Chris de Freitas: Carbon scheme may be costly symbol

Forests need to be replaced regularly. Photo / Alan Gibson

Five years ago, the current Minister for Climate Change Issues, Dr Nick Smith, said of the Labour Government's proposed greenhouse gas policy:

"The madness of the Government's new carbon tax is that ... it will drive up the costs of living and undermine competitiveness of New Zealand business for negligible environmental gain".

Much the same can be said of the present Government's emissions trading scheme due to come into effect on July 1. First, our greenhouse gas emissions make up less than one half of 1 per cent of the global total, so our scheme will have no effect on global temperatures or global climate.

Second, the emissions scheme is unlikely to be effective in reducing emissions.

Every tree planted removes carbon dioxide from the atmosphere and stores it in the form of wood. But the effectiveness of planting a whole forest to create a reservoir or "sink" for carbon depends on several things.

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

New Zealand's radiata pine forests can average eight or nine tonnes of carbon per hectare per year. 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.

To maintain a strong sink of atmospheric carbon within forests, the forests need to be replaced regularly and the timing of replacement requires clever management.

If the average age of the trees and the average carbon stored as biomass per hectare is allowed to fall, the forest becomes a carbon source and contributes further to carbon dioxide concentrations in the atmosphere.

Effective forest management alone is not sufficient, as the fate of the harvested material determines whether afforestation can serve as a long-term sink for atmospheric carbon.

To be successful, the approach used must ensure the carbon removed stays stored in trees and does not return to the atmosphere.

For a typical radiata pine aged 30, the standard harvesting time for this species of tree, there are 230 tonnes of biomass carbon per hectare stored in tree wood.

Only about half is in the form of usable wood. The rest - of stumps, roots, branches, needles, unsaleable litter - soon decomposes and returns to the atmosphere as carbon dioxide.

About half of the residual sawlog 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.

Research by the former Forest Research Institute showed that, in the end, only 12 per cent of the carbon temporarily stored by forests has a life expectancy of longer than five years.

In forests used solely for pulpwood, most of the end products will be broken down by decay within five years. Even some of the short-term benefits will be lost if fossil fuels are used to process the wood.

The effectiveness of "carbon forests" will also depend on the amount of land that is not now forested but could be used, and whether or not the land area available is sufficient.

To absorb the carbon that would be emitted from a medium-sized (400 MW) 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.

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.

Over the first 12 months of the emissions trading scheme the Government will have given out to foresters well over $1 billion in carbon credits. Presumably this money will come from carbon suppliers such as Solid Energy and these costs will have to be passed on to the public.

The emissions scheme will have negligible environmental gain or impact on emissions so its purpose can only be symbolic. Are we as consumers prepared to pay for a symbol?

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