Understanding Greenhouse Gases in Forestry

Greenhouse gases (noun) - Any gas that limits the escape of heat from the earth's atmosphere.

The three key greenhouse gases in forestry and agriculture are - carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). To understand how greenhouse gases play a role in forestry, it helps to understand how these different gases compare.

Each of the 3 gases has a different global warming potential (GWP), which is a measure of their ability to trap heat in the earth's atmosphere. As CO₂ is the weakest of the greenhouse gases, the other gases are compared to CO₂ as a factor of how much stronger they are. This is known as CO₂ equivalents (ie, CO₂e).

Carbon Dioxide	Methane	Nitrous Oxide
CO2 = 1 CO2e	CH4 = 25 CO2e	N2O = 298 CO2e

These gases are often associated with a variety of processes in agriculture and forestry.

CO₂ is emitted whenever organic matter is burnt or decomposes, but is also absorbed by plants as they grow, converting it to carbon and using it to create plant tissues and energy. By virtue of their size and longevity, trees can absorb significant amounts of CO₂.

CH₄ is released as a part of organic decomposition and ruminant livestock production (beef & sheep).

N₂O is produced when nitrogen fertiliser or urine are exposed to the atmosphere. N₂O emissions are common in livestock or fertilised industries.

Comparing forestry and agricultural emissions

In contrast to other agricultural systems, forests often act as a greenhouse gas sink for two reasons.

Forests emit relatively few greenhouse gases

Under typical growing conditions, forests emit very little CO₂ and CH₄. Even when fertiliser is applied, N₂O emissions are small considering the application usually takes place only once or twice over a forest's lifetime. Fire can result in a large loss of CO₂ stored in a forest.

Forests absorb large amounts of greenhouse gases from the atmosphere

Any emissions from forests are often counteracted by a trees' ability to absorb CO₂ as they grow. Absorption of CO₂ will continue from establishment through to maturity. At maturity, net gain of CO₂ in the trees will stop, and other CO₂ accumulation may continue (eg, soils).

Because of the ability of forests to absorb CO₂ and emit very few greenhouse gases, they can be a very effective at reducing emissions compared to other agricultural businesses.

Emissions of CO₂e t/ha/yr (Indicative only)

Calculating a forest's emissions

Calculators are available for forest managers interested in estimating the greenhouse gas profile of their forests.

For more information on measuring and reducing emissions in agriculture, go to DPI's Carbon Toolkits in Agriculture Network page.