John Cross of Dunedin asks :-

How best should we use the land in order to mop up carbon dioxide most efficiently?

Ian Payton, an ecologist at Landcare Research, responded.

Put another way this question could read, how do we maximise the size of our carbon sinks, and how do we ensure their permanence? Both plants and soils store carbon. Retaining our existing forests and developing new ones offers the best prospect of maximising the potential of land to mop up carbon. New Zealand is in the fortunate position that most land below treeline is sufficiently warm, moist and fertile to support forest development.

Below the climatic treeline most of New Zealand was forested before human settlement. Wood is approximately 50 per cent carbon and the bulk of the carbon in a forest is contained in the big trees. Planting forests is an expensive business, and isn’t likely to be a viable option for large-scale reforestation, except where there is an expectation that the trees will be harvested. Where harvesting involves soil disturbance this also depletes the carbon stored in the soil. Monocultures are inherently more vulnerable to pests and diseases.

Natural regeneration offers better prospects for the development of larger-scale carbon sinks, because it avoids the need for the costly infrastructure required to propagate and establish tree stocks, it encourages species diversity, and offers a wider range of biodiversity and sustainability options. Naturally regenerating forests accumulate carbon more slowly than plantations, but over the longer term will achieve higher levels of carbon storage.

Apart from ensuring the availability of appropriate seed sources, the main requirement for natural forest regeneration is the removal of herbivores (e.g., farm stock, deer, goats, possums) which hinder the regeneration process. In the current economic environment where the use of privately owned land is largely driven by market forces, it will be important to concentrate reforestation initiatives in areas that are marginally economic for other activities, and to incorporate a covenanting regime to ensure responsibility of maintaining the carbon stock is not lost when the economic climate or the ownership of the land changes.

Further information on regenerating forests as carbon sinks can be obtained from www.EBEX21.co.nz