by Bill Schlesinger
The current heated debate about the importance of saving old-growth forests is muddled by confusion about what constitutes an “old-growth” forest. Is old growth a forest that has never been cut? That definition is not perfect because it would classify some areas mostly with small trees as old forests, if these areas were burned or subject to hurricane-force winds in their past. It seems reasonable to expect that an old growth forest should contain large trees, if only a small percentage of the population.
Confusion about what constitutes old growth underlies the difference in inventory of such areas in the United States. The U.S. Forest Service and Bureau of Land Management recognize 72 million hectares of mature (45%) or old growth (18%)—much higher values than in most inventories by the conservation community. It is not unreasonable to suggest that the higher estimates from the Federal agencies are meant to diffuse our concern about logging remaining old growth stands.
Regardless of the definition of old growth and about how much remains, there is clear evidence that logging large trees will not enhance carbon sequestration in line with the Biden administration’s desire to mitigate global warming during the critical period of the next few decades. Mature forests continue to store carbon, albeit at lower rates than younger forests. But, replacing old forests with young forests to increase carbon sequestration is a fool’s errand.
When an old-growth forest is removed, most of the carbon it contains is released to the atmosphere, even if the goal is to produce saw-timber. The younger forest that replaces old growth has a greater uptake of carbon, but not enough to make up for the huge amount lost due to logging. Typically, the carbon pay-back period for old growth ranges from 50-100 years. All efforts to burn forest biomass instead of coal are destined to increase carbon dioxide emissions to the atmosphere for decades.
Some argue that logging old growth can be allowed if the entire landscape shows a net increase in forest biomass due to growth of adjacent stands. This overlooks the fact that these other forests were sequestering carbon before the decision to log. It is double counting to now include these in an inventory of new carbon uptake that is helpful in climate-change mitigation—it is not additional. See: Additionality – Translational Ecology (duke.edu).
Saving old-growth forests with large trees provides critical habitat preservation for wildlife. These areas also provide a benchmark of what constituted a healthy forest ecosystem before the onslaught of human activities. (See: What Makes a Healthy Ecosystem? – Translational Ecology (duke.edu))
The definition of old growth is fuzzy, but when an area has large trees, I know it’s old-growth when I see it.
References
Jonker, J.C.G, M. Junginger, and A. Faaj. 2014. Carbon payback period and carbon offset parity point of wood pellet production in the southeastern United States. Global Change Biology Bioenergy 6: 371-389.
Law, B.E., O.J. Sun, J. Campbell, S. Van Tuyl and P.E. Thornton. 2003. Changes in carbon storage and fluxes in a chronosequence of ponderosa pine. Global Change Biology 9: 510-524
Sterman, J., W. Moomaw, J.N. Rooney-Varga and L. Siegel. 2022. Does wood bioenergy help or harm the climate. Bulletin of the Atomic Scientists 78 (3) , pp.128-138.
Stephenson, N.L, et al. 2014. Rate of tree carbon accumulation increases continuously with tree size. Nature 507: 90-93.
