The Role of Agriculture and Forestry in Climate Change

This is the fourth in a series of documents explaining international efforts 10 address climate change, New Zealand's response and the implications for agriculture and forestry.

The Role of Agriculture in Climate Change

Agriculture is the main source of greenhouse gas (GHG) emissions in New Zealand. In 1995 the agricultural sector was responsible for more than half (58%) of New Zealand's total greenhouse gas effect on the atmosphere. The energy sector contributed 34 percent and industrial processes four percent. New Zealand has a unique GHG emissions profile in that it is the only developed country where agriculture is the main contributor to the national greenhouse gas effect - the energy sector is the major contributor in most other countries.

Agriculture is New Zealand's main GHG emitter because it is the main source of methane and nitrous oxide emissions in New Zealand. Although in tonnage terms New Zealand emits more carbon dioxide than methane or nitrous oxide in a given year, methane and nitrous oxide are more potent gases in terms of their "global warming" effect. When measured in terms of contribution to the greenhouse effect, methane is New Zealand's dominant greenhouse gas.

Methane

Ruminant livestock (i.e. cows, sheep and goats) are the primary sources of methane emissions in New Zealand accounting for almost 90% of total methane emissions. Ruminant livestock produce methane in the rumen (a large forestomach) and the gas is released through the mouth and nostrils of the animal. Sheep are the major source (60%) of ruminant methane in New Zealand with beef cattle second (20%) and dairy cattle a close third (18%). On a per animal basis dairy cattle emit the most methane. Animal waste is also a source of methane although much smaller in New Zealand's case. Non-agricultural sources of methane in New Zealand include landfills and energy.

NZ Dairy Cows

Nitrous Oxide

Agriculture is also the main source of nitrous oxide (N₂O) emissions in New Zealand. Emissions of nitrous oxide from the agricultural sector are primarily due to the processes of nitrification and denitrification in the soil. Although the actual biological processes are not well understood it is clear that nitrogen from synthetic fertilisers, animal wastes, nitrogen leaching and run-off contribute to these emissions.

Options for reducing methane emissions from agriculture

A number of options are available for reducing methane emissions from agriculture. These include reducing livestock numbers, something which has already happened to some extent, improving animal productivity, and other methods.

A "source" of greenhouse gases is any process (natural or man-made) which re/eases greenhouse gases into the atmosphere. Significant sources of emissions include factories, thermal electricity generation, deforestation, cars and sheep

A greenhouse gas "sink" is any process which removes greenhouse gases from the atmosphere, and in the climate change context is generally applied to forests.

Reducing livestock Numbers

Projections for methane emission levels in 2008-2012 (which are expected to be about 8% below 1990 levels) are based on a predicted reduction in livestock numbers over this period. However, reducing livestock numbers further would have significant implications for the livestock sectors and the communities that support them. Therefore, from an industry viewpoint, it may well be more attractive to consider options that don't require a further reduction in livestock numbers.

Improving animal productivity

The amount of enteric methane (methane produced in the stomach) produced by any individual ruminant varies according to the quality of the diet. Low quality pastures can be expected to produce a greater amount of methane per unit of production. Supplementation of animals' grazing forage is one way of increasing the efficiency of the digestive process. Another way to increase the productivity of grazing animals is to improve the nutritional value of the pasture plants. This can be done through the fertilisation of soil, replacement of grass with legumes and the development of more leafy pasture. Research has also identified that some animals emit less methane than other animals of the same species. It may therefore be possible to selectively breed animals that have this characteristic.

Modifying rumen bacteria

Research is being undertaken into how to modify the bacteria in the rumen in order to reduce methane formation without adversely affecting the digestive process. While these measures are considered to have considerable potential they are very much in the research stage and it is difficult to predict their long term effectiveness.

Options for reducing nitrous oxide emissions from agriculture

Increasing the efficiency of the use of nitrogen supplied is one way in which agriculture may reduce nitrous oxide emissions (another is to reduce livestock numbers). Specific techniques include the use of slow release products and nitrification inhibitors. Codes of agricultural practice that guide the frequency, timing and placement of fertiliser use could assist in increasing nitrogen use efficiency.

The Role of Forestry in Climate Change

NZ Forest

Forests, and all vegetation play an important role in reducing greenhouse gases in the atmosphere because trees and other plants absorb carbon dioxide from the air. When a forest is increasing in size it absorbs C0₂ as part of the process of increasing its biomass and is referred to as a carbon "sink". Once the forest reaches maturity the carbon density remains approximately constant. This is called a carbon "reservoir". When a forest is cleared much of the stored carbon is rapidly converted to carbon dioxide and the forest is a source of C0₂.

Maintenance of existing biomass stocks (e.g. forests and forest products) is a key to avoiding further emissions of greenhouse gases. This includes protection of native and exotic forests which are both substantial carbon reservoirs. If the area under these land uses increases, or their condition improves, this can represent a significant carbon sink.

Plantation Forests

New Zealand's plantation forests have increased significantly since 1990 and the role these forests play in removing carbon has been an important component of New Zealand's climate change response strategy. During the commitment period 2008-2012, forests planted since 1990 are expected to remove around 130 million tonnes of C0₂.

Indigenous forest and scrub

Indigenous forest covers approximately four times the area that is under plantation forest, and contains a significant stock of carbon locked up in trees, understorey, forest floor and soil. The total area may currently be expanding due to the abandonment of marginal pastures which are reverting to scrub and which, if allowed, may eventually develop into high forest.

Methods to maintain and enhance forest sinks

Increasing afforestation

An option to continue to increase C0₂absorption is to plant new forests. The rate of carbon accumulation, and the maximum at maturity, will depend on the species, site and management system used. Depending on the objectives and constraints of each situation, trees may be planted in farm forestry systems (e.g. shelterbelts) or in continuous blocks, in a single year or over successive years. Each system will have different characteristics as a sink and reservoir.

Managing forests to store carbon

There are several ways to increase a forest's carbon sink (the rate at which carbon is sequestered or absorbed from the air and turned into carbon in a plant) or reservoir (its capacity to store carbon).

Increasing the rotation age allows the trees more time to grow and increases the carbon reservoir in the mature forest. A typical stand of radiata pine could be expected to contain 250tC/ha after 30 years, but could increase its carbon content to 280tC/ha if the rotation was extended to 35 years. Changing the species planted may increase either the sink or the reservoir, or both. For example, Douglas fir grows slower than radiata pine and has a longer rotation period, but it contains more carbon at the end of the rotation. A change in forest management may also increase carbon storage potential, for example a regime with no thinning or pruning may contain more carbon than a more intensive regime.

It is possible to retain a forest as a carbon reservoir and not harvest it. Some species are more suited to this than others. If the trees are not harvested, the carbon content will not increase past a given point. However, if they are harvested they may be turned into wood products, and could thus extend the time before the carbon is released back to the atmosphere.

The Role of Bio-energy

Bio-energy is a term used to describe energy sources from the conversion of plant material such as crops. Wood and plant waste. A term commonly used to describe all plant material is "biomass". Bio-energy, when used in place of fuels containing fossil carbon such as oil, can help reduce the risks of global warming. This is because the carbon from living material remains within the natural carbon cycle.

Bio-energy Opportunities in Forestry and Agriculture

The forestry industry' produces considerable volumes of "waste" both in the forest and the processing plant. Prunings, thinnings and harvesting waste is either burned or left in the forest to decay or, while processing, residues are often taken off-site and left to decay. Wood products, once they have served a useful life, also become 'wastes'. If this waste is utilised to produce energy, the carbon is released more quic~y, but it can prevent the emissions associated with the use of the fossil fuel.

There are a number of options available for growing dedicated bio-energy crops in addition to using waste products. Some countries are encouraging farmers to grow oil seed crops (e.g. soya, linseed) for the production of transport fuels. An alternative option is to grow fast growing crops such as miscanthus or eucalyptus which produce large volumes of biomass very quickly Some of the tree species being considered can be grown on coppice cycles of up to 5 years; every time the above-ground biomass is harvested, new shoots emerge from the cut stumps to produce the next crop, hence avoiding the need for replanting.

The most direct way to produce energy from biomass is through combustion, i.e. burning it to produce heat or steam. In some cases this can be used to generate heat and power, e.g. the Kinleith cogeneration plant. Biomass can also be converted via various methods to solid, liquid or gaseous biofuels. Such intermediate biofuels may have advantages over the 'raw' biomass in terms of easier transport and handling, greater control over its use, a wider variety of end uses, and perhaps less air pollution.

Mill

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