Retorts

Forest Carbon Sinks

In Science Digest [May], a visiting climatologist (Kevin Trenberth) is quoted as saying "The trouble with trading off trees for CO₂ is that means you've got to keep planting trees and they aren't allowed to die or be used or you will lose the benefit". I find such ignorance understandable in the general public, but quite depressing when attributable to an "expert" and quoted in a Royal Society newsletter.

The reality is that global deforestation has been a contributor to enhanced levels of atmospheric CO₂ and reforestation is merely the reverse of this process. A landscape of low carbon-density is being converted to one of higher carbon-density, a one-off operation. Whether the individual trees die in the forest or are extracted and used by humans is not the main issue here. For example, Kaingaroa Forest is a large area of land that is logged every day, but it undoubtedly contains more carbon than the bare landscape of sparse shrub that it replaced in the 1930s. When evaluating the carbon gain per hectare, one sums the carbon in all the various age-classes, divides by the total area and subtracts the carbon present prior to human intervention.

It is true that, for offset forestry to provide long-term benefit, new land (i.e. with a low carbon density) must continuously be planted in trees. Eventually, we will run out of plantable land. I estimate that, in New Zealand, this may not occur for 50-100 years. If our planning horizon is longer than a century, then afforestation is undoubtedly not a permanent solution. But in the meantime, trees displace methane-emitting livestock and possibly reduce emissions of nitrous oxide. The products of forestry also provide a low-energy alternative to substitutes like aluminium, steel and plastic.

Piers Maclaren, NZ Forest Research Institute, Rangiora