Carbon sequestration and storage

Highlights

BC’s old coastal forests store huge amounts of carbon (up to 1,300 Mg ha^-1) which is more than in tropical or boreal areas and second in the world only to old multi-layered Australian temperate moist mountain ash forests.
On sites like Fairy Creek, old forests are estimated to store twice as much carbon as mature forests and six or more times as much as clearcuts. Productive coastal old forests can store up to six times more carbon than old forests in drier climatic areas.
As forests age, the rate of net carbon uptake by trees levels off or decreases but they still remove carbon dioxide from the air and total storage continues to increase unless a stand-replacing disturbance occurs.
Logging old-growth forests releases 40–65% of the ecosystem carbon to the atmosphere, even when off-site storage of carbon in wood products is factored in.
For a second growth stand to recover the amount of carbon stored in a 300-year-old stand (i.e., to get back to the same carbon density per hectare) could take 200 years or more, therefore crucial short-term greenhouse gas mitigation objectives will not be met by converting old growth to second growth.
During stand development, part of the carbon sequestered by trees and other vegetation is transferred to the carbon stock of the soil in the form of litterfall and root turnover. Organic matter and carbon steadily accumulate in the forest ﬂoor and mineral soil horizons as stand age.

Climate change and greenhouse gases

Most scientists agree that climate change is occurring on Earth and that it is primarily due to an increase in ‘greenhouse gases’ including carbon dioxide, which has resulted from increased emissions as the world’s population and industrial activities including timber resource extraction have increased. The increased level of greenhouse gases is changing our climate by trapping heat in the atmosphere.

Carbon sequestration by forests

Forests take in carbon dioxide from the atmosphere and convert it into stored carbon, thus they have an important role to play in reducing atmospheric carbon dioxide levels and thereby lessening the degree of climate change. Temperate forests represent one-third of the global forest and are accumulating carbon in large enough quantities to affect the global carbon budget (; ). Forests are of particular interest for offsetting atmospheric carbon dioxide because they do not require new technologies or infrastructure to mitigate climate change (). Carbon in forests is stored above-ground in live and dead trees (stems, bark, branches, leaves), understory vegetation including shrubs, herbs and , downed woody debris (logs, branches, and twigs), and below-ground in roots, fungi, soil microbiota, forest floor and mineral soil.

The process of carbon storage in trees, which is eventually transferred to snags and logs, begins with photosynthesis. Old trees are a major site for photosynthesis, or production of the food base, on which the rest of the old growth forest system depends. The amount of leaf area and biomass that is needed for photosynthetic production is generally large and intact in old growth forests (). Although gross is maintained at high levels in most old growth stands, it is usually balanced by mortality, with merchantable volume and living biomass remaining constant for centuries, or gradually decreasing due to increasing defects, although this balance may change in extremely old forests (>750 years). Old forests can continuously operate as carbon-rich banks because in addition to the living tree carbon, over time they accumulate large amounts of carbon in litter, the soil, and woody debris (snags, down logs) which accumulates more rapidly than it decomposes (; ). Total organic matter (live plus dead) keeps increasing as old forests age and is thought to peak around 800 or 1000 years.

Release of carbon through respiration and decomposition

Forests also return carbon to the atmosphere through the processes of respiration and decomposition. Organic matter is composed of the remains of plants and animals, and it decomposed by the soil foodweb containing myriad organisms such as bacteria and fungi. In decomposition, these organisms derive energy and nutrients by feeding on the organic matter. The bacteria and fungi use cellular respiration to extract the energy contained in the chemical bonds while feeding on (decomposing) the organic matter, and so release carbon dioxide into the atmosphere.

Carbon balance in forests

The net balance between sequestration and losses is important because it determines whether the forest is currently a net carbon source or sink. Carbon sequestration in old coastal forests in B.C. has resulted in steady accumulations of terrestrial carbon for centuries and sudden removal of old forests immediately releases large amounts of carbon, even after accounting for carbon stored in forest products with a long life (). As forests age, the rate of net carbon uptake by trees levels off or decreases but they still remove carbon dioxide from the air and total storage continues to increase unless a stand-replacing disturbance occurs.

Effect of logging on the carbon balance

Logging old-growth forests and replacing them with vigorous second-growth stands will not improve the forest carbon balance, at least in the short-term. For a second growth stand to recover the amount of carbon stored in a 300-year-old stand (i.e., to get back to the same carbon density per hectare) could take 200 years or more, therefore crucial short-term GHG mitigation objectives will not be met with this replacement strategy (Pojar, 2021Pojar, J. 2021. Old-growth forests of Fairy Creek, Vancouver Island, British Columbia.).

A large pulse of carbon is lost immediately after clearcut logging due to the removal of all or most trees. Respiration is also increased after logging because disturbance to the soil and original vegetation, and sometimes warming of the site, results in an increased rate of decomposition of coarse woody debris, litter, and soil organic matter. As well, sequestration rates are lower for one to several decades after logging until rates of net carbon uptake in the secondary forest return to pre-harvest rates. The net effect is that losses of carbon dioxide due to respiration exceed the amount fixed through photosynthesis by the regenerating forest for at least a decade (Fredeen et al., 2007Fredeen, A.L., Waughtal, J.D., Pypker, T.G. 2007. When do replanted sub-boreal clearcuts become net sinks for CO2? For. Ecol. Manag. 239:210-216. https://doi.org/10.1016/j.foreco.2006.12.011; Luyssaert et al., 2008Luyssaert, S., Schulze, E.-D., Börner, A, Knohl, A., Hessenmöller, D., Law, B.E., Ciais, P., Grace, J. 2008. Old-growth forests as global carbon sinks. Nature 455:213-215.)

Amount of carbon stored in forests

Australian temperate moist mountain ash. Photo by John Benson.

BC’s old coastal forests store huge amounts of carbon per hectare (up to 1,300 Mg ha^-1) which is second in the world only to old multi-layered Australian temperate moist mountain ash (Eucalyptus regnans) forests which have been estimated to store 1,819 Mg C ha⁻¹ in living above-ground biomass and 2,844 Mg C ha⁻¹ in total biomass (). It has been suggested that some long-gone old growth on the Pacific west coast could have stored up to 2,000 Mg C ha^-1 (). Tropical and boreal forests store less carbon than old growth coastal forests (). In the wet, productive CWHvm subzone in coastal BC, which is where Fairy Creek occurs, old forests have been estimated to store twice as much carbon as mature forests and six or more times as much as clearcuts (; ; ) (Table 1). Also in the , new clearcuts stored no carbon in aboveground tree boles while old growth was estimated to store an average of 545 Mg C ha^-1 and mature forests 149 Mg C ha^-1 (; ).

Influence of climate on carbon stored in forests

Regional climate has a huge influence on total carbon stored in forests. Coastal productive old forests in Oregon stored up to six times more carbon than old forests in inland Oregon where the climate is drier (). Similarly, mature coastal forests stored about 20% more ecosystem carbon than mature inland rainforest forests and about three times as much as mature dry interior Douglas-fir forests (; ). Ecosystem carbon storage in white spruce-subalpine fir forests in the central interior of B.C. was intermediate between the dry Interior Douglas-fir stands and inland cedar-hemlock rainforests (). In mature forests, tree boles accounted for just 44 Mg C ha^-1 in the dry Douglas-fir type, but more than twice that amount in the inland rainforest and three times that much on a productive coastal cedar-hemlock site ().