James D. Johnston1*
R. Keala Hagmann2,3
S. Trent Seager4
Andrew G. Merschel1Jerry F. Franklin2K. Norman Johnson1,3- 1College of Forestry, Department of Forest Ecosystems and Society, Oregon State University College of Forestry, Corvallis, OR, United States
- 2College of Environment, School of Environmental and Forest Sciences, University of Washington College of the Environment, Seattle, WA, United States
- 3Applegate Forestry LLC, Corvallis, OR, United States
- 4Sustainable Northwest, Portland, OR, United States
A Commentary on
Large Trees Dominate Carbon Storage in Forests East of the Cascade Crest in the United States Pacific Northwest
by Mildrexler, D. J., Berner, L. T., Law, B. E., Birdsey, R. A., and Moomaw, W. R. (2020). Front. For. Glob. Change 3:594274. doi: 10.3389/ffgc.2020.594274
Introduction
The U.S. Forest Service (USFS) recently made revisions to an interim prohibition on cutting trees ≥53 cm diameter at breast height (DBH) in seasonally dry, fire-prone forests of eastern Oregon. 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.
Mildrexler et al. err in assuming that prohibiting logging of relatively large but young shade-tolerant trees will enhance forest carbon storage over time in seasonally dry, fire-prone landscapes. Carbon stores in these forest communities are increasingly vulnerable to the combined effects of more than a century of fire exclusion and a warming climate (Hessburg et al., 2019). Mildrexler et al. disregard the ecological benefits of thinning projects that remove young shade-tolerant trees to enhance the resistance of old shade-intolerant trees that can store carbon over longer periods in the face of a warming climate (Henson et al., 2013; Bradford and Bell, 2017; Stephens et al., 2020). 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.
Table 1. Summary of key errors and misrepresentations in Mildrexler et al. (2020).
Mildrexler et al. Misrepresent Forest Ecology and Carbon Dynamics in Seasonally Dry Forests
At the heart of Mildrexler et al.'s argument is the conviction that current carbon stocks can be maintained, and even more carbon can be stored in seasonally dry forests of eastern Orgon if thinning is limited to trees <53 cm DBH (or, alternately, 30 cm DBH). This argument ignores the fact that current carbon stores in eastern Oregon forests accumulated because fire was effectively excluded from the landscape for more than a century (Parks et al., 2015; Reilly et al., 2017; Haugo et al., 2019). In particular, the number of shade-tolerant fir ≥53 cm DBH increased substantially over the last century as a consequence of fire exclusion (Hagmann et al., 2013, 2014; Merschel et al., 2014; Johnston, 2017; Johnston et al., 2018). Mildrexler et al. ignore research showing that dry forests have overshot their carbon-carrying capacity and that thinning treatments, although they reduce carbon stocks in the short term, will tend to stabilize carbon stocks over multi-decadal time scales in the face of a warming climate (e.g., Hurteau et al., 2019; Krofcheck et al., 2019). Mildrexler et al. assert without evidence that large shade-tolerant fir are not over-represented on the landscape and that forests of eastern Oregon have “low future climatic vulnerability.” But deepening drought and increasing fire extent and severity throughout eastern Oregon (Reilly et al., 2017; Parks and Abatzoglou, 2020) have made it clear that much of the carbon currently stored on this landscape is increasingly vulnerable to loss over the next several decades if stand densities remain at their current levels (Halofsky et al., 2018; Kerns et al., 2018; Stephens et al., 2020).
The USFS's proposal to allow cutting of some large but young shade-tolerant trees is designed to restore ecosystem resilience to fire and drought and increase the resistance (and long-term carbon storage potential) of shade-intolerant old-growth trees, especially ponderosa pine. Old-growth ponderosa pine has extensive heartwood and exceptional drought, insect, and fire tolerance when freed from competition with fast-growing shade-tolerant fir with high leaf area and transpiration demands (Hessburg et al., 2020). Mildrexler et al. assert that extant populations of young shade-tolerant fir can provide “centuries of long-term carbon storage” and that removal of relatively large young trees facilitated by Forest Service policy change represents a net emission to the atmosphere over all spatial and temporal scales. In fact, relative to the old pine and larch they endanger, large young fir that were off-limits to removal are far more prone to heart rot, which results in significant greenhouse gas emissions (Aho, 1977; Covey et al., 2012). They are also far more prone to mortality from drought, insects, and root diseases than pine. A number of studies investigating mortality of grand fir in eastern Oregon report 100% mortality of large fir over 10–20 years of observations (i.e., Cochran, 1998; Filip et al., 2007).
Throughout their paper, Mildrexler et al. assert that prohibitions on cutting large but young fir in eastern Oregon convey significant benefits to wildlife, water quality, and fire and drought resilience. But the literature cited in support of these claims either speaks to management of old-growth trees in highly productive mesic forests of western Oregon or actually makes the case for the USFS's proposal to remove large but young fir to reduce competition with fire- and drought-tolerant old-growth pine and larch. There is little doubt that conserving the most productive structurally complex older forests in western Oregon achieves carbon storage, water quality, and wildlife habitat benefits without risking uncharacteristically extensive mortality from fire and drought (Halofsky et al., 2018). But in seasonally dry forests of eastern Oregon, research demonstrates that providing a wide range of wildlife habitat, protecting old-growth trees, and enhancing stream and watershed health is best achieved by judicious removal of young trees, including large shade-tolerant trees, that established while fire was excluded from the landscape (Lehmkuhl et al., 2007; Fontaine and Kennedy, 2012; Hessburg et al., 2020).
Discussion
Avoiding catastrophic effects of rising global temperatures is the most important challenge facing human civilization (IPCC, 2018). Forests have an important role in sequestering carbon to offset anthropogenic emissions. For instance, deferring harvest or increasing rotation ages in mesic forests currently below their carbon storage capacity has tremendous potential for offsetting emissions (Hudiburg et al., 2009). But relying on seasonally dry, fire-prone stands that are currently well above historical levels of aboveground tree carbon is likely to destabilize carbon stocks and forfeit the multiple ecological benefits associated with restoration treatments, especially as the climate warms (Hurteau et al., 2016; Liang et al., 2018; Foster et al., 2020; Stephens et al., 2020). We urge policy makers to rely on comprehensive and accurate accounts of carbon dynamics when crafting policy for dry forests.
Author Contributions
JJ, RH, SS, AM, JF, and KJ contributed research and writing. All authors contributed to the article and approved the submitted version.
Conflict of Interest
NJ was employed by Applegate Forestry.
The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Acknowledgments
The authors are indebted to Harold Zald, Matt Reilly, the associate editor, and two reviewers for helpful comments that improved a draft manuscript.
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Keywords: carbon storage, climate change mitigation, dry forests, eastern Oregon, eastside screens, forest restoration, 21-inch rule
Citation: Johnston JD, Hagmann RK, Seager ST, Merschel AG, Franklin JF and Johnson KN (2021) Commentary: Large Trees Dominate Carbon Storage in Forests East of the Cascade Crest in the United States Pacific Northwest. Front. For. Glob. Change 4:653774. doi: 10.3389/ffgc.2021.653774
Received: 15 January 2021; Accepted: 26 February 2021;
Published: 23 March 2021.
Edited by:
Marco Mina, Université du Québec à Montréal, CanadaReviewed by:
Dario Martin-Benito, Centro de Investigación Forestal (INIA), SpainKerry Kemp, The Nature Conservancy, United States
Copyright © 2021 Johnston, Hagmann, Seager, Merschel, Franklin and Johnson. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: James D. Johnston, james.johnston@oregonstate.edu