How trees and forests fit in

Trees, like all green plants, are organisms that can build their own organic material by harnessing the energy of sunlight using photosynthesis. Plants use carbon dioxide and water as raw materials to build simple sugar molecules during photosynthesis and these are then combined to produce cellulose – and lignin in the case of woody plants and trees. Oxygen is produced as a by-product of photosynthesis. Expressed simply, the chemical equation for photosynthesis can be written as:

This process of taking ‘free’ carbon from the atmosphere and combining it into living organic material is called ‘fixing’ carbon, and the process of building living material by fixing carbon is known as primary production. (Animals cannot fix carbon and so can live only by consuming primary producers, either directly or indirectly). The total amount of carbon fixed in this way by plants is called gross primary production and is usually expressed as grams of carbon (gC) or kilograms of carbon (kgC).

Some of the carbon dioxide taken up by plants is returned to the atmosphere through the process of respiration, which is essentially the reverse of the equation shown above. Respiration releases the energy stored in organic matter and provides the energy for the plant to sustain itself. The remaining carbon is stored in leaf, root, seed, wood and branch tissues – (often referred to as ‘biomass’) and represents the net primary production. In other words, net primary production is the carbon that becomes incorporated into plant tissue. In terms of net primary production, tropical rainforests are by far the most productive ecosystems.

At an annual timescale, the carbon associated with short-lived components of woodland is returned to the atmosphere through decomposition, with only a proportion of fixed carbon being retained in the longer term as wood.