Forest ecosystems play a crucial role in global carbon cycles, acting as a sink and a source. Forests form an active carbon pool that accounts for 60 per cent of carbon storage in the earth’s land surface. Therefore, dynamics of carbon in forest vegetation and soils are significant in terms of global climate change policy framework. The rate of carbon absorption is greatest in the earliest stages of growth and regeneration, and declines as forests mature. Tropical forests dominate the dialogue in the global carbon flux, and carbon stocks. It is thus such regions that require dedicated research to estimate its carbon sequestration potential. The tropical forests, both moist and dry types, account for approximately 60 per cent of global forests. While covering only 22 per cent of potential vegetation by area, tropical forests have been estimated to account for 75 per cent of the world’s terrestrial net primary productivity (NPP). However, under elevated CO2 conditions a possibility of decomposition occurring more than net primary production (NPP) leading to a loss of carbon is predicted in some forest regions. In others, elevated CO2 and N deposition tend to increase NPP more than decomposition, leading to carbon storage. Given the uncertain scenario, a micro level carbon flux examination of different forest types alone can give a clear picture.
The two significant drivers of forest carbon flux are biophysical processes operating at various spatial and temporal scales; and the local anthropogenic disturbances. In addition to this, global climate change and other multiple stressors such as ozone, sulphur and nitrogen depositions also influence the productivity and carbon stock, which has been largely ignored till date in the Indian scenario. Many studies in the nation have pointed out forest degradation and productivity loss due to regional climate anomalies and trends, fires, cultivation, mining, biomass extraction and cattle grazing. The challenge for the scientific and policy making community now lies in identifying the major factor that affect the carbon flux in the forest at micro level.
In one such attempt we focussed on carbon stock variations in the Kolli Hills forest regions of Namakkal, Tamil Nadu, covering an area of about 500 km2. The forest occupies 44 per cent of the total geographical area, agricultural activities take place in 51.6 per cent and other activities occupy less than 5 per cent of the total geographical area. Annual rainfall is between 300 to 750 mm and the soil type varies between red to black clay. The highest point in the region is marked by Kollimalai, 1400 m above sea level, but the general elevation of the Kolli Hills is not more than 1000 m. As per Census 2001, the population residing in the area is about 37 thousand.
As described in classical Tamil literature, under varied nomenclature – Agananooru, Silappathigaram, Manimekalai, Purananuru and Ainkurnuru, Kolli Hills, historically witnessed a good forest cover (75 per cent) which gradually dwindled. However, even today the area is important for its farm products which include coffee, tea, jackfruit, pineapple, black pepper and other spices. Rice and other minor millets form the primary food of the tribal people who inhabit these mountains. Kolli Hills are also well known for their medical herbs and plants.
Evergreen and semi-evergreen forests in this area occurs in upper plateau region with an elevation of 900 m and above, while the slopes are occupied by deciduous and thorn forests. Land use and land cover analysis showed abundance of different forest cover in the order deciduous > mixed > evergreen > open scrub > plantation with total area under forest cover extending up to 26587.8 ha. Anthropogenic disturbances such as mining, exotic plantation, agriculture extension, shifting cultivation, over grazing, tourism developments and firewood collection occurred at several places. An earlier study also hinted extensive mining activities in Kolli Hills removing about 600 mg of soil per day for cement and aluminium factories.
Our examination for carbon stock was limited to the forest cover that is protected under reserved forests. For comparative assessment, sample studies were also undertaken in the open scrub and plantation forests. Research revealed that the carbon stock is undergoing a gradual change in the forest system both above ground and within the soil. For instance, carbon stock in the above ground biomass increased from 4.4 teragram (Tg) in 2009 to 5.68 in 2010, while the soil carbon stock decreased in the same proportion (Table 1), which implies Kolli Hills forest is neither a sink nor source. However, the increase of carbon stock in woody debris and surface litter indicates a possible addition to future soil carbon stock, provided similar biophysical conditions prevail without any anthropogenic disturbances. Periodical examination of this and similar areas is required to reveal if a particular region in the forest ecosystem is undergoing any significant change. Subsequently, the predominant drivers behind the change can also be identified.