Plantations as carbon sinks: the case for and against
Eucalypts are the most widely planted hardwood in the world. Brazil, China, India, Portugal, Spain, South Africa and Chile all have large plantation estates, mostly of a handful of fast-growing species and hybrids. The trees are grown for pulp, poles, firewood and increasingly for engineered timber. Because they grow quickly and put on a great deal of wood in a few years, they are also promoted as a way to pull carbon out of the atmosphere. The claim is not false, but it needs qualifying.
The case for
A well-managed eucalypt plantation on a good site can add biomass faster than almost any other tree crop. On a per-hectare, per-year basis this is a real and measurable uptake of carbon. If the wood ends up in long-lived products, in buildings for instance, some of that carbon stays out of the atmosphere for decades. If the plantation replaces degraded pasture or bare ground, the soil may also gain carbon over time.
The case against
The first problem is the rotation. Pulpwood plantations are cut every seven to twelve years. Carbon stored in pulp, paper and packaging returns to the atmosphere within a few years, so the net effect over several rotations is a modest standing stock that goes up and down, not a growing store. Counting the growth phase and ignoring the harvest phase produces impressive numbers that do not survive contact with a whole rotation.
The second problem is water. Eucalypts transpire heavily when water is available, and catchment studies in several countries have measured reduced streamflow after plantations replaced grassland or native vegetation. In regions that are already drying, this is not a side issue.
The third problem is what was there before. Replacing native forest or woodland with plantation almost always releases more carbon than the plantation will recapture for a long time, quite apart from what it does to biodiversity. The carbon case for plantations only holds when they are established on land that was not forest.
A reasonable position
Plantations are a wood supply, and a good one. As a climate measure they are a partial and reversible store, useful when the wood goes into durable products and when the land was not forest and is not short of water. They are not a substitute for keeping existing forests standing, and the accounting used to promote them should cover the harvest as well as the growth.
Further reading
- FAO, Global Forest Resources Assessment
- IPCC, Special Report on Climate Change and Land
- CSIRO, plantation water use and catchment studies