In this article we will discuss about:- 1. Introduction to Carbon Sequestration in Forests 2. Strategies of Forest Carbon Sequestration 3. Assessment.
Introduction to Carbon Sequestration in Forests:
1. Carbon and Forests:
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The process of photosynthesis combines atmospheric carbon dioxide with water, subsequently releasing oxygen into the atmosphere and incorporating the carbon atoms into the cell of plants. Trees, unlike annual plants that die and decompose yearly, are long-lived plants that develop a large biomass, thereby capturing large amounts of carbon over a growth cycle of many decades. Thus, a forest ecosystem can capture and retain large volumes of carbon over long periods.
Forests operate both as vehicles for capturing additional carbon and as carbon reservoirs. A young forest, when growing rapidly, can sequester relatively large volumes of additional carbon roughly proportional to the forest’s growth in biomass. An old-growth forest acts as a reservoir, holding large volumes of carbon even if it is not experiencing net growth. Thus, a young forest holds less carbon, but it is sequestering additional carbon over time.
An old forest may not be capturing any new carbon but can continue to hold larger volumes of carbon as biomass over long periods of time. Managed forests offer the opportunity for influencing forest growth rates and providing for full stocking, both of which allow for more carbon sequestration. Forest systems operate on a scale of many decades and centuries, rather than annually or over a few years as would be the case with most crops and non-tree vegetation.
2. Forests as Carbon Sink:
The world’s forests store more than 650 Gt of carbon, 44 per cent in the biomass, 11 per cent in dead wood and litter, and 45 per cent in the soil. Globally carbon stocks are decreasing as a result of the loss of forest area, however, the carbon stock per hectare has remained almost constant for the period 1990-2010. FAO (2010a) estimates that the world’s forests store 289 Gt of carbon in their biomass alone.
While sustainable management, planting and rehabilitation of forests can conserve or increase forest carbon stocks, deforestation, degradation and poor forest management reduce them. Carbon stocks in the world’s forest biomass decreased by an estimated 0.5 Gt annually during the period 2005-2010.
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It estimates an average carbon stock of 161.8 tons per hectare in the vegetation of the world’s forests for 2010. Each cubic metre of growing stock equals different amounts of biomass and carbon (in biomass) in different regions. Globally, each cubic metre of growing stock equals, on an average, 1 ton of above-ground biomass, 1.3 tons of total biomass and 0.7 ton of carbon in biomass.
India’s forest and tree cover accounts for about 23.4 per cent of the total geographical area of the country. Over the past decades, national policies of India aimed at conservation and sustainable management of forests have transformed India’s forests into a net sink of CO2.
India’s forests serve as a major sink of CO2, the estimates show that the annual CO2 removals by India’s forest and tree cover is enough to neutralize 11.25 per cent of India’s total GHG emissions (CO2e) at 1994 levels and to neutralize 9.31 per cent of total annual emissions of 2000.
From 1995 to 2005, the carbon stocks stored in Indian forests and trees have increased from 6,245 to 6,662 million metric tons registering an annual increment of 38 million metric tons of carbon or 138 million metric tons of CO2e. FAO (2010b) estimates that the India’s forests store 6,923 million tons of carbon, 40 per cent in the living biomass, 1.5 per cent in dead wood and litter, and 58 per cent in the soil.
The carbon stock for the period 2006-30 is projected to increase from 8.79 to 9.75 GtC with forest cover becoming more or less stable, and new forest carbon accretions coming from the current initiatives of afforestation and reforestation programme.
Strategies of Forest Carbon Sequestration:
Afforestation and reforestation of marginal cropland and pastures which include planting trees on land previously used for other purposes could result in substantial gains in carbon storage in biomass and soils comparing to reforestation (planting trees on land recently devoted to forestry). Carbon storage can also be improved by changing silvicultural practices.
Since these practices are usually developed and applied for purposes other than carbon sequestration, it may be difficult to quantify the magnitude of increased total carbon storage when practices change. For instance, increasing timber growth will not necessarily increase biomass growth and soil carbon storage. Conversion of forest land to non-forest use usually means permanent loss of all or a substantial part of live biomass and reduction of organic matter in soils and in the forest floor.
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Some carbon may be sequestered in wood products if the harvested biomass is utilized. Carbon dioxide and other GHGs are emitted when the remaining biomass and organic matter is burned or decomposed. Protecting and conserving forests should maintain or increase carbon pools in the short term.
Carbon in wood and paper products remains sequestered and is emitted to varying degrees depending on how products are made, used and disposed. Sequestration in products and uses can be increased by altered processing methods, shifts in products used, end-use durability and landfill management. Sequestration in forests and products can be maximized by coordinated understanding of forest ecosystems and product utilization.
Table 39.5: Carbon Sequestration Practices
The Intergovernmental Panel on Climate Change (IPCC) estimated that it may be possible, over the course of the next 50-100 years, to remove between 40 and 80 Gt of the carbon by sequestering it through agroforestry. Thus Agroforestry would be one of the interesting areas of research in land-use related to carbon sequestration, for various reasons.
First, the surface involved is considerable and the rate of carbon gain is relatively high, 0.2- 3.1 ton per hectare per year.
Second, it can mitigate the important CO2 emission resulting from deforestation.
Third, it could provide a sustainable system from technical, ecological and economic points of view.
Assessment of Carbon Sequestration in Forest Ecosystem:
Monitoring and verifying carbon storage can be expensive depending on the level of scientific validity needed. A system of cost-effective methods is necessary for monitoring and verification on a commercial basis for three types of land use- forest plantations, managed natural forests and agroforestry. It assesses changes in four main carbon pools- above ground biomass, below ground biomass, soils and forest litter.
It aims to assess the net change in each pool for land use project and non-project (or pre-project) areas over a specified time period. Carbon monitoring efforts require specialized equipment, methods and trained personnel and most carbon assessment activities are likely to be performed infrequently – once every two to five years.
Carbon assessment system involves the following components:
i. Baseline determination of pre-project carbon pools in biomass, soils and forest litter.
ii. Establishment of fixed area permanent sample plots or carbon inventories for periodic measurement of changes in carbon pool in land use project areas.
iii. Assessment of carbon stock in above ground biomass, below ground biomass, soils and forest litter.
iv. Plot less vegetation survey methods (quarter point or quadrat sampling) to measure carbon stored in non-project areas or areas with sparse vegetation.
v. Calculation of the net difference in carbon accumulated in project and non-project land uses.
vi. Use of satellite images as gauges of land-use changes, and as base maps for a microcomputer based geographic information system.
vii. Software for calculating minimum sample size, assigning sample unit locations (either in a systematic grid or randomly), determining the minimum spacing for plots and optimizing site-specific monitoring plans.
viii. Computer modelling of changes in carbon storage for periods between field measurements.
ix. A database of biomass partitioning (roots, wood and foliage) for selected species.
i. Carbon Inventory:
For carbon inventory, stratified random sampling generally yields more precise estimates. Each stratum can be defined by vegetation type, soil type, or topography. Useful tools for defining strata include satellite images, aerial photographs and maps of vegetation, soils or topography. These should be combined with ground measurements for accurate result. A GIS can automatically determine stratum size and the size of exclusions or buffer zones. Permanent plot locations can be selected either randomly or systematically.
ii. Estimation of Plant Biomass:
For this, diameter of all woody vegetation above the minimum diameter at breast height (e.g. >2 cm), is measured. Subsamples of smaller diameter (< 2 cm) woody vegetation and herbaceous plants are taken using small quadrats or circular plots. Individual dbh values for each plot are converted to biomass using single-entry biomass table or general biomass equations.
Where single-entry tables do not provide adequate estimates of biomass, double-entry tables based on dbh and height are used. When weight and biomass tables do not exist for certain tree species, it is needed to develop and construct weight and biomass tables using a minimum of 30 well-selected trees or with a mean tree technique. Regression analysis should be performed for biomass of each component.
Partitioning and Weighing:
Each individual tree or shrub should be harvested and measured for diameter, length, height and divided into major components. Individual components (stem, branches, leaves, roots etc.) should be divided into several size classes for convenient handling and sub- sampling.
Sub-samples should be taken to determine moisture content and specific gravity. The best way is to partition and weigh only one tree at a time and measure green weight in the field. To calculate moisture content, sub-samples should be taken, dried at 80° C and re- weighed.
Non-Destructive Method:
Although harvesting is the preferred way to develop weight tables, it is not always possible due to conservation or regeneration considerations. For biomass tables from nondestructive samples, calculate stem-wood volumes and convert them to biomass using specific gravity for wood and expansion factors for canopy biomass.
Estimation of Carbon Stock in Trees, Soil, Litter and Herbaceous Vegetation:
Total tree biomass is converted to total carbon using conversion factors or standard formula. Carbon analysis in samples of soil, herbaceous vegetation and forest litter is usually carried out in laboratories using Walkley-Black procedure for determination of soil organic carbon. Other methods include dry combustion and wet combustion.
