Title: Fourth National Climate Assessment, Chapter 6: Forests

Agency: U.S. Global Change Research Program

Comments Close: January 31st, 2018

Summary Index: 

  1. Purpose
  2. Context: The National Climate Assessment & US Global Change Research Program; Chapter 6 Writing Process; Tips for Chapter Review
  3. Details: Key Messages; Summary of Key Messages



Volume II of the Fourth National Climate Assessment “summarizes the impacts of climate change on the United States, now and in the future” (GlobalChange.gov).

Chapter 6, “Forests,” summarizes the observed and expected impacts of climate change on forests, the relevance of these impacts to humans, and approaches and challenges associated with adaptive management. Forests provide benefits to both humans and the natural environment, which include storing C02, regulating water resources, and fueling the wood industry. Changes in climate are causing rapid and long-term forest disturbances including increased wildfires, increases in tree-harming insect populations, increased atmospheric C02, and changes in the timing of snowfall and snow melt. These climate impacts increase the likelihood of wildfires, alter hydrology, and decrease forest growth rate and thus carbon sequestration capacity.  Adaptation measures are being implemented to increase forest resistance and resilience to changes in climate.




The National Climate Assessment & The Global Change Research Program

The National Climate Assessment is a scientific report synthesized by the US Global Change Research Program.  The Program, established by Presidential Initiative in 1989 and mandated by Congress in 1990, is made up of 13 federal agencies that “conduct or use research on global change and its impacts on society." The purpose of the Program is “to assist the Nation and the world to understand, assess, predict and respond to human-induced and natural processes of global change” (GlobalChange.gov).

Chapter Writing Process

Chapter 6: Forests was written by two coordinating lead authors, six chapter authors, and five technical contributors from the U.S. Forest Service and several universities. Multiple teleconferences were held between chapter authors and technical contributors. Targeted consultation with external experts was conducted in addition to elicitation of public feedback. 'Considerable emphasis was placed on recent scientific findings as reported in the literature and relevance to specific applications in the management of forest resources.'

Tips for Chapter Review

Each chapter is broken down into sections by “Key Messages.” Chapter 6 begins with a Summary Overview and an overview of the 'State of the Forest Sector' before delving into each Key Message. Each one- to two-sentence Key Message is followed by a short section that contains a summary of the literature and data that support and expand upon the message.

At the end of each chapter is a final “Traceable Accounts” section, which gives a brief overview of how the chapter was developed. This section then goes on to address the following for each Key Message: (1) describes where evidence for that key message was drawn from, including major projects and particularly critical studies, (2) acknowledges major uncertainties in conclusions and related data, and (3) classifies scientists’ confidence in conclusions.




Chapter 6 Key Messages

  • Ecological Disturbances and Forest Health
    • Rapid Forest Change - Wildfire
    • Rapid Forest Change - Insects and Pathogens
    • Long-term Forest Change
  • Ecosystem Services
    • Forest Carbon Dynamics
    • Water Resources
  • Adaptation


Key Message 1: Ecological Disturbance and Forest Health

‘It is highly likely that more frequent extreme weather events will increase the frequency and magnitude of severe ecological disturbances, driving rapid (months to years) and often persistent changes in forest structure and function across large landscapes. It is also likely that other changes, resulting from gradual climate change and less severe disturbances, will alter forest productivity, health, and the distribution and abundance of specie at longer time scales (decades to centuries).’

Rapid forest changes have the highest potential to alter forest ecosystems. The resilience of forests depends largely on the intensity and frequency of such changes, and societal responses. The interaction of rapid changes, including pathogen and insects, and long-term changes, such as droughts, has the potential to exacerbate ecosystem responses. For example drought can be a stressor, making trees more susceptible to disease, while tree mortality caused by disease increases production of dead fuels and contribute to wildfire intensity. These changes have the potential to impact forest productivity.

Cited data and figures include:

  • Increases in US area burned and federal suppression spending over time for wildfires (Figure 6.2)
  • Total area of forests burned compared to severity of wildfires (Figure 6.3)
  • Wildfires-Changes in Burn Area and Cost: A warm, dry climate has increased the area burned by wildfires across the nation; in some parts of the western US the burn season has increased by up to 80 days
  • Rapid Forest Change-Insect and Pathogens: Tree mortality from the bark beetle has increased in the western US in the past 30 years and the range of other tree-damaging insects is increasing; warmer winters has increased observed cases of eastern white pine leaf diseases in New England
  • Long-Term Forest Change: While increased C02 can increase tree growth, drought and extreme heat can cause stress on vegetation, decreasing productivity

Data sources include:

  • Peer-reviewed scientific literature published since the Third National Climate Assessment
  • Long-term scientific observations

Uncertainties include:

  • Especially at  smaller than regional scales, both temporal and spatial patterns in fires are difficult to project. For example, ‘as the spatial extent of wildfires increases, burned areas may provide fuel breaks’ which can influence pattern, extent, and severity of burns
  • Effects of warmer climate on many forest species, with the exception of a few well-studied cases (e.g., the bark beetle), are not well known
  • Limited research has been done on the effect of climate change on fungal pathogens
  • Drought responses are dependent on natural ecosystem limitations and community structure



Key Message 2: Ecosystem Services

‘It is highly likely that climate change will mostly decrease the ability of forest ecosystems to provide ecosystem services to society. Tree growth and carbon storage are expected to decrease in most locations as a result of higher temperature, more frequent drought, and increased disturbances. The onset and magnitude of climate change effects on water resources in forest ecosystems will vary but are already occurring in some regions.’

Key ecosystem services of forests include carbon dynamics and sequestration, and moderating effects of extreme events such as droughts and rainfall. Currently, the US has a net gain of forest cover; storage by forests offsets 11% of US CO2 emissions. However, increasing disturbances (Key Message 1) will overall decrease storage rates. Changes in hydrology and persistent snowpack have large impacts on downstream ecosystems and human communities (flooding, reduced water quality), and forests have the potential to moderate these impacts.

Cited data and figures include:

  • Map of the types of forest disturbances across the US (Figure 6.4)
  • Forest Carbon Dynamics: The storage by forests have ‘offset approximately 11% of US CO2 emissions’, however US forests are projected to store carbon at declining rates (35% less than 2013 levels by 2037)
  • Water Resources: Changes in tree species in forest watersheds are altering streamflow and water quality; the role the that tree species play  in regulating water resources is affected by changes in snowfall, melt timing, and wildfires

Data sources include:

  • Peer-reviewed scientific literature
  • 2017 Report the US Environmental Protection Agency

Uncertainties include:

  • ‘Uncertainties about the effects of climate change on carbon sequestration are high, because it is difficult to project future trends in both natural and socioeconomic systems’
  • Empirical evidence that increased CO2 promotes tree growth comes from young trees, however no long-term data exists for mature trees
  • High spatial variability in forest structure makes it challenging to identify changes in forest conditions in specific geographic locations and at fine scales
  • Future  land-use conversions and changes in the production of wood are hard to predict


Key Message 3: Adaptation

‘Forest management activities that increase the resilience of US forests to climate change are being implemented, with a broad range of adaptation options for different resources, including applications in planning. The future pace of adaptation will depend on how effectively social, organizational, and economic conditions support implementation.’

Some practices that mitigate stressors could increase forests’ resistance and resilience to climate change, such as reduction in stand density. Current forest management practices are already considered ‘climate smart’ and climate change is being incorporated into management plans, assessments and monitoring programs.

Cited data and figures include: 

  • Climate vulnerabilities and corresponding adaptation options (Figure 6.5)
  • Land management practices, environmental assessments, and monitoring programs that are incorporating aadaptation actions
  • Ongoing evaluations by forest managers to increase forest resistance and resilience to fire, insects and drought
  • Challenges  to adaptation measures presented by the declining forest sector workforce and receding timber outputs

Data sources include:

  • 2016 Report by the US Forest Service
  • Peer-reviewed literature on increasing resistance and resilience to climate change;  on-the-ground implementation efforts

Uncertainties include: 

  • ‘Physical and biological conditions of the future are uncertain, and interactions among multiple ecosystem stressors could have unforeseen outcomes’
  • Adaptation strategies come from the current understanding of how climate change will affect forest ecosystems, though conditions in the future are uncertain
  • ‘The future pace of adaptation and barriers to implementation,' which are likely to persist



Are you a scientist at or near the University of Washington? Attend the UW Program on Climate Change's workshop for this chapter's review. More details here



Note that all single quotes ‘’ cite from the Chapter draft text; material in double quotes “” may come from any of the sources below.

"Chapter 6: Forests." In Fourth National Climate Assessment. US Global Change Research Program. Chapter Leads: James Vose & David Peterson, US Forest Service.

"About USGCRP." United States Global Change Research Program. Retrieved from: www.globalchange.gov/about









Contributors: PhD Candidate, Earth and Space Sciences; MS Candidate, Aquatic & Fishery Sciences