Do young forests sequester more carbon in the Pacific Northwest?

Age-class and carbon storage are positively correlated across forests in the PNW.

The forests of the Pacific Northwest Coast Range and Western Cascades are some of the most productive in the world containing many large and longed lived tree species such as Douglas fir, western hemlock, western red cedar, and redwoods in the south. Due to this productivity, forests in the Pacific Northwest (PNW) are some of the best methods of carbon capture available, able to store up to 1673.5 mtCO₂/ acre (total ecosystem carbon), including as much as 600 to +700 mtCO₂e /acre in above ground live carbon (1). To put that in perspective, Oregonians emit an average of around 10 mtCO₂ a year per capital. Current and historic forest management practices sequester far less than their ecological potential, as demonstrated in Table 1 below.

Table 1: Area and carbon totals across ownerships for all of Oregon, the OR Coast Range and West Cascades ecoregions from 2007-2016. Source: USFS FIA

Age-class and carbon storage are positively correlated across forests in the PNW. In Oregon, nearly two-thirds (63%) of private forestland in the Coast Range are less than 40 years old. Increasing age class diversity and average age in this region could have profoundly positive carbon impacts. Recent analysis found that extending rotations on the average growing site in the Coast Range from 40 to 90 years could sequester an additional 216 mtCO₂e/acre over the interim 50 years (gross estimate). Moving the 862,000 acres of private coastal timberland in Oregon currently between the ages of 31 – 50 years to the 81 – 90-year age class could result in a gross estimated overall gain of over 200,000,000 mtCO₂e over the next 40 to 50 years.

Advocates of short rotation forestry have focused their message on the fact that ‘healthy, younger’ forest sequester carbon faster than older forests. While this message is not necessarily untrue, as usual, the devil is in the details. In this case, the key detail is how you define a ‘young’ versus an ‘old’ forest and the timeframe over which you are comparing stand-level carbon sequestration. Dr. Andrew Gray of the USFS Forest Inventory and Analysis program has studied carbon accumulation rates in the PNW at length and published several papers on the topic. In a 2016 paper, Gray and others found that the average PNW forest stand accumulates 75% of its maximum carbon by age 127, plus or minus 35 years (2). In another publication, Watts, Gray and others show that stands over 150 years old do not accumulate additional carbon as quickly as younger stands due to increasing mortality rates, however, these stands do store significant amounts of carbon for long periods.

Proponents of short-rotation management are using the ‘younger is better’ mantra to promote current harvest rotations of 30-60 years (40 year average for coastal Douglas Fir). The reality is these forests are just beginning to realize their carbon sequestering potential when they are harvested. This is where the concept of substitution becomes very important in forest carbon accounting. To close the carbon gap between short and long rotations, studies have relied heavily on the substitution carbon pool (3). This is a hypothetical carbon pool created by the assumption that in-leu of long-term wood products, such as dimensional lumber, more carbon intense building materials like concrete and steel would be used. Only as the modeled benefits of carbon in wood products and substitution accumulate over time do short rotations start to look more climate friendly than allowing forests to reach their prime growing years (4).

Of course, older forests also provide a host other environmental and social non-carbon co-benefits such as wildlife habitat, clean water, and biodiversity. The take-away is that positioning short-rotation, high-utilization forest management as climate positive relies on many assumptions regarding harvest and utilization rates, end product usage, and potential substitute materials. Allowing forests to reach their optimal growth years is known to accumulate substantially more carbon in forests while eventually providing an increased volume of higher quality wood products.

For more information on the climate impacts of rotation length in the PNW I invite readers to check out Kate Anderson’s work at Sightline Institute:   https://www.sightline.org/series/forest-rotations/

1. Smithwick et al. 2002. Available at: https://www.jstor.org/stable/3099973?seq=1#page_scan_tab_contents
2. Gray, A. N., Whittier, T. R., & Harmon, M. E. (2016). Carbon stocks and accumulation rates in Pacific Northwest forests: role of stand age, plant community, and productivity. Ecosphere, 7(1), e01224
3. E.g., Perez-Garcia, et al., 2005a & b, 2006, Gustavsson et al., 2017
4. Managing Moist Forests if the Pacific Northwest United States for Climate Positive Outcomes. https://www.mdpi.com/1999-4907/9/10/618