Climate change

NZ’s response

The New Zealand Government has embarked on a programme to reduce our collective carbon footprint.

This is proposed to involve all sectors of the economy, and all greenhouse gases.

The idea is for each sector to take responsibility for its own emissions either by reducing them or by buying “New Zealand Emission-Reduction Units” (NZUs) from someone else who has done so - emissions will be traded.

This is intended to ensure that market forces will discover the most cost-effective ways of reduction, and minimise harm to the economy. Starting from July 1st  2008, forestry is the first sector to be phased into the new legislation, but other sectors will be added later.

Those intending to deforest land (change the land cover from forest to non-forest) will need to obtain NZUs equivalent to the carbon that they are responsible for releasing.

Exemptions include approved owners of smaller woodlots of less than 50 hectares or owners who have allowed the government to retain all their NZUs and liabilities.

Those who have planted trees on non-forested sites after 1989, and those who intend to plant trees, can choose to receive units corresponding to the gain in carbon in their woodlots – and sell them on the market. But if they select this option, they will need to take responsibility for any loss in carbon that occurs, whether it be from natural disaster, deforestation or routine harvesting.

They must also cover the costs of measuring, verifying, insuring, registering and trading their units.

Absorbing CO2

All vegetation absorbs atmospheric carbon through leaves and needles by the well-known process of photosynthesis.

Plants also release carbon dioxide by respiration, either as part of the living organism or as a result of decomposition by microbes. This is particularly the case at night, in winter, and following an event such as wind, fire or harvest activity. It is the balance of the carbon entering the system and the carbon leaving it that determines the net effect on the atmosphere.

When averaged over a long time period, vegetation types can be described as “carbon neutral” in that the outflow of carbon dioxide exactly balances the inflow. For example, the carbon stocks in a natural alpine meadow would be much the same if measured on a given date in successive years – despite the fact that vegetation has grown in the interim, it has also decayed, with the net effect of there being no sequestration.

The same would be true of a natural forest, such as Fiordland National Park. If the forest was even a very slight source or a very slight sink of carbon, on a yearly but ongoing basis, then after tens of thousands of years there would either be no forest remaining, or there would be a wall of wood that would be impossible to penetrate.

The conversion of a non-forest such as pasture to a forest results in a move from short vegetation to tall vegetation, with an inevitable gain in carbon stocks. The conversion of a forest to a non-forest causes a loss in carbon stocks.

The growth of a stand of trees is not a straight line; growth starts slowly, builds up momentum, and then levels off as the trees reach maturity. Put in mathematical terms, growth is sigmoidal not linear. Therefore, it is not possible to assess the amount of carbon that a particular sort of tree sequesters per hectare per year. The carbon growth of a harvested stand over many crop cycles can be represented by a sawtooth graph. The “teeth” of this graph are the high points reached just prior to harvest.

A similarity exists with a non-harvested stand, for example an area of reverting indigenous forest. This is because, at best, growth will cease when the trees have reached their maximum size limits (heights, stockings, diameters) and when the parcel of land has become “carbon saturated”. At worst, the forest will collapse – New Zealand indigenous forest is well-known for its patchwork composition provided by periodic catastrophic events – resembling the sawtooth graph of a harvested stand.

When looked at in this way, it becomes obvious that offsetting emissions by tree-planting has its limitations: there is a maximum quantity of carbon that be held on any given area. In an unharvested forest, trees will eventually die to balance the carbon absorbed by actively growing trees. In a harvested forest, patches of trees are removed by chainsaw and logging truck and in large clearfelling coupes, but – as far as the atmosphere is concerned – there is no difference between the two sorts of forest.

The key questions are: how much carbon is retained over the long-term in a certain type of forest, and how long does it take to reach that level?

But forests are far more than mere passive reservoirs of carbon: they produce wood. This wood can be allowed to decay on the forest floor, and release its carbon to the atmosphere naturally, or it can be extracted for a range of useful products and then release its carbon once it has fulfilled those tasks.

Greenhouse Benefits

Carbon is retained in both forests and buildings. This carbon is not in gaseous form and is not available to cause global warming.

So why is carbon in the form of wooden objects not treated in the same way as carbon in the form of trees?

The answer is that it is the balance of carbon entering the “wood products pool of carbon” and the carbon leaving it, that determines the net removal from the atmosphere.

So is the amount of wood in service expanding, shrinking or staying the same? We can easily calculate this balance with living forests, but it is harder to do this with timber products.

By taking on the liability for the carbon at harvest, wood is universally accepted as being carbon neutral. It is made by natural processes using solar energy. It is recycled atmospheric carbon, as opposed to fossil carbon that represents additions from below the ground.

This concept is accepted internationally and it means that wood is free of the carbon penalties that will eventually accompany alternative products.