The Role of the Biosphere including Forests and Grasslands in the Carbon Cycle Given Geological Time Scales
The biosphere comprises all living things on the planet and it stores around 600 Gt of carbon. Major components are forests and grasslands.
The consensus on forestry as a carbon sink, seems to be that mature forests are not large net carbon sinks – in our time scales at least. Given longer time scales however old growth forests are very important for sequestration because of the flow of carbon to more permanent forms.
Grassland on the other hand is less of a carbon sink but still important in this long term process.
The soils that form in grasslands or forests are important sinks. See The Role of Soils in Sequestration.
The Permanence of Carbon Sinks 
Forests are part of a much bigger ecosystem – nature as a whole - and it is this system that ultimately puts carbon away more permanently in the form of peat, coal, petroleum and carbon in soil as humus and soils like Terra Preta. (See The Role of Soils in Sequestration.and Newsletter 67) . The whole concept of an ecosystem involves linkages and the linkages from forests stretch far and wide. They cannot, as most people assume, be considered as a closed system and so it is important that rich natural growth such as found in forests is encouraged.
There are also other benefits to having trees and other plant life. They are vital to eco-systems. The formation of oxygen through photosynthesis is just a little vital as is the creation of new soil, the sequestering, filtering and release of water, the provision of food and homes for animals, etc, etc. Trees provide an amazing and irreplaceable array of functions and beauty.
Consider an uninhabited forest. Is the carbon cycle in such a closed system neutral? Trees die and decompose, while other trees are growing, and eventually the forest reaches an steady state where the two are almost equal. But not quite because there is a carbon leakage all the time to more permanent forms. Much of this sequestration process if the result of microbial activity and as forests mature the nutrient deficits and increased acidity result in slower decomposition with a resulting rise in carbon concentration in various forms including humus and eventually, and if there is enough dead plant matter, coal.
The critical problem on the planet is the short term release of carbon which is flooding the atmosphere with more than the planet can handle through natural sequestration processes. The composition time for a tree is equal to the amount of time that the tree has lived. This is significant given the existence of trees that live upwards of a thousand years. Trees can take several times longer to decompose - say upwards of two or three thousand years. It follows that locking more carbon up in forests has a short term effect. Increasing the plant based carbon sink from 600 to say 650 gigatons in the short term (See the diagram above) would reduce the amount of CO2 in the air by 50 gigatons which is equivalent to about 10 years of current fossil fuel emissions.
We need to reduce and eliminate the short term release of carbon to deal with the green house effect or sequester massive amounts or better still – do both. When a tree decomposes on the forest floor, it is consumed by decomposing bacteria, insects and saprophytic organisms. Nutrients are recycled back to the forest and through linkages to the whole ecosystem. Much is transported deep into the soil by microbes where it can remain for millennia - that is until it is disturbed and severely depleted by practices including mono-cultural forestry and agriculture and in particular the associated burning off and ploughing. Given geological time some carbon is permanently sequestered through this process.
The most obvious result of the long term sequestration of carbon by forests is the formation of coal deposits. It is considered by most that the "coalification" process occurs in a waterlogged environment where plant and tree debris accumulate. As the layer of debris is thickened overtime, with the additional weight the floors of these vast swamps subsided slowly and fungi and bacteria decomposed the plant material. The first stage of the process is characterised by extensive biochemical reactions. Proteins, starches and cellulose undergo more rapid decomposition than woody material (lignin) and the waxy parts of plants (the leaf cuticles and the spore and pollen walls). We know this because the remains of many types of vegetation, including tree stumps, leaves, spores, seedpods, and resin are found in brown coal.
To varying degrees, and depending upon climatic conditions, plant constituents are decomposed under aerobic conditions to carbon dioxide, water and ammonia. This process is called "humification" and partial completion results in the formation of peat. The second stage of coalification occurs when the peat becomes covered with layers of sediment which exclude air, introducing anaerobic conditions. In this second stage the combined effects of time, temperature and pressure convert the peat firstly into brown coal (lignite) and then into sub-bituminous coal, bituminous coal and finally to anthracite. These three latter coals are usually called black coals.
It was not until the Carboniferous some 300 million years ago that land plants developed sufficiently to form the forests which produced the major coal deposits of the Northern Hemisphere. In Queensland and New South Wales Australia, coal formation occurred much later in the Permian Period some 250 million years ago. In Victoria Australia, the coals are much younger; being deposited 15 to 50 million years ago during the Tertiary Period.
Forests are also important for bio-diversity and preventing soil erosion and play an important role influencing weather patterns. Cutting them down leads to climate change. CO2 is not the only potentially harmful by product of burning trees. There are other particulates that have harmful often carcinogenic substances formed and released as well and so burning off is not a good idea.
In summary, forests must be considered as part of the overall eco-system and their importance in geological time as opposed to our times scales realised.
All plants,to varying degrees,take carbon dioxide from the atmosphere,and bio- logically tie-up or "sequester" carbon in the soil. Most people probably associate carbon sequestration with forests however grassland covers some 25 percent or more of the Earth's land area and provide important terrestrial carbon dioxide sequestration. In forests the majority of the carbon mass is in the above-ground vegetation whereas with grassland the major accumulations are located in the soil.
Carbon is sequestered in two ways in soils. One way is by a physical process: Carbon can be tied up by soil particles so that microorganisms cannot access it and release it back into the atmosphere as carbon dioxide. Some of the carbon in organic matter on the other hand, such as found in roots, break down into stable compounds many of which do not readily degrade further, and therefore tie up the carbon for decades or more.
 Modified from Figure 2 in Ziock, H. J. and D. P. Harrison. "Zero Emission Coal Power, a New Concept." from http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/2b2.pdf. by the inclusion of a bar to represent sedimentary sinks.