- Lead Authors:
- Grant Domke, USDA Forest Service
- Christopher A. Williams, Clark University
- Contributing Authors:
- Richard Birdsey, Woods Hole Research Center
- John Coulston, USDA Forest Service
- Adrien Finzi, Boston University
- Christopher Gough, Virginia Commonwealth University
- Bob Haight, USDA Forest Service
- Jeff Hicke, University of Idaho
- Maria Janowiak, USDA Forest Service
- Ben de Jong, El Colegio de la Frontera Sur
- Werner A. Kurz, Natural Resources Canada, Canadian Forest Service
- Melissa Lucash, Portland State University
- Stephen Ogle, Colorado State University
- Marcela Olguín-Álvarez, Consultant, SilvaCarbon Program
- Yude Pan, USDA Forest Service
- Margaret Skutsch, Centro de Investigaciones en Geografía Ambiental
- Carolyn Smyth, Natural Resources Canada, Canadian Forest Service
- Chris Swanston, USDA Forest Service
- Pamela Templer, Boston University
- Dave Wear, USDA Forest Service
- Christopher W. Woodall, USDA Forest Service
<b>Domke</b>, G., C. A. <b>Williams</b>, R. Birdsey, J. Coulston, A. Finzi, C. Gough, B. Haight, J. Hicke, M. Janowiak, B. de Jong, W. A. Kurz, M. Lucash, S. Ogle, M. Olguín-Álvarez, Y. Pan, M. Skutsch, C. Smyth, C. Swanston, P. Templer, D. Wear, and C. W. Woodall, 2018: Chapter 9: Forests. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 365-398, https://doi.org/10.7930/SOCCR2.2018.Ch9.
Forests
SUPPORTING EVIDENCE
KEY FINDINGS
Key Finding 1
Net uptake of 217 teragrams of carbon (Tg C) per year by the forest sector in North America is well documented and has persisted at about this level over the last decade. The strength of net carbon uptake varies regionally, with about 80% of the North American forest carbon sink occurring within the United States (high confidence, very likely).
Description of evidence base
Net carbon uptake in North American forests, as documented in national inventory reports from Canada (ECCC 2016), Mexico (INECC/SEMARNAT 2015), and the United States (U.S. EPA 2018), is in broad agreement with results from a wide range of sources (Hayes et al., 2012; King et al., 2015). These sources include atmospheric inversion models (Peylin et al., 2013), syntheses of forest inventory and land-change data (Pan et al., 2011), measurements of forest-atmosphere carbon exchange with eddy covariance (Amiro et al., 2010), and ecosystem process models (Sitch et al., 2015).
Major uncertainties
Regions differ widely in their source and sink patterns and drivers. For example, in the United States, the Northeast has a prevailing legacy of carbon uptake from historical land clearing; in the Southeast, carbon uptake is dominated by regrowth from contemporary harvesting; and the West has increasing carbon releases from the recent rise in environmental stresses (e.g., droughts, insects, and pathogens) and disturbances (Williams et al., 2016). Fluxes also exhibit large spatial variability at landscape scales (Turner et al., 2016; Williams et al., 2014), with neighboring stands ranging from sources to sinks because of a host of factors including time since disturbance, disturbance type and severity, forest type, local climate, site fertility, topographic position, and other edaphic factors.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
While some uncertainty remains about the spatial patterns and drivers of carbon sources and sinks across the continent, multiple lines of evidence converge to provide high confidence regarding the magnitude of net carbon uptake across North America’s forests in recent decades.
Summary sentence or paragraph that integrates the above information
It is highly likely that North American forests represent a net sink of carbon, given the convergence in evidence across multiple inventory, scaling, and modeling approaches in Canada, Mexico, and the United States.
Key Finding 2
Forest regrowth following historical clearing plays a substantial role in determining the size of the forest carbon sink, but studies also suggest sizeable contributions from growth enhancements such as carbon dioxide (CO2) fertilization, nitrogen deposition, or climate trends supporting accelerated growth (medium confidence). Resolving each factor’s contribution is a major challenge and critical for developing reliable predictions.
Description of evidence base
Although the use of remote sensing (e.g., Landsat) has led to major advances over the past decade in monitoring aspects of disturbance and land-use change (Bachelet et al., 2015; Hansen et al., 2013), critical research gaps remain. Disturbance histories at the stand scale and attribution to disturbance type and severity remain poorly characterized, as are rates of forest conversion.
Major uncertainties
Improved estimates of the location, severity, and timing of natural disturbances are needed, particularly in Mexico. Degradation of forest stocks (e.g., from selective logging, low-severity disturbances, and stress) also remain poorly characterized at the scales needed for assessing carbon dynamics and managing forest carbon. Also needed are landscape-scale records of management practices such as replanting, selective harvesting, cyclical use, and agroforestry. Integration of a range of remote-sensing technologies, including light detection and ranging (LIDAR), with field plot data and carbon cycle modeling, promises to substantially improve the ability to measure and monitor forest carbon dynamics at large scales. Addressing these and other gaps ultimately will lead to spatially explicit estimates of carbon stocks and fluxes that comprehensively assess impacts of disturbance, management, and environmental changes on carbon fluxes.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
While the evidence base strongly supports the finding of net carbon uptake by North American forests, attribution of this carbon uptake to driving factors remains less well understood. This is in part because each factor’s contribution is likely to change across diverse forest settings and conditions.
Summary sentence or paragraph that integrates the above information
Attributing carbon fluxes in North American forests to specific natural and human activities remains a challenge given the diversity of forest types, land-use changes, disturbance dynamics, and human activities that influence these fluxes.
Key Finding 3
Annual harvest removals from forestry operations in select regions decrease forest carbon stocks, but this decline in stocks is balanced by post-harvest recovery and regrowth in forestlands that were harvested in prior years. Removal, processing, and use of harvested biomass causes carbon emissions outside of forests, offsetting a substantial portion (about half) of the net carbon sink in North American forests (high confidence).
Description of evidence base
Recent trends in natural disturbance rates indicate that the strength of net forest uptake has diminished across much of North America. Net loss of forest carbon stocks from land conversions also reduces sink strength across the continent, with carbon losses from forest conversion exceeding carbon gains from afforestation and reforestation. These findings are supported by 1) national inventory reports of greenhouse gas emissions and removals in the forestland category in Canada (ECCC 2016), Mexico (INECC/SEMARNAT 2015), and the United States (U.S. EPA 2018); 2) atmospheric inversion models (Peylin et al., 2013); 3) syntheses of forest inventory and land-change data (Pan et al., 2011); 4) measurements of forest-atmosphere carbon exchange with eddy covariance (Amiro et al., 2010); and 5) ecosystem process models (Sitch et al., 2015).
Major uncertainties
Intensively managed forests are among the most well understood ecosystems in North America. Decomposition dynamics associated with harvested wood products are less well understood, however, and changes in forest use and climate may alter these dynamics in the future. Furthermore, basic understanding of carbon flux and stock dynamics following disturbance is still limited, with some studies suggesting a substantial impact to fluxes (Edburg et al., 2011) and others reporting a more muted response (Moore et al., 2013; Reed et al., 2014). Predictions of future disturbance trends are hampered by limited understanding of disturbance interactions from legacies of flammability, host species presence and absence, and active management responses such as fuel reduction treatments or preemptive and salvage logging.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
The carbon balance impacts of harvesting are well observed and well understood thanks to a wide range of observations that are compiled, analyzed, and reported in detailed accounts.
Summary sentence or paragraph that integrates the above information
Intensive forest management in select regions is widely known to cause large annual reductions in forest carbon stocks. Less understood is how forest regrowth (which often takes decades) compensates for these losses.
Key Finding 4
Recent trends in some disturbance rates (e.g., wildfires and insects) have diminished the strength of net forest carbon uptake across much of North America. Net loss of forest carbon stocks from land conversions reduced sink strength across the continent by 11 Tg C per year, with carbon losses from forest conversion exceeding carbon gains from afforestation and reforestation (medium confidence).
Description of evidence base
Carbon impacts of disturbance vary with several key features, including disturbance type and severity, temporal sequence of events, and biotic and climatic conditions of forest regeneration (Hicke et al., 2012; Williams et al., 2016). The extent, severity, and frequency of natural disturbances have increased in recent decades (Allen et al., 2010; Hicke et al., 2013), likely influenced by recent climate change and human activities.
Major uncertainties
Basic understanding of carbon flux and stock dynamics following disturbance is still limited, with some studies suggesting a substantial impact to fluxes (Edburg et al., 2011) and others reporting a more muted response (Moore et al., 2013; Reed et al., 2014). Predictions of future disturbance trends are hampered by limited understanding of disturbance interactions from legacies of flammability, host species presence and absence, and active management responses such as fuel reduction treatments or preemptive and salvage logging.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Patterns and trends of major disturbances and forest conversions are well documented, however, their effects on carbon uptake and release can be diverse, presenting a significant challenge for assessing impacts on the carbon cycle.
Summary sentence or paragraph that integrates the above information
Detection and quantification of natural disturbance and land-use change in forest ecosystems have improved over the last decade. However, basic understanding of carbon dynamics following these events is still limited. Nevertheless, evidence suggests that recent trends in natural disturbance rates have diminished the strength of net forest uptake across much of North America.
Key Finding 5
Several factors driving the carbon sink in North American forests are expected to decline over coming decades, and an increasing rate of natural disturbance could further diminish current net carbon uptake (medium confidence).
Description of evidence base
Accounting for land-use change, management, disturbance, and forest aging, U.S. forests are projected to continue to uptake carbon but at declining rates, largely because of land-use dynamics and aging forests (USDA-OCE 2016; Wear and Coulston 2015). After 20 years of net gains, forest area is projected to level and then decline gradually after 2030 because of ongoing population growth and declining afforestation on agricultural lands (U.S. Forest Service 2012; Wear and Coulston 2015). In the western United States, aging forests coupled with disturbance dynamics are projected to diminish carbon uptake to negligible levels by midcentury. Younger productive forests in the East are expected to take up atmospheric carbon at a high rate, though harvest-related emissions substantially reduce the net effect on atmospheric carbon.
Major uncertainties
Basic understanding of carbon flux and stock dynamics following disturbance is still limited, with some studies suggesting a substantial impact to fluxes (Edburg et al., 2011) and others reporting a more muted response (Moore et al., 2013; Reed et al., 2014). Predicting disturbance trends into the future is challenging because of limited understanding of disturbance interactions from legacies of flammability, host species presence and absence, and active management responses such as fuel reduction treatments or preemptive and salvage logging. Forest regrowth following historical clearing plays a role, but studies also suggest sizeable contributions from growth enhancements such as CO2 fertilization, nitrogen deposition, or climate trends supporting accelerated growth. Resolving each factor’s contribution is a major challenge and critical for developing reliable predictions.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Although projections vary depending on future climate and land-use scenarios, theory, observations, and modeling all support the expectation that today’s carbon uptake from aging forests and from forest expansion will begin to decline in coming decades, and that natural disturbances will become more frequent and severe, releasing more forest carbon to the atmosphere.
Summary sentence or paragraph that integrates the above information
Although detection and quantification of natural disturbance and land-use change in forest ecosystems have improved over the last decade, basic understanding of carbon dynamics following these events is still limited. Several factors driving the forest carbon sink are expected to decline over coming decades, and although predicting disturbance trends into the future is challenging, an increasing rate of natural disturbance could further diminish the current estimated net carbon uptake by North American forests.
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