Large Trees Dominate Carbon Storage in Forests East of the Cascade Crest in the United States Pacific Northwest

Abstract

Large-diameter trees store disproportionally massive amounts of carbon and are a major driver of carbon cycle dynamics in forests worldwide. In the temperate forests of the western United States, proposed changes to Forest Plans would significantly weaken protections for a large portion of trees greater than 53 cm (21 inches) in diameter (herein referred to as "large-diameter trees") across 11.5 million acres (∼4.7 million ha) of National Forest lands. This study is among the first to report how carbon storage in large trees and forest ecosystems would be affected by a proposed policy. We examined the proportion of large-diameter trees on National Forest lands east of the Cascade Mountains crest in Oregon and Washington, their contribution to overall aboveground carbon (AGC) storage, and the potential reduction in carbon stocks resulting from widespread harvest. We analyzed forest inventory data collected on 3,335 plots and found that large trees play a major role in the accumulated carbon stock of these forests. Tree AGC (kg) increases sharply with tree diameter at breast height (DBH; cm) among five dominant tree species. Large trees accounted for 2.0 to 3.7% of all stems (DBH ≥ 1" or 2.54 cm) among five tree species; but held 33 to 46% of the total AGC stored by each species. Pooled across the five dominant species, large trees accounted for 3% of the 636,520 trees occurring on the inventory plots but stored 42% of the total AGC. A recently proposed large-scale vegetation management project that involved widespread harvest of large trees, mostly grand fir, would have removed ∼44% of the AGC stored in these large-diameter trees, and released a large amount of carbon dioxide to the atmosphere. Given the urgency of keeping additional carbon out of the atmosphere and continuing carbon accumulation from the atmosphere to protect the climate system, it would be prudent to continue protecting ecosystems with large trees for their carbon stores, and also for their co-benefits of habitat for biodiversity, resilience to drought and fire, and microclimate buffering under future climate extremes.

Older Eastern White Pine Trees and Stands Accumulate Carbon for Many Decades and Maximize Cumulative Carbon

“…In contrast, volume (and therefore carbon) shows the smallest increment in the first 50 years and the biggest in the 50-yr epoch between 100 and 150 years. This superior carbon sequestration in older trees is consistent with recent reports of recent rapid sequestration of older oak trees in Massachusetts (Finzi et al, 2020) and the outsized forest accumulation in large trees (Stephenson et al, 2014;Mildrexler et al, 2020). Here, the largest pine measured in Massachusetts (by volume) achieved 6.62 tC at 190 years old, and we found very large pines at ages ranging up to 350 years at dozens of sites in the Eastern United States.…”

“…To avoid ever more serious consequences of a changed climate, the goal must be to become net carbon negative as soon as possible. Growing suitable existing forests is an effective and low cost means for reducing the atmospheric stock of carbon as others have noted (Fargione et al, 2018;Hudiburg et al, 2019;Moomaw et al, 2019;Mildrexler et al, 2020) and will be demonstrated by the findings reported in this paper. Natural regeneration of forests has recently been found to accumulate more carbon in the first 30 years than managed reforestation (Cook-Patton et al, 2020).…”

Impact of land use and land cover changes on carbon storage in rubber dominated tropical Xishuangbanna, South West China

“…Because large trees dominate carbon storage in natural forests (Mildrexler, Berner, and Law et al 2020), and trees are generally larger in natural forests than plantations, such loss of natural forest cover would hugely reduce carbon storage capacity. We showed that Xishuangbanna's rapid land use change has also reduced its overall ability to store carbon.…”

Commentary: Large Trees Dominate Carbon Storage in Forests East of the Cascade Crest in the United States Pacific Northwest

“…This policy change is designed to allow cutting of young (<150 years) shade-tolerant fir ≥53 cm DBH to facilitate the conservation and recruitment of old (>150 years) shade-intolerant pine and larch (United States Department of Agriculture (USDA) Forest Service, 2020). Mildrexler et al (2020) criticize this proposal based solely on evidence that large trees (i.e., trees ≥53 cm DBH) store more carbon than small trees (i.e., trees <53 cm DBH). Without any analysis of tree-, stand-, or landscape-scale carbon fluxes, Mildrexler et al argue that forest-based climate change mitigation goals can best be served by maintaining prohibitions on cutting young trees ≥53 cm or even extending prohibitions to include trees as small as 30 cm DBH.…”

“…The errors, oversights, and misrepresentations in Mildrexler et al summarized below and in Table 1 make this study an unsuitable basis for evaluating policy change. Mildrexler et al (2020).…”