0405-B2

Carbon sequestration by rehabilitating degraded forests in India

Meenakshi Joshi ¹ , Preet Pal Singh ²

Abstract

Earlier studies estimating the carbon mitigation potential achievable by rehabilitation of degraded forests are based on assuming rates of reforestation and single biomass productivity for the entire country. The present study considers carbon storage in a context in which it might succeed: that of using C flux and densities derived from Indian data taking into account the current policy of reforestation of open forests. This study is based on detailed inventory data collected by Forest Survey of India from 1,70,000 sampling units distributed all over the country. Carbon storage in above ground enhanced biomass, litter and forest product pool have been considered. This paper highlights that in Indian context, reduction in growing stock because of degradation is very significant and hence is an important factor deserving attention in any carbon sequestration programme. This study estimates that additional carbon storage of 1008.49 Tg C over a period of 75 years (13.45 Tg C/yr) is possible given the current trends in forest management policy of the government.

Introduction

Greenhouse gases (CHGs) have increased significantly in recent years. A major challenge facing the nations of the world is the need to reduce emissions of greenhouse gases. The main component of CHGs is carbon dioxide (CO₂) forming 0.03% of the atmospheric volume. The concentration of CO₂ alone has increased by 25% since pre-industrial times (IPCC 1992). It is expected to account for more than 50% of the radiative forcing of greenhouse gases released because of human activity in this century. Currently, approximately 75% of the emissions of CO₂ result from the combustion of fossil fuels and about 25% result from changes in land use, largely deforestation (Houghton et al 1993). The range of global warming is predicted to be 1.7-4.9 ^o C. This calls for reduction of CO₂ emissions to acceptable levels such as 450 parts per million by volume by 2100 (Vuuren et al. 2001). Drastic reductions (of the order of 60% or more) are required to stabilize concentrations of greenhouse gases in the atmosphere at today's level (Houghton et al. 1990).

Global concern over accumulation of greenhouse gases in the earth's atmosphere has simulated interest in land-use sector mitigation options. One mechanism, among many others that have been advocated to lower the concentration of atmospheric CO2 is to increase the forest biomass. The main arguments for this measure are, first, that tree biomass fixes large quantities of atmospheric CO₂ over long periods. Second, forest biomass can be used for energy purposes and thereby reduce the use of fossil fuel or can substitute for other materials such as aluminium or steel constructions whose production consume large quantities of fossil fuels. Finally, increasing the standing stock of forest biomass in many ecosystems may give several environmental benefits other than carbon sequestration.

Earlier studies estimating the carbon mitigation potential in forestry sector are based assumption of single biomass productivity for the entire country (Ravindranath et al. 1997, Lal et al. 2000). This assumption completely fails to take into account even the major climatic and edaphic factors affecting the biomass productivities. The approach here is to consider carbon storage in a context in which it might succeed: that of using C flux and densities derived from Indian data taking into account the current policy of reforestation of open forests. This study is based on detailed inventory data collected by Forest Survey of India (FSI, 1995) from 1,70,000 sampling units distributed all over the country. This paper does not discuss the financial and economic aspects of achieving this potential.

Carbon emissions, deforestation, and forest degradation

To estimate the carbon sequestration potential of Indian forest it is necessary to under the dynamics of both deforestation and forest degradation. Deforestation refers to conversion of any forest to other uses e.g. croplands, pastures, or urban land. Degradation refers to reduction in productivity and/or diversity of a forest due to unsustainable harvesting , fire (except for fire dependent eco-systems), pests, and diseases, removal of nutrients and pollution or climate change (RERI 1998). Subsequent to enactment of Forest Conservation Act, 1980, rate of diversion of forestland has declined drastically to around 0.021 million ha (mha) per annum (ICFRE, 2000). Loss of natural regeneration, low growing stock and low productivity are important parameters indicating forest degradation.

Loss of natural regeneration is loss of future potential biomass. Over-grazing and repeated fires eventually affect relatively hardy species too and their ability to regenerate. The soil is also rendered less fertile because of destruction of organic matter. Forest fires also result in loss of natural regeneration. An FSI sample survey conducted in 1995 found that annually fires affect some 53 to 54 percent of forest areas. Majority of fires are deliberate to facilitate collection from ground of commercially important non-timber forest produce as `mahua' (Madhuca indica) and Shorea robusta seeds. It also results in new flush of grass or Diospyros melanoxylon tendu used for rolling country made cigarettes (TERI 1998). As a consequence, natural regeneration is either absent inadequate in 53% of the country's forest .

Effectiveness of the forest to sequester carbon is proportional to mean annual increment. High productivity, therefore, leads to high rate of carbon storage in biomass and wood-products. The productivity of Indian forest is also low. The current productivity of forest is 1.37 cu m/ha, calculated on the bases of net annual increment of 87.62 million cu m and forest cover of 63.7 M ha (FSI 1995), is low when compared with the global average of 2.1 cu m/ha/yr.

FAO estimates 93 t/ha of biomass in natural forests in India. This is low in comparison to 181 and 131 t/ha of biomass in natural forests of tropical Asia and World respectively (FAO 1995). The area under open forest gives an idea of the effect of degradation of the forests. FSI has listed the per hectare value of biomass for various forest types/strata for the year 1995 (13) for canopy densities of >70%, 40-70% and 10 to less than 40%. Average growing stocks in forest of these canopy densities have been used to find the relationship between canopy density and growing stock per ha as shown in Fig-1. The forest with canopy density of 40-70% and 10 - 40% show average growing stock as 74.1% and 28.2% respectively of the growing stock with canopy density of more than 70%. This shows that forest degradation lowers growing stock significantly and is an important factor to be considered in any carbon sequestration strategy.

Fig-1: Growing stock under different canopy density

Source: (FSI 1995)

Methodology

This quantification is based on the satellite data, aerial photography and processing of forest inventory data collected from 1,70,000 sampling units carried out by Forest Survey of India (FSI). Analyzing the data, FSI classified forest in each state into three density classes - D1, D2, and D3 (Table-1)

Table - 1: Classification of forests

Enhancement of above ground biomass: For each state, potential addition in growing stock achievable by increasing the growing stock density of D2 and D3 to D1 was calculated. For converting standing stock in volume to biomass in weight, the volume was multiplied by default (IPCC 1992) Conversion and expansion factor (0.95), to account for small branches of vegetation. The additional carbon storage in tree biomass is calculated by multiplying enhanced biomass by 0.45. Half of this additional carbon sequestered would be stored for an indefinite period. The rotation period followed for management of most of the forest strata is 120 years (FSI 1995). As all the strata have more than 38.0 % and 17.5 % of total number of stems per hectare in 10-20 and 20-30 cm diameter class respectively (Haripriya 2000), the enrichment period is assumed to be 75 years.

Litter layer: The amount of carbon in detritus depends on the decomposition period. A fraction of litter accumulated on the forest floor is decomposed by the decomposing community and the remaining portion is added to the next litter fall. Thus there is a built up forest floor material. In course of time, the income equals the loss and a steady state accumulation is attained. The various forest strata were grouped into tropical, subtropical and temperate forest types based on Champion and Seth's (Champion 1968) classification of Indian forests. This was done keeping in view the limitation of availability of data for litter fall and decomposition constants under Indian conditions. The litter accumulated after time t is given by:

dQ/dt = L-kQ (Olson 1963)

where dQ/dt is rate of accumulation of litter, L is litter fall rate and kQ is rate of litter decomposition. Under steady state condition, dQ/dt = 0, i.e. when rate of decay is equal to rate of litter input, amount of litter present on the forest floor is given by Q = L/k. Maximum litter fall available from the literature was considered to be the litter fall in very dense forest category (Table-2). 74.1% and 28.2% (same as proportion of growing stock already present) of average litter storage was already assumed to be present in the dense and open forest category.

Table - 2: Litter fall rates and decomposition constant for different forest strata.

* Since no study was available on temperate forests of India, it was assumed that litter fall in this forest would be similar to litter fall of sub-tropical forest and the value of decomposition constant (K) was taken from Swift et al. 1979.

Forest Products: Biomass is harvested from the forests has diverse end-uses, which leads to different amounts of carbon storage depending on the conditions and nature of product use. As 10-15% of timber is left on site during harvesting, carbon transferred to the forest products is taken to be 85% of the aboveground biomass. Of the wood that enters forest products, on an average 30% is assumed to be fuelwood production (Rai et al. 1996). The remaining 70% is assumed to be used for industrial use in forest products that can be classified into three lifespan categories: short, medium and long (Table-3). The decay of wood products is assumed to be exponential with half life of 3, 12 and 30 years for short, medium and long category of forest products. The biomass available on harvest for industrial use was distributed based on the proportion of total projected demand for wood in 2020 (MoFF, 2001) by forest products in different lifespan categories. The amount of wood remaining (Q) after time t is given by :

Q = Q₀ exp(-kt)

where Q₀ is initial quantity of wood and k is decay constant ( = 0.693/half life). Average amount of wood stored <Q> over a period T is given by :

Table - 3 : Forest products in different life-span categories

Discussion and conclusions

The additional carbon storage of 1008.49 Tg C (Table-4) by biomass enrichment of forest land having forest cover less than 70% presents an indication to the amount of carbon that could be stored in enhanced biomass, litter and forest product pool. Given the low biomass density of Indian forests and considering that more than 38.0 % and 17.5 % of total number of stems per hectare exist in 10-20 and 20-30 cm diameter class respectively, the best strategy would be to carry out selective thinnings and protect the forest having low biomass content from grazing and fire to allow for natural regeneration and re-growth. This potential is achievable given the present forest management trend of rehabilitation of degraded forest land as reflected in the official policy of the government. The current National Forestry Action Programme (NFAP, 1999) of improving forest cover density of about 31 M ha forest areas having less than 40% canopy over a 20 year period (1996-2016) provides an important mechanism to achieve increased carbon storage by biomass enrichment of existing forests. With effective measures to arrest degradation of forests and rehabilitation of degraded forest areas, the Indian forests have potential to act as a net carbon sink in future.

Table - 4: Potential of carbon storage in different pools

A number of factors such as increasing population pressure, poverty, lack of effective land-use policy, socio-economic power play and vested interests, lack of mechanism to internalize the externalities of forest, distortionary subsidies and inadequate infrastructure may limit the realization of these potentials. These calculations do not take into account the other economic and environmental benefits of sustainable forest uses. Further, carbon sequestration in forest soils undergoing biomass enrichment and reduced use of fossil fuel inputs by increased use of forest biomass is not considered in this study. In this context, regard, the potential carbon storage and benefits are underestimated. The assumption that within a state the growing stock and productivity would be similar may result in some error but it will be considerable lower than assuming a constant productivity for the entire country. Despite this caveat and others, the findings reported here indicate that even if moderate efforts towards managing existing forest areas are carried out, the potential of additional carbon storage is enormous.

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¹ Indian Forest Service, Government of India Mailing address: Meenakshi Joshi, C/o Mrs. Shanti Joshi, 105/I-4, New Colony, Ballupur, Dehradun, Uttranchal-248001
Phone: (0135)627368, (0135)764491
E-mail: [email protected]

² Corresponding author, Indian Forest Service, Government of India
Mailing address: Preet Pal Singh C/o Ms Jolly Enterprises, 100 Feet road, Bhatinda city, Bhatinda, Punjab.
Phone: (0164)213272
E-mail: [email protected]