Dialogue: Govt's forest plating plan a short term solution only

CHRIS DE FREITAS*

The steadily rising price of petrol is not all bad news. The effect will be the same as a carbon tax. It will help us fight global warming as required by the Kyoto climate treaty.

It is fair to say that it is not a popular environmental strategy.

For this reason, the Government has adopted a novel and far less painful approach: increased forest planting.

Prime Minister Helen Clark has committed us to meet the Kyoto target for the reduction of greenhouse gas emissions to 1990 levels by 2010. Given that our emissions in 2010 are predicted to be almost 40 per cent higher than 1990 levels, the Kyoto Protocol is clearly a big ask.

But the protocol allows countries to meet their targets by planting forests to soak up carbon dioxide instead of making cuts. It's called carbon sequestration. The problem for us is that carbon sequestration is emerging as one of the most contentious issues in the Kyoto climate treaty negotiations.

In light of the Government's stance, the critical question for New Zealand is whether a reliance on a tree-planting policy will have the desired effect.

Every tree that is planted removes carbon dioxide from the atmosphere and stores it in the form of wood. The problem is that the effectiveness of planting a whole forest to remove carbon dioxide from the atmosphere and create a reservoir or so-called sink for carbon depends on several things.

The first is the rate at which carbon can be taken from the atmosphere and stored within growing trees. Rates of carbon accumulation in forests vary according to tree and soil type, temperature and rainfall.

The good news is that New Zealand's radiata pine forests can average as much as nine tonnes of carbon a hectare a year. The bad news is that this rate of carbon storage can be maintained for only a short period. Eventually the carbon obtained by the ageing trees by photosynthesis is exceeded by carbon lost by respiration, at which point the forest becomes a net source of carbon dioxide.

Thus, to maintain a strong sink of atmospheric carbon within forests, the forests need to be replaced regularly. And the timing of replacement requires careful management. If the average age of the trees is allowed to fall, the forest becomes a carbon source and contributes further to carbon dioxide concentrations in the atmosphere. The fate of the harvested trees also determines whether forest planting reduces carbon dioxide in the atmosphere.

Only about half of every tree is usable wood. The remainder consists of unsaleable litter, such as stumps, roots, branches and leaves, which soon decomposes and returns to the atmosphere as carbon dioxide.

What is more, about half of the usable component is lost as offcuts and sawdust at sawmills. And further loss continues at each stage of use. For example, 15 per cent of timber delivered to building sites is lost through offcuts.

In the case of forests used solely for pulpwood, most of the end-products will be broken down by decay within five years and end up adding to the concentration of carbon dioxide in the atmosphere.

The effectiveness of planting forests to offset carbon dioxide emissions also depends on whether or not the land area available is sufficient. The space requirements are enormous.

To absorb the carbon that would be emitted from a medium-sized (400-megawatt) gas-fired power station, more than 4000ha of land would have to be acquired and planted with trees.

Not all land in New Zealand is capable of supporting forests and the consequences of removing agricultural land from production could be economically undesirable.

There are other problems. Conservationists fear that private owners of native forests could be tempted to replace ancient trees with faster-growing varieties, or that foresters will chop down existing natural forests to make way for fast-growing, carbon-guzzling trees.

It is becoming clear that planting forests to offset carbon emissions can only be, at best, a short-term, stop-gap measure. But the Kyoto Protocol's targets are clearly set out. Signatories to the treaty need to understand their longer-term commitment as well as the consequences of not meeting that commitment.

European Union ministers recently announced that countries will face financial penalties if Kyoto targets are not met. Negotiations will take place later this year to determine the rules for complying with the treaty.

The Government would be well advised to follow these developments closely.

* Chris de Freitas is an associate professor in geography at the University of Auckland.