 3.32–3.89 m from the ground), and then decreased gradually to the 31–37th internode, after which a slight increase was again recorded (Fig. 1). Internode length of culms in the large diameter class (DBH of 4–5 cm) increased from the base of the culm to the 16–20th internode (at heights of 4.81–5.09 m from the ground), and then decreased gradually to the 39–46th internode, after which a slight increase was again recorded (Fig. 1). The relationship was best represented by a third order polynomial regression model in all diameter classes of each age class (Fig. 1).

Internode length varied from base to top and this variation was specific to culm diameter class. The relationship between internode length and number of internodes was best represented by a third order polynomial regression model in three culm diameter classes of each culm age class. Shigematsu (1958) reported that internode length of bamboo is species specific but there is also intraspecific variation.

Internode diameter and internode number

Internode diameter was greatest at the first internode and gradually declined towards the tip of the culm of the smaller and intermediate diameter class in all age classes (Fig. 2). A slight increase in internode diameter was recorded in higher diameter class culms at the basal portion to the 12–15th internode, beyond which it gradually declined towards the tip of the culm (Fig. 2). A second order polynomial regression model (Fig. 2) accounted for 96.4–98.6, 98.4–100 and 98.1–99.2 % of the variation in smaller, intermediate and higher diameter class culms, respectively (Fig. 2). For current-year culms:Model:

$ \begin{array}{*{20}{l}} {{Y_1} = a - \left( {{b_1}{X^2}} \right) + \left( {{b_2}X} \right)}\\ {{Y_1} = 2.8104 - \left( {0.0037{X^2}} \right)}\\ { + \left( {0.0111X} \right)...\left( {{\rm{for}}\;\;{\rm{small}}\;\;{\rm{diameter}}\;\;{\rm{class}}\;\;{\rm{culm}}} \right)}\\ {{Y_2} = a - \left( {{b_1}{X^2}} \right) + \left( {{b_2}X} \right)}\\ {{Y_2} = 3.6646 - \left( {0.0015{X^2}} \right)}\\ { + \left( {0.0111X} \right)...\left( {{\rm{for}}\;\;{\rm{intermediate}}\;\;{\rm{diameter}}\;\;{\rm{class}}\;\;{\rm{culm}}} \right)}\\ \begin{array}{l} {Y_3} = a - \left( {{b_1}{X^2}} \right) + \left( {{b_2}X} \right)\\ {Y_3}{\rm{ = }}4.2828 - \left( {0.0018{X^2}} \right)\\ + \left( {0.0063X} \right) \ldots \left( {{\rm{for }}\;\;{\rm{large }}\;\;{\rm{diameter}}\;\;{\rm{ class}}\;\;{\rm{ culm}}} \right) \end{array} \end{array} $

where, Y₁, Y₂, Y₃ internode diameter (cm); X internode number; a constant; b₁, b₂ regression coefficients.

The regression models for other culm age classes are given in Fig. 2. Unlike internode length, which increased until a considerable height from ground level, internode diameter gradually decreased towards the tip of the culm in smaller and intermediate diameter class culms. Internode diameter varied from base to top of the culm and this variation was specific to culm diameter classes. Pattanaik et al. (2004) developed a third order polynomial regression model for internode diameter that yielded an R² value of 0.826. However, in this study the relationship between culm diameter and internode numbers is best represented by a second order polynomial regression model for all culm age classes (with R² values of 0.96–0.99, 0.98–0.99 and 0.98–0.99 for small, intermediate and large diameter class culms, respectively, of all culm age classes).

Culm height and culm DBH

Mean culm height was highest for three-year old culms (with 9.15 m and maximum of 14.58 m), but four-year old culms had maximum mean culm height of 15.3 m (Table 1). Lowest mean culm height was recorded for current-year culms (8.37 m). The regressions developed for the five age classes of height (HT) on DBH are shown in Table 2. Pattanaik et al. (2004) developed a third order polynomial regression for culm height as the best fit with a correlation coefficient r = 0.69 (culm diameter was taken at the eighth internode). Watanabe and Ueda (1976) developed an exponential model for culm height of Japanese bamboo Phyllostachys bambusoides. In our study the relationship between culm height and DBH was best represented by a linear regression model and the model explained 79.7–93.3 % of variability in the dependent variable.

Culm green weight and culm DBH

Culm green weight was highest in three-year-old culms (mean = 5.42 kg/culm and maximum = 12.87 kg/culm) and lowest in current-year culms (mean = 3.66 kg/culm and maximum = 7.61 kg/culm) (Table 1). Regression of culm green weight on culm DBH is shown in Table 3. All regression equations and intercepts of culm green weight were significant at p < 0.01. DBH was highly correlated (r = 0.93–0.98) with total culm green weight within culm age classes. A linear regression model best fitted culm green weight and was highly significant (p < 0.01). The model accounted for 87.1–96.8 % of the variation in the dependent variable.

Culm volume and culm DBH

Mean culm volume was highest in one-year old culms (6533.93 cm³/culm) followed by three-year (6438.62 cm³/ culm), four-year (6222.43 cm³/culm), two-year (5903.35 cm³/ culm) and current-year-old culms (5735.65 cm³/culm) (Table 1). For five age classes of muli bamboo the regression equations developed between culm volume and culm DBH are presented in Fig. 3. A third order polynomial regression model was fitted for current-, two-and three-year old culms, whereas a second order polynomial regression model best fitted the oneand three-year old culms (Fig. 3). The models accounted for 94.6-98.4 % of the variation in the dependent variable.

Culm green weight and culm volume

The highest coefficient of determination (R² = 99.2 %) was obtained for three-year old culms followed by fouryear (R² = 98.8 %), two-year (R² = 97.2 %), one-year (R² = 97.2 %) and current-year culms (R² = 92.4 %) (Table 4). The regression results shown in Table 4 indicate a highly positive linear relationship between culm volume and culm green weight for all culm age classes. The fitted regression lines are shown in Fig. 4. Tandug and Torres (1985) and Mohamed et al. (1991) suggested that this type of information could be applied in determining the pulp yield of bamboos for paper manufacture.

Conclusions

M. baccifera has high economic value and therefore, culm characteristics studied for five different culm age classes can help in culm selection procedures for better utilization of the species. Lower green weight and solid volume in currentyear culms reflects their unsuitability for diverse uses. Age specific regression models for culm DBH on culm green weight and culm solid volume will guide researchers, forest departments and resource managers in sustainable stand management and accurate yield prediction of the species.

Acknowledgments

Senior author is grateful to Mohan, Appu and Mukta for their help during the field study. Financial assistance from University Grants Commission, New Delhi is acknowledged. We thank two anonymous reviewers for their comments to improve the manuscript.