Ecological implications of village bamboo as global climate change mitigation strategy: A case study in Barak Valley, Assam, North East India Open Access

Since 1750, the atmospheric concentration of carbon dioxide has increased by about 32 percent (from about 280 to 376 parts per million in 2003), primarily due to the combustion of fossil fuels and land use changes (IPCC, 2007). Approximately 60 percent of that increase (60 parts per million) has taken place since 1959 (MEA, 2005). Terrestrial ecosystems were on average a net source of CO₂ emissions during the nineteenth and early twentieth centuries, but became a net sink around the middle of the last century, and thus in the last 50 years the role of ecosystems in regulating global climate through carbon sequestration has also been enhanced (MEA, 2003).

Homegardening is the oldest land use activity next only to shifting cultivation. It evolved through generations of gradual intensification of cropping in response to increasing human pressure and the corresponding shortage of arable lands (Kumar and Nair, 2004). The rural lives in Assam are intricately linked with the bamboos of homegarden (Nath and Das, 2008). Under traditional homegarden management system bamboo plantation development is the inherent ecological consistency of local farming practices. For practical purpose farmers have divided the homegarden system into different landforms depending on the suitability of the species for different land quality. Mostly the farmers in these locations are subsistence‐oriented and they maintain multistrata homegardens including trees, shrubs, and herbaceous plants. Bamboo is one of the more important components of the homegardens, which provides the villagers with a wide range of goods and services. Bamboos have socio‐economic and ecological values and its management can provide benefits on a local, national and global level through livelihood, economic and environmental security for many million of the rural people (Nath et al., 2009). Bamboo plantation development and its management in homegarden have both ecological and economical benefits to the rural life and can become an effective choice for climate change mitigation strategy. However, Liese (2009) and Düking et al. (2011) raised their concern on false expectations on the significance of bamboo forests as carbon sinks. They further emphasized the misunderstandings about the growth of bamboo culms that can lead to highly exaggerated expectations on the productivity of bamboo.

Only a few studies have demonstrated the potential of bamboos to function as carbon storage and carbon sinks (Isagi et al., 1997; Das and Chaturvedi, 2006). Therefore, understanding of C storage and C sequestration potential of bamboos is crucial to evaluate the role of bamboo in homegarden in environmental and economical sustainability. The paper combines the potential for small‐holder bamboo farmer in C stock management and C mitigation through sequestration along with growth strategy and population status of the bamboos in homegarden.

The present study was conducted in Irongmara and Dargakona village, in Cachar district of Barak Valley, Assam, North East India and is situated between longitude 92°24′ E to 93°15′ E and latitude 24°22′ N to 25°8′ N (Figure 1).

The study villages' dates back from the British colonial rule and most of the inhabitant of villages are tea garden labourers. Bamboo forms one of the important components in homegardening system of study area and its occurrence was observed in all the homegarden.

About 100 homegardens and 40 bamboo groves were selected from the study site. Selection criteria for homegarden was its size (<1 ha). Since majority of the homegarden owners were small holders and large holders represents only a small fraction of the study village, sampling was done mostly for small holders. Bamboo groves are the extended part of the homegarden and 50 percent of the villagers in the study area manage this land use for commercial utilization. About 40 bamboo groves were randomly selected from 100 homegarden owners that were considered for the present study. Inventorization and prioritization of village bamboos was studied by surveying the selected homegardens and bamboo groves and subsequently noting the number of clumps of different bamboo species.

Culm growth was observed during July‐November. Because bamboo is a quick growing plant and its height is stabilized over a comparatively short span of time, longitudinal study was preferred over random samples at different times (Hills, 1974). About 25 newly sprouted culms of each species was selected randomly and identified with numbered aluminum foil. Since all the 25 culms began their life almost together, their average height was considered to study the pattern of growth.

To study the growth rate in height, the average rate of change in height between any two time epochs, say t₁ and t₂, is measured as: Equation 1 where y₁ and y₂ are heights at time t₁ and t₂.

Sub‐samples of culm, branch and leaf from different culm ages for the three species were ground in a Wiley mill and analyzed for carbon content determination. A total of 50 percent of the ash free mass was calculated as the carbon (C) content. The ash content was determined by igniting 1 g of powdered litter sample at 550°C for 6 h in a muffle furnace (Allen, 1989). The carbon storage in the different culm component was determined by multiplying the biomass with carbon content. Detailed of the estimation of biomass C and C sequestration rate is described in Nath et al. (2008, 2009).

Litter floor mass was collected at three‐month intervals, from five different plots of 50 cm×50 cm sized quadrate laid randomly at each occasion. From each plot floor mass was separated into leaf, sheath and branch litter and weighed separately. Sub‐samples were collected and oven dried at 70°C to a constant weight. Oven dried samples were powdered for further analysis.

Soil samples from three replicates were collected at depths of 0‐10, 10‐20 and 20‐30 cm. A composite sample was prepared for each depth, air‐dried, ground and passed through a 2 mm sieve and stored in plastic container. Organic carbon content of the soil samples was examined by Walkley and Black's rapid titration method (Jackson, 1958). Soil organic carbon (Mg ha⁻¹) was computed by multiplying soil weight per given soil volume with mean C values.

One way ANOVA was performed to analyze the significant difference between the number of bamboo clumps in homegarden and bamboo groves, culm density among the species, biomass C stock among the species and biomass C accumulation among the different culm components.

The climate of the study site is sub‐tropical warm and humid with average annual rainfall of 2,226 mm, most of which is received during the Southwest monsoon season (May‐September). Southwest monsoon usually operates for a longer spell in the North Eastern region compared to the other parts of India. Average maximum and minimum temperatures were 30.5°C and 20.3°C, respectively. The average relative humidity varied between 48 percent (January) and 97 percent (June).

Inventorization of village bamboo species are described in Table I. Traditional homegardens and bamboo groves of Barak Valley are rich in bamboo resources as also in other parts of the North East India. Among the village grown bamboo species, 74 and 90 percent bamboo growers are growing B. cacharensis, 68 and 78 percent growing B. vulgaris and 53 and 70 percent are having B. balcooa in their homegardens and bamboo groves, respectively (Figure 2). Bamboo growers growing, B. nutans, S. dullooa, B. assamica and G. albociliata are comparatively few in number in both homegardens and bamboo groves. In the study area G. albociliata was found only in the homegarden. Depending on the villagers' species preference, B. cacharensis, B. vulgaris and B. balcooa comprised the priority village bamboo species of the study site. Number of bamboo clump per homegarden and bamboo grove represented, B. cacharensis grown in highest number (3.84 and 10.56 clump), followed by B. vulgaris (1.50 and 4.26 clump) and B. balcooa (1.07 and 3.13 clump). B. nutans, S. dullooa, B. assamica and G. albociliata are grown in very few numbers by the bamboo grower in both homegarden and bamboo grove (Table I). Total number of bamboo clumps in bamboo grove (20 clumps) is three fold higher than the total clump number in homegarden (seven clumps).

Inventorization of village bamboos revealed the richness of different bamboo species in the traditional land use system of homegarden. The traditional homegardens of the study site are rich in bamboo resources as also in the homegardens of other parts of India, Bangladesh, Malaysia and Indonesia (Randhawa, 1980; Widjaja, 1991). The traditional bamboo growers of the study site have prioritized three bamboo species under one genus against 14 species under five genera from the rest of India and 38 species under 11 genera from the world as a whole (Williams and Rao, 1994). Among the three priority bamboo species, B. cacharensis is the highest preferred and this species is frequently cultivated and widely distributed in the home gardens of this region. Adaptability to different agro climatic conditions, durable culm quality, high pulping quality, high biomass productivity and quick recovery of clumps after felling with rapid extension growth, desirable growth architecture and multiple uses (Nath et al., 2004) made the species as highly preferred in the study site. Prioritization of bamboo species by bamboo growers in homegarden and bamboo grove largely depends on their needs, site characteristics and adaptability of a bamboo species. The common bamboo species of Bangladesh homestead are B. balcooa, B. nutans, B. tulda, B. longispiculata and B. vulgaris. The selection and cultivation of a bamboo species is mainly dependent on people's choice and its economic evaluation (Banik, 2000). The other two priority species of study site B. balcooa and B. vulgaris are also the priority bamboo species for national and international action. Among the village grown bamboo species of Bangladesh B. balcooa and B. vulgaris are at the top of the list (Banik, 2000). As the local bamboo growers completely rely on B. cacharensis to fulfill their basic household, commercial and craft requirements, this species is grown in maximum number of clumps in both homegardens and bamboo groves. Abundance of village bamboo clumps with holding size is in consistent with the reports of bamboo occurrence and abundance is strongly dependent on the size of the holdings (Kumar et al., 2005). Because of the great importance of bamboo in the rural socioeconomy, such regional level inventorization and prioritization of bamboos can provide the basic information for developing scientific ways and means to increase the income of rural farmers from bamboo resources.

The height growth curve of the culms for all the three species has a smooth S shape (Figure 3). The shape of the growth curve is described by the rate of change of culm growth at different times. The total culm extension growth in B. cacharensis, B. vulgaris and B. balcooa was 135, 113 and 118 days, respectively. From the growth rate curve of culm height it was observed that for all the three species during the first 15 days height growth per day ranged from 1 to 3 cm. However, over the next 40‐50 days the growth rate was very fast, and ranged from 3.0 to 28 cm in B. cacharensis, 4.0 to 33.0 cm in B. vulgaris and 2.5 to 26 cm per day in B. balcooa. From the first week of August to second week of September growth rate varied between 15.0 and 28.0 cm per day in B. cacharensis, 18.0 and 33.0 cm per day in B. vulgaris and 15.0 and 26.0 cm per day in B. balcooa, and this period may be termed as peak growth period. From the first to second week of September growth rate starts declining in all the species and varied between 0.5 and 3.0 cm per day and finally showing zero growth by the middle of November in B. cacharensis and end of October in both B. vulgaris and B. balcooa.

Studies on the culm growth extension revealed the brief periodicity of culm growth nature of bamboo. Periodicity of culm extension growth in the present study is comparable with the 135 days in B. bambos (Shanmughavel and Francis, 1996). However, compared to M. baccifera (Nandy et al., 2004) the periodicity of culm extension growth in the present study is much lower. The shorter extension period could be a strategy to allocate more resources towards culm thickness in thick walled culms as in B. balcooa. Growth rate curve exhibited initial slow growth rate that gradually peaked and retained for about two months and declined thereafter with finally no further culm height increment during the last week of October to middle of November. During the rapid culm growth period, 66, 73 and 78 percent of the total height growth is attained in B. cacharensis, B. vulgaris and B. balcooa, respectively, within 25‐30 percent of the total culm extension period. Such a peak elongation period was also reported by Rao et al. (1990) in N. dullooa. Peak culm elongation rates must be physiologically demanding but necessary because culms that failed to achieve mature length of 10‐20 m before the onset of dry season were unable to subsequently renew the process (Franklin, 2005).

Biomass C stock was estimated from the culm density in homegarden (Table II). Biomass C stock in homegarden ranged from 7.60 to 10.76 Mg ha⁻¹ from 2003 to 2007. Species wise comparison revealed B. cacharensis contributed 40‐44 percent of the total C stock in homegarden. The corresponding figure for B. vulgaris and B. balcooa was 30‐33 percent and 22‐24 percent. Biomass C stock among the three species varies significantly at 5 percent level of significance. Biomass C in different culm component for three bamboo species in homegarden is depicted in Table III. Total biomass C stock in different culm component in homegarden was correlated with five year study period (Figure 4).

The consistent trend of increase in above ground C stock in homegarden might have resulted from the increase in culm density over the study period under the traditional management system. The farmers in the study area fell less number of culms per clump than it produce annually. Since the culm density of bamboo in homegarden is progressively consistent, C stock in them is increasingly stable. Estimated C stock of 4.87 and 14.62 Mg ha⁻¹ in agricultural and agroforestry systems in terai zone of India has been reported (Koul and Panwar, 2008). C stock in agroforestry practices has been estimated as 9, 21, 50, and 63 Mg C ha⁻¹ in semiarid, subhumid, humid, and temperate regions (Montagnini and Nair, 2004). Biomass C stock ranged from 0.7 to 54.0 Mg C ha⁻¹ in traditional and improved agroforestry systems in the West African Sahel (Takimoto et al., 2008). Since bamboo is one of the components in multistrata mixed species homegardening system, bamboo farming system in homegarden had relatively smaller C stock than other agroforestry systems. Moreover, in comparison to other tree species bamboo is relatively low biomass plant that conversely reduces its ability to store more C.

The rate of leaf biomass C accumulation in homegarden ranged from 0.017 to 0.089 Mg ha⁻¹ yr⁻¹. The corresponding rate of C accumulation in branch, culm and total biomass was 0.025‐0.071, 0.391‐1.018 and 0.432‐1.178 Mg ha⁻¹ yr⁻¹, respectively. Biomass C accumulation varies significantly among the different components at 5 percent level of significance for the three species. Comparison among the species showed rate of leaf biomass C accumulation was highest for B. vulgaris (0.015 Mg ha⁻¹ yr⁻¹) while branch (0.018 Mg ha⁻¹ yr⁻¹), culm (0.292 Mg ha⁻¹ yr⁻¹) and total (0.327 Mg ha⁻¹ yr⁻¹) was highest for B. cacharensis (Table IV).

In bamboo the C sequestration potential is determined by the new culms produced annually. In homegarden under farmers' management system new culms are restricted from felling and hence almost all C sequestrated through it can be assumed as a net gain, that further demonstrates the small‐holder bamboo farming systems can sequestrate C while also fulfilling basic rural needs from harvesting mature culms. In homegarden under selective felling system although the C stock is low, but represents a permanent C stock as C export through harvesting of mature culms are balanced by C gain from new culms produced in the clump (Nath and Das, 2011). Long rotation systems such as agroforests and homegardens can sequester sizeable quantities of C in plant biomass and in long‐lasting wood products (Albrecht and Kandji, 2003) besides having other secondary environmental benefits (Pandey, 2002) and important role in reclamation of marginal, sloping agricultural land and degraded land through bamboo plantation (Mertens et al., 2008). Lower C/N ratio in bamboo soil compared to pasture soil signifies higher availability of soil nutrients in the former (Tian et al., 2007) which further strengthens the role of bamboo in soil reclamation. For smallholder agroforestry systems in the tropics, potential C sequestration rates range from 1.5 to 3.5 Mg C ha⁻¹ yr⁻¹ (Montagnini and Nair, 2004). C sequestration at abandoned agricultural land and degraded forest land sites in Central Himalayan region was 1.79‐3.13 Mg ha⁻¹ (Maikhuri et al., 2000). Although the amount of C sequestered is less in bamboo farming system of homegarden than any other agroforestry system, bamboo also meet the felt needs of rural household other than providing the villagers with wide range of economic and environmental services. Therefore, potentiality of homegarden bamboo offers a feasible alternative for rural land use management as Tian et al. (2007) emphasized benefits of bamboo in land use conversion from their study in Montane Ecuador.

Litter floor mass of the bamboo stand studied was 300.43 kg ha⁻¹. Leaf, sheath and branch components of the floor mass recognized the C concentrations to be 37.02, 41 and 45.73 percent, respectively. Carbon storage in the floor mass was 115.3 kg ha⁻¹, of which leaf litter made up the highest amount (76.32 kg C ha⁻¹) followed by sheath (35.34 kg C ha⁻¹) and branch (3.64 kg C ha⁻¹) (Table V). Carbon storage in the soil up to 30 cm depth was 50.1 Mg C ha⁻¹. C storage in the soil decreased with increase in depth (Table VI). About 43 percent of the total carbon storage in the soil was confined to the upper 10 cm of the soil surface.

C storage of 115 kg ha⁻¹ through litter floor mass reflects its importance as an important C sequester source, which in turn has positive impact on environmental services and biodiversity aspect of the ecosystem (Nath and Das, 2009). The consequences of an increased litter floor mass can increase the soil fertility, the land productivity for greater production and the prevention of land degradation. Soils are one of the largest carbon reservoirs of the terrestrial carbon cycle. The quantity of C stored in the soil decreased with increase in depth and most of it is confined to the organic matter bound soil layer (0‐10 cm). The average carbon content for closed forest is assumed to be 133 Mg ha⁻¹ in the top 100 cm and 72 Mg ha⁻¹ in the top 40 cm (Bolin et al., 1986). For open forest, the equivalent estimates are 80 Mg C ha⁻¹ in top 100 cm and 49 Mg C ha⁻¹ for top 40 cm (Bolin et al., 1986); 50.1 Mg C ha⁻¹ up to top 30 cm in the present study is comparable with the C storage in the top layer of closed forest. C storage in soil is the balance between the input of dead plant material and losses from decomposition and mineralization processes. C storage in the soil up to 30 cm in the present study is greater than that of the C storage in the above ground biomass. Therefore, bamboo soil has important implications in the manipulation of atmospheric CO₂ as does the soil of tropical forests.

Village bamboos have been used extensively for household and craft preparation by the villagers for long time. Villagers are required to depend substantially on village bamboo resources for construction of their new houses as well as repairing of old houses. Bamboo provides the major framework of traditional houses in the study village. Different components are used for constructions of rural houses include pillars, walls, ceiling and roof. Nath et al. (2009) reported traditional craftsperson in the study village prepares a total of 40 different fishing and agriculture related products to fulfill their own needs and to sell in the market throughout the year. Therefore, utilization of village bamboo offers the opportunity to maintain and increase carbon stocks through carbon sequestration as long as the products are not burnt and are instead used for durable products (Nath et al., 2009).

Climate change across the world certainly needs particular attention for its mitigation. To mitigate the increasing concentration of GHGs in atmosphere cost‐effective methods for emission reduction have been emphasized. Finding low‐cost methods to sequester carbon is emerging as a major international policy goal in the context of global climate change (Montagnini and Nair, 2004). The role of agroforestry systems across the world has been prioritized in C sequestration while bamboos in particular remain unexplored. Rapid phase of culm growth and subsequent greater resource acquisition resulted into higher shoot productivity and consequently greater biomass carbon storage in the village bamboo stand. Short periodicity of culm growth, rapid culm elongation rate and brief clump development period makes village bamboo as a prospective resource to sink atmospheric CO₂ (Nath et al., 2008). It was found that 43 percent of the total soil carbon is stored in the upper soil layer implying the role of village bamboo management in prevention of soil erosion in village landscape. Mean total carbon storage (vegetation + litter floor + soil) in homegarden is 55.22 Mg ha⁻¹.

All these suggest that village bamboo management can help to mitigate climate change while simultaneously providing subsistence needs of the rural people and other ecosystem services. Consistent standing stock of C in homegarden bamboo management may provide small‐holder farmers an option of selling carbon credits under climate change agreement. In December 2003, the 9th Conference of the Parties to the UNFCCC resolved to include small‐scale forestry as an eligible activity under the Clean Development Mechanism of the Kyoto Protocol. Under villagers management system, C storage and sequestration values obtained suggest bamboos in homegarden is cost effective in terms of carbon sequestered. Sustainable management and utilization of bamboo resources can further increase the amount of carbon sequestered which may increase the carbon storage capacity within the rural landscape in the short‐term, and in long‐term through uses of durable products.

Carbon credit under the climate change agreement provides an opportunity to reduce the emission of green house gases. The rural landscape of Barak Valley endowed with bamboos has a good potential to earn carbon credit if we can utilize its ability to sequester carbon. Therefore, more emphasis is needed to utilize the bamboo resources of the region for social, ecological and economical upliftment of the region.

This work was supported by a research grant from the G.B. Pant Institute of Himalayan Environment and Development, Almora sponsored project. The authors thank anonymous reviewers for their comments to improve the manuscript.

Arun Jyoti Nath is an Assistant Professor in the Department of Ecology and Environmental Science, Assam University, Silchar, India and is a graduate in Botany and postgraduate in Ecology. He did his doctoral work on ecology and management of village bamboos in the rural landscape of North East India and his present field of research interest is on carbon storage and sequestration by village bamboos in the rural landscape. Currently, he is also associated with Centre for Climate Change and Disaster Management at Assam University.

Ashesh Kumar Das teaches in the Department of Ecology and Environmental Science, Assam University, Silchar, India. He is an ecologist working in the fields of plant biodiversity management and conservation by smallholders and carbon sequestration in different land use systems. Ashesh Kumar Das is the corresponding author and can be contacted at: asheshdas684@hotmail.com