Using forests to manage carbon: a heated debate

The best way of managing trees and forests for climate change and accounting for contributions of forests and forestry activities in carbon budgets remains hotly contested. Forests can either take up carbon dioxide (CO2) or release more CO2 into the atmosphere. Wood can substitute fossil fuels or energy-intensive materials, but forests are also large carbon reservoirs that add emission peaks if disturbed.

The atmospheric concentration of CO2 has increased from a pre-Industrial 280 ppm (volume parts per million) to just above 407 ppm and will reach 550 ppm by 2050. As the main greenhouse gas, CO2 drives human-induced climate change, with most global CO2 emissions coming from burning fossil fuels, but net deforestation still adds about around 1 billion metric tons of CO2 per year.

Global deforestation is mainly determined by large-scale clearing of tropical forests, still progressing at some 3 million hectares a year. In contrast, European forests have been cleared over many centuries and are expanding, having grown by about 11 million hectares since 1990. Re-growing forests on deforested land creates carbon sinks which remove CO2 from the atmosphere.

Wood can avoid carbon emissions by substituting for materials such as cement or metals and replacing fossil fuels in energy generation. The CO2 released when wood is burnt can be recouped with new trees, making wood a renewable source of energy.

Accounting for forests and forestry activities in carbon balance sheets is a controversial task. For example, the amount of timber harvesting that can be seen as sustainable is regularly contested, even among European countries, or increasing use of wood fuels in energy generation is creating debatable and often perverse outcomes.

Such controversies often boil down to a choice between locking up the existing carbon reservoirs in trees and forests, or growing forests into wood products that replace fossil fuel-intensive alternatives.

Young, rapidly growing forests remove atmospheric carbon quickly, but have relatively small carbon reservoirs. Ageing forests capture carbon at decreasing rates, but build up large carbon reservoirs. When an older forest is logged, soil carbon is released back into the atmosphere, creating a ‘carbon debt’. Especially large old trees store most carbon, but are often over 100 years old. Repayment of the carbon debt may, therefore, take a long time.

Theoretically, older forests reach an equilibrium, when carbon taken up into new growth is balanced by carbon released through decomposition processes. This has been proven wrong. Even 800-year-old forests still continue to take up carbon, and, perhaps more surprisingly, individual large old trees maintain high growth rates, too. Old forests are not only large carbon reservoirs worth maintaining, but actively continue to capture atmospheric carbon.

So, is it better to protect all older forests from harvesting?

There are risks. Firstly, we do not know for how long mature forests will continue to soak up additional CO2 as atmospheric concentrations increase further. Only large-scale experiments such as the Free Air CO2 Enrichment (FACE) programme initiated by the Birmingham Institute of Forest Research can supply real-world data needed to answer this question.

Secondly, large-scale disturbances such as bushfires, drought dieback or pest epidemics, stop trees from taking up more carbon and also mobilise carbon from soils and decaying or burning trees. For example, forests in British Columbia, Canada, have turned from a carbon sink to a net carbon source following large-scale outbreaks of a native pine beetle.

On the upside, in a country with low forest cover such as the UK, any reforestation is beneficial for carbon balance. Yet it will also be important to increase areas of mature, natural woodlands, and protect existing ones from threats.

Apart from carbon, older trees and forests provide a raft of services, including biodiversity, flood mitigation, clean water and human wellbeing benefits. Any policy incentives must aim at balanced outcomes for all forest goods and services. Incentives that commodify one service but not others, too often create unintended consequences. Where forests are concerned, such mistakes are expensive, because it takes a long time to reverse adverse effects on old trees and forests.