Introduction 

Forest ecosystems play a crucial role in global carbon cycling acting as sink and source. Forests form an active carbon pool that accounts for 60 per cent of carbon storage in the earth’s land surface ([28]). Tropical forests dominate the role of forests in the global carbon flux and stocks, and therefore require researchers and policy makers to estimate the carbon sequestration potentials. The tropical forests, both moist and dry types, account for approximately 60% of global forests ([9]). While covering only 22% of potential vegetation by area, tropical forests have been estimated to account for 43% of the world’s terrestrial net primary productivity ([17]).

Unprecedented increase in anthropogenic activities since globalization and subsequent industrialization and urbanization has reportedly lead to a corresponding decline and degradation of tropical forest ecosystems of the world ([10], [18], [25]).

Current population growth and rapid infrastructure development are expected to put additional pressures on tropical forests leading to rapid rise in the release of CO₂ into atmosphere and decline in atmospheric carbon sequestration. Despite unresolved controversies over observed changes in biomass and gas fluxes, current observations indicate the likelihood that additional climate change would have substantial impacts on tropical forests ([11]).

India has a diverse range of forests covering 64 Million hectare, of which 72% are tropical moist deciduous, dry deciduous, and wet evergreen forest ([22]). Unlike developed countries, India does not have any carbon inventories and databank to monitor and enhance the carbon sequestration potential in diverse forests. Currently attempts are made to evaluate carbon stock assessment at macro level, mostly based on secondary data. Only few attempts have been made so far to assess above ground biomass and soil carbon sequestration at micro-level in any specific forest system in India ([3], [13], [19], [7]). Such kinds of studies are essential to understand the carbon stock potential in the forest system. This paper aims to evaluate the carbon stock of above ground biomass, litter, woody debris and soils in different forest systems of Kolli hills, Eastern Ghats, India.

  Materials and Methods 

Study area

The Eastern Ghats are uneven terrain running nearly parallel to eastern coast of India. Eastern Ghats are divisible into three zones such as northern Eastern Ghats, middle and the southern Ghats ([21]). Kolli hills, the present study area, is one among eight hills in southern region (Fig. 1). It lies at a longitude of 78°20’ to 78°30’ E and a latitude of 11°10’ to 11°30’ N, covering about 500 km². Forests cover 44 % of the total geographical area, agricultural activities take place in 51.6% and other activities cover less than 5% of the area. Annual rainfall varies between 300 to 750 mm and the soil type varies between red to black clay. Highest point in Kollimalai is 1400 m a.s.l., but the general level of the upper surface of the hill is not more than 1000 m. The altitude range of Kolli hills varied from 200 to 1415 m a.s.l. The geology of area is characterized by charnockite associated with gneisses and metamorphic rocks. In order to estimate the carbon stock in above ground biomass, litter, woody debris and soil, about 26 quadrates of 25 x 25 m size were established during September 2009 in different forest types, i.e., evergreen, deciduous, mixed, open scrub and plantation forest.

Fig. 1 - Study area and sampling sites.

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Evergreen forests are represented by forest patches in high-elevation sites (above 1000 m a.s.l) which receive high rainfall (about 2000 mm year^-1). In general, trees of these forests do not shed their leaves in any season of the year. Major evergreen tree species of Kolli forests are Terminalia paniculata, Myrstica dactyloides, Scolopia crenata, Prunus ceylanica, Syzygium cumini. Deciduous forests are generally present in regions with annual rainfall ranging between 750-2000 mm. They are called “deciduous forests” because trees shed their leaves in the dry season of the year. Common deciduous trees in Kolli forests are Tectona grandis, Terminalia chebula, Spondias pinnata, Anogeissus latifolia, Albizia anara, Albizia lebbeck. Mixed forest patches analyzed in this study contain both evergreen and deciduous tree species. Scrub forests are confined to areas with a rainfall lower than 750 mm. Thorn, cactus, Lantana camara, Euphorbia species, are common among scrub. Plantation forests of Kolli include silver oak, eucalyptus, pines, etc.

The study plots were representative of forest types relating to species abundance, altitude and other factors such as stand density, microclimate and crown conditions. Results obtained in the plots were extrapolated to respective forest types and geographical area. Statistical analysis of the results was carried out using SPSS^® and MS-Excel^®. The land use and cover map (Fig. 2) for Kolli forest was generated based on secondary data and maps obtained from forest department (Govt. of Tamilnadu) and earlier satellite observations ([14]). Mapping techniques including classification of forest cover were followed to prepare the forest cover density map of the study area. Normalized Difference Vegetation Index (NDVI) was prepared and recoded into four classes based on the density, i.e., very dense (>70%), dense (40-70%), open (10-40%) and degraded (<10% - [23]).

Fig. 2 - Land use and land cover map of Kolli Hills.

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Estimation of above ground biomass carbon

Aboveground biomass (AGB) and its existing carbon stock was estimated in 26 quadrates during September 2009. All trees with diameter at breast height (DBH) greater than 0.3 m were enumerated, measured, and classified. Ravi’s altimeter was used to measure tree height and AGB was calculated by the allometric equation developed by Mani & Parthasarathy ([16]). Carbon content of AGB was calculated by the conversion factor 0.5 ([2], [5], [12], [9], [6]).

Estimation of soil organic carbon, surface litter and coarse wood debris

Soil samples were collected at three depths (0-10 cm, 10-20 cm and 20-30 cm). Samples were air dried and ground to fine powder using an agate pestle and mortar. The SOC content in the 0.5 mm sieved soil samples was estimated following Walkley & Black ([27]) wet oxidation method.

Coarse wood debris and surface litter samples were collected as per standard procedures ([26], [8], [4], [20]). Estimation of organic carbon storage in woody debris and litter was performed as per Allen et al. ([1]). In brief, after taking the fresh weight collected samples (woody debris, leaf litter) were dried in the oven for 48 hours at 65 °C and dried weight was measured. Oven-dried samples were taken in pre-weighed crucibles. The samples were ignited at 550 °C for one hour in muffle furnace. After cooling, the crucibles with ash were weighed and percentage of organic carbon was calculated as follows (eqn. 1, 2):

where C is the organic carbon, W _a the weight of crucible, W _b the weight of oven dried grind samples + crucibles, W _c the weight of ash + crucibles.

  Results and Discussion 

In Kolli hills, evergreen forests and semi-evergreen occurs in upper plateau of 900 m a.s.l., while slopes are covered by deciduous and thorn forest types. Land use and land cover analysis showed abundance of different forest cover in the order deciduous > mixed > evergreen > open scrub > plantation (Fig. 2), with total area under forest cover extending up to 26 587.8 ha. Anthropogenic disturbances such as mining, exotic plantation, agriculture extension, shifting cultivation, over grazing, tourism developments and firewood collection occurred at many places. An earlier study also reported that the extensive mining activities in Kolli hills take away about 600 Mg of soil per day for cement and aluminum factories ([24]).

In the present investigation, AGB analysis using DBH and height of trees yielded different size classes of trees (Fig. 3). In general, DBH of trees in class size 30-60 cm was found dominant followed by 60-90 cm, above 120 cm, 90-120 cm and less than 30 cm. Stand density observed in the Kolli hills varied from plantation> deciduous> evergreen > mixed forest > open scrub. Maximum stand density was observed in the forest plantation (2 220 trees ha^-1) followed by deciduous (1 960 trees ha^-1) and evergreen forest (1 680 trees ha^-1). Tree height was recorded in the range of 3 to 19.88 m. Aboveground biomass (AGB) in different forest types ranged between 15.61 to 597.13 t ha^-1 (Tab. 1) with minimum in open scrub and maximum in evergreen forest. Carbon stock in AGB varied between 7.80 to 298.56 t ha^-1, with average value of 170.65 t ha^-1. Overall of extrapolation analysis showed 4.49 Tg C stock in aboveground biomass for entire Kolli hills.

Fig. 3 - Distribution of above ground biomass with respect to DBH.

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Surface litter carbon stock varies between 0.16 to 3.26 t ha^-1 (Tab. 2). The maximum mean of surface litter carbon stock was recorded in evergreen forest (1.58 t ha^-1), followed by forest plantation (1.38t ha^-1) and deciduous forest (1.37 t ha^-1). The total carbon stock of surface litter after extrapolation for the entire Kolli forest was found to be 34.13 Gg C in Kolli Hills. The Coarse Wood Debris (CWD) carbon stock ranged between 0.0006 to 0.0977 t ha^-1, with the higher values recorded for deciduous forests (52%), followed by mixed (21%) and evergreen forest (18%). Irrespective of forest types, SOC content ranged between 1.71 to 12.59% and decreased with soil depth. The SOC concentration was observed in the order: deciduous > mixed > evergreen > open scrub > plantation (Tab. 4). Analysis of variance showed statistically significant difference among forest types (p< 0.05). The total soil carbon stock of Kolli Hills after extrapolation accounted to 5.54 Tg C. High carbon content of the surface layer (0-10 cm) is indicative of rich inputs from litter fall, wood and fine roots ([15]).

Despite the rich biodiversity of Kolli forests, a comparatively lower carbon stock reflects potential threat from anthropogenic pressures, including mining, shifting cultivation and other tourism-related activities. Unregulated and unsustainable exploitation for fuel wood and other wood and non-wood products and excessive overgrazing in many regions of Kolli forests may also account for forest degradation.

  Conclusion 

Regional terrestrial carbon accounting is very important to address global climate change mitigation, particularly in tropical areas. In this perspective, the current study attempted an estimation of carbon stock in different forest Types of Kolli hills of Tamilnadu State (India), with emphasis on above ground standing biomass, litter, woody debris and soil by terrestrial field-based experiments. The land cover analysis revealed a total area of 26 587.8 ha covered by forests in Kolli Hills. Among different forest types, mixed deciduous forest cover the major portion of forested land. Total carbon stock distribution varied in the following order: deciduous > mixed > evergreen > open scrub > plantation. Above ground carbon stock of Kolli hills recorded 4.49 Tg C, while soil C stock showed 5.54 Tg C and surface litter and woody debris carbon stock registered 34.125 Gg C and 1.02 Gg C, respectively.

  Acknowledgments 

The authors would like to thank the University Grant Commission (UGC), Government of India, for financial support.

  References