Influence of spacing and nutrient regimes on aboveground biomass production of Dendrocalamus brandisii (Munro) Kurz. in abandoned rice field of Kodagu, Karnataka

Introduction

Bamboo is an exceptionally fast-growing, non-timber forest resource that plays an increasingly vital role in global sustainable development, bioenergy production, and land restoration (Liese and Kohl, 2015) [9] . As a perennial woody grass, bamboo exhibits biomass accumulation rates that rival or exceed many fast-growing hardwood plantation species (Nath et al., 2015; Yen, 2016) [13,22] . Beyond its economic utility, bamboo provides critical ecosystem services including soil stabilization, watershed conservation, and rapid carbon sequestration, making it a key candidate for climate change mitigation frameworks (Song et al., 2011; FAO, 2020) [19,6] . Worldwide, bamboo covers over 35 million hectares across tropical and subtropical zones, supporting complex rural value chains and industrial applications (FAO, 2020) [6] . India represents one of the primary centers of bamboo diversity, hosting approximately 136 species across 36 genera (National Bamboo Mission, 2020) [14] . Among indigenous and naturalized tropical species, Dendrocalamus brandisii (Munro) Kurz., commonly known as Sweet Bamboo or Velvet Leaf Bamboo, is distinguished by its large dimensions, thick culm walls, high tensile strength, and edible young shoots (Tewari, 1992; Bisht et al., 2010) [21,1] . Cultivated widely across South and Southeast Asia, D. brandisii is increasingly integrated into plantation forestry and agroforestry systems aimed at pulp production, engineered structural timber, biochar, and bio-energy feedstocks (Singh et al., 2018) [17] . The productivity of managed bamboo plantations is governed primarily by silvicultural decisions regarding stand density (spacing) and soil fertility management (nutrients) (Kleinhenz and Midmore, 2001) [8] . Spacing directly controls light interception, canopy closure, root spatial competition, and culm recruitment rates (Shanmughavel and Francis, 1996; Nath et al., 2009) [16,12] . High planting densities maximize initial stand-level biomass per unit area, whereas wider spacings promote individual clump vigor, diameter growth, and lower mortality due to reduced intra-specific competition (Kleinhenz and Midmore, 2001; Emberton, 2008) [8,4] . Simultaneously, nutrient management is paramount during early establishment. Sympodial bamboos demand substantial macro-nutrients (N, P, and K) to sustain rapid seasonal shoot emergence and rapid culm elongation (Rao et al., 1990; Suos et al., 2020) [15,20] . Nitrogen drives vegetative vigor and photosynthetic activity, phosphorus facilitates subterranean rhizome branching and root expansion, and potassium improves osmotic regulation and structural tissue lignification (Nath and Das, 2011; Mailappa et al., 2017) [11,10] . Although the standalone responses of bamboo species like Bambusa bambos, Dendrocalamus strictus, and Phyllostachys pubescens to fertilization or spacing are well-documented (Shanmughavel and Francis, 1996; Mailappa et al., 2017) [16,10] , empirical research evaluating the interactive effects of spacing and nutrient regimes on D. brandisii-particularly when established on marginal or degraded agricultural lands such as abandoned paddy fields remains sparse. Therefore, this study investigated the influence of two spacing treatments and three NPK nutrient levels on the aboveground biomass partitioning and total yield of D. brandisii in the humid tropics of Kodagu, Karnataka, India.

Materials and Methods

The experiment was conducted during 2024-2025 at Halligattu village near the College of Forestry, Ponnampet, Kodagu district, Karnataka, India (12°08′ N, 75°55′ E; 867 m above mean sea level), situated in the Hilly Zone (Zone 9) of Karnataka, which experiences a humid tropical climate with an annual rainfall of 2,500-3,500 mm received mainly during the southwest monsoon (June-September). The study site comprised an abandoned lowland rice (Oryza sativa) field of approximately 0.96 ha with sandy loam soil, acidic pH (5.1-5.4), and poor natural drainage. To prevent seasonal waterlogging, a drainage system consisting of main collector trenches (1.0 m × 1.5 m) and secondary sub-trenches (0.35 m × 0.45 m) was established before transplanting. The experiment was laid out in a Split-Plot Design with three replications using healthy, uniform tissue-cultured seedlings of Dendrocalamus brandisii, transplanted in June 2020 at the onset of the monsoon. At the time of destructive sampling during 2023-2024, the plantation was three years old. The treatments consisted of two spacing regimes, namely M1 (4.0 × 5.0 m; 500 clumps ha -1 ) and M2 (5.0 × 5.0 m; 400 clumps ha -1 ), combined with three nutrient regimes: S 1 (recommended dose of fertilizer, 25 g N: 9 g P 2 O 5: 25 g K 2 O plant -1 year -1 ), S 2 (31.25 g N: 11.25 g P 2 O 5: 31.25 g K 2 O plant - 1 year -1 ), and S 3 (37.50 g N: 13.50 g P 2 O 5: 37.50 g K 2 O plant - 1 year -1 ). Nutrients were supplied through urea (46% N), single superphosphate (16% P 2 O 5 ), and muriate of potash (60% K 2 O), applied in two equal splits during May and September using a circular trench method at a 30 cm radius from the clump margin. Aboveground biomass (AGB) was estimated using destructive harvesting following standard procedures for sympodial bamboo (Brown, 1997; FAO, 2004) [2,5] . Culms were classified into four age classes (<1, 1-2, 2-3, and 3-4 years) based on sheath retention, culm colour, and branch development. Representative culms from each age class and treatment combination were harvested at ground level (<10 cm stubble height) and separated into culm, branch, and leaf components. Fresh weights were recorded using a digital hanging balance (±0.01 kg), after which approximately 1 kg subsamples of each component were oven-dried at 75 °C to constant weight for determination of moisture content and dry biomass. Total culm biomass was obtained by summing the dry biomass of culm, branches, and leaves. Clump biomass was estimated by multiplying the mean dry biomass of each age class by the corresponding number of culms per clump and summing across all age classes, while stand biomass (t ha -1 ) was calculated by multiplying clump biomass by the planting density of the respective spacing treatment (500 clumps ha -1 for M1 and 400 clumps ha -1 for M2). The observations recorded on growth parameters of bamboo and soil properties were statistically analyzed following the procedures outlined by Jayaraman (2001) [7] . The experimental data were subjected to analysis of variance (ANOVA) appropriate for a split plot design using GRAPES statistical software.

Results and Discussion

Main Effects of Spacing and Nutrient Regimes

ANOVA revealed that neither spacing regimes, nutrient levels, nor their interactions produced statistically significant differences (p > 0.05) in total aboveground stand biomass of three-year-old D. brandisii (Table 1). Despite the lack of statistical significance, clear numerical patterns were observed across treatments. Closer spacing (M 1: 4.0 × 5.0 m) achieved a higher mean stand biomass (60.75 t ha -1 ) than wider spacing (M 2: 5.0 × 5.0 m, 59.39 t ha -1 ). The 2.3% yield increase in M 1 was largely attributable to greater initial stem density (500 vs. 400 clumps ha -1 ) rather than greater individual clump weight. Individual clumps in wider spacing (M 2 ) demonstrated higher average clump biomass, indicating reduced lateral light competition and broader root expansion room, which partially compensated for lower stocking density. These findings align with Shanmughavel and Francis (1996) [16] for Bambusa bambos and Nath et al. (2009) [12] for Dendrocalamus hamiltonii, where stand density acted as the primary driver of total area-based yield during early establishment phases. The application of enhanced fertilizer levels resulted in a positive numerical response in total biomass accumulation. The S 3 treatment achieved the highest biomass production (64.10 t ha -1 ), followed by S 2 (59.82 t ha - 1 ) and S1 (56.29 t ha -1 ). The 13.88% increase in biomass from S 1 to S 3 underlines the responsiveness of young bamboo stands to increased macro-nutrient inputs. Nitrogen and potassium supplementation accelerate leaf area development and net assimilation rates during annual shoot expansion cycles, as previously highlighted by Rao et al. (1990) [15] and Mailappa et al. (2017) [10] . The interaction treatment M 2 S 3 (5.0 × 5.0 m) registered the highest absolute biomass (64.20 t ha -1 ), closely followed by M 1 S 3 (64.00 t ha -1 ). Under wider spacing, individual clumps faced less canopy overlap, enabling them to convert higher fertilizer availability into structural biomass more efficiently. This interaction trend suggests that as plantations mature, wider spacing coupled with adequate fertilization may optimize individual clump yield and simplify long-term harvest management (Das and Chaturvedi, 2006; Singnar et al., 2017) [3,18] .

Age-Class Biomass Allocation Patterns: Biomass contribution varied significantly across culm age classes. Young culms (<1 year) accounted for over 52% of total aboveground stand biomass across all treatments. The 1-2 year age class contributed approximately 26%, while older culms (2-3 and 3-4 years) contributed 14% and 8% respectively. This structural allocation reflects the physiological growth strategy of sympodial species: young shoots complete their primary height and diameter growth within a single growing season (3-4 months) using stored rhizome reserves and current-year photosynthates (Liese and Kohl, 2015) [9] . Consequently, initial stand productivity is heavily dictated by annual shoot emergence rates and survival during early growth stages.

Conclusion

This study evaluated the effects of spacing and nutrient management on the aboveground biomass of Dendrocalamus brandisii established on an abandoned paddy field in Kodagu, Karnataka. Spacing, nutrient regimes, and their interactions did not yield statistically significant differences in stand-level biomass at three years of age (p > 0.05). However, biological trends showed that close spacing (4.0 × 5.0 m) produced higher total stand biomass (60.75 t ha -1 ), while higher fertilizer application achieved maximum biomass yield (64.10 t ha -1 ). Culms under one year of age accounted for more than half of the total aboveground biomass. These results provide useful baseline parameters for establishing commercial D. brandisii plantations and integrating bamboo into tropical carbon offset and land rehabilitation programs.

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