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Disturbances in U.S. Forests

Disturbance in forests is a natural ecological process, although human actions can and have altered patterns of forest disturbance across the United States in recent decades. When management intentionally mimics natural disturbances, the ecological functioning and climate resilience of the forest can be enhanced.

 

The most common types of natural forest disturbance in the U.S. include fire, insects and disease, drought, heat, and wind. In addition to changing disturbance regimes, many forests are experiencing heightened stress under climate change (USGCRP 2023). The changing climate is increasing the frequency and intensity of many disturbances and has become a “threat multiplier” for many ecosystems.  When exposed to repeated disturbances, forests become more sensitive and less able to cope with and recover from disturbances over time. For example, forests that are experiencing drought stress can be more susceptible to some insect and disease outbreaks (Kolb et al. 2016).  

Effects of Disturbance on Forest Carbon and Ecosystem Function

Disturbances can significantly impact forest carbon uptake and storage. The effect of a disturbance on forest carbon cycling depends on the type and extent of the disturbance, as well as local site characteristics such as tree species composition and diversity. In general, the more severe and extensive a disturbance is, the greater the loss in both carbon storage and future carbon uptake potential (Figure 1). Forests vary in how quickly carbon storage and uptake will return to pre-disturbance levels, with the effects from a changing climate also influencing the ability of forests to recover and regenerate (Dey et al. 2019). The biggest disturbance effects on carbon cycling happen when a forest converts to a different forest type or even to a different ecosystem type all together, such as a forest converting to a shrubland. 

Disturbances not only affect carbon cycling, but can also significantly impact other ecosystem properties such as nutrient cycling and biodiversity. The effects of disturbance on ecosystem functions can then feed back into the system to alter carbon uptake and storage. Managing forests for diverse ecosystem benefits can help mitigate the effects of disturbance and maintain forest carbon cycling (Ontl et al. 2020).

Illustrated comparison of less severe, less extensive disturbance on the left side and more severe, more extensive disturbance on the right side. The less severe and extensive disturbance has a bar graph and trees shaded to indicate more carbon storage and potential future carbon uptake. The more severe and extensive disturbance has a bar that is shaded to indicate that carbon storage and potential future carbon uptake is less.
Figure 1. Illustration of the difference in carbon storage and potential future carbon uptake depending on the severity and extensiveness of a disturbance.

Types of Disturbance

Fire Disturbance

For many ecosystems, fire is a common disturbance that enhances local native plant biodiversity. Fire emits carbon into the atmosphere immediately during combustion and later when plant biomass affected by fire decomposes. The severity and extent of a fire influences its impact on carbon cycling. High-severity, stand-replacing fires generally lead to large losses of carbon from the ecosystem that are not quickly recovered, while low-severity fires that primarily burn the forest understory and litter layer generally result in lower immediate carbon emissions, faster carbon recovery, and often increase long-term forest resiliency to future high-severity fire events.

Large fires are relatively rare in the Midwest and Northeast U.S., even as large fires have increased in recent decades in the West. However, the length of the fire season is expanding across all regions in the U.S (cite NCA 2023). In the coming decades, the Upper Midwest and Northeast are projected to experience an increase in wildfire activity compared to historical observations and a higher frequency of wildfire than other regions (Dye et al. 2024); this may increase forest carbon emissions over time.

Illustration of fire-dependent systems with and without prescribed fire. Panel A shows characteristics of fire-dependent systems such as canopy gaps create areas of partial and full sun, fire-tolerant trees, shrubs, and ground cover, and some downed woody biomass. Panel B shows the immediate effects of prescribed fire such as combustion of non-woodby biomass, litter layer and dead wood, and nitrogen volatilization leading to greenhouse gases released into the amosphere and the mortality of young trees and fire intolerant species. Panel C shows the near-term effects of prescribed fire such as increased resilience to drought, canopy gaps create areas of partial and full sun and reduced down woody biomass. Panel D shows the system without fire with increased vulnerability to drought, increase in shade-tolerant mesic species, shading alters microclimate, and lower structural diversity.
Figure 2. Illustration of fire-dependent systems with prescribed fire and without fire (Keller and Handler 2024). Frequent use of prescribed fire can help maintain a fire-dependent ecosystem (A–C), whereas long-term removal of fire from the landscape can result in significant changes in the microclimate and plant community (D).

Insects and Disease

In recent decades, non-native insects and diseases have impacted forest mortality in the Upper Midwest and Northeast U.S. more than in other regions of the country (USDA FS 2023). Insects and disease reduce forest growth, which in turn reduces carbon uptake. Continued impacts from insects and diseases can eventually increase tree mortality and decrease carbon storage in a forest.

Photograph of a tree stripped of bark that shows larval paths from emerald ash borers.
Figure 3. A dead black ash tree with evidence of Emerald Ash Borer larval galleries where larvae fed on the inner bark. USDA Forest Service photo by Nate Siegert.

Drought and Extreme Heat

Drought and extreme heat (“hot droughts”) can reduce photosynthesis, tree growth, and carbon uptake if trees are exposed to conditions beyond their tolerance zones. In the upper Midwest and Northeast U.S., trees rarely die from drought alone, but rather in combination with other disturbances, such as insect outbreaks. The combination of drought, extreme heat, and other disturbances can lead to significant mortality and reduced carbon storage (Coble et al. 2017), which can be detrimental to forests. 

Over the last century, many of the region’s forests have experienced mesophication, which is when the composition of a forest shifts to include more tree species that prefer wetter conditions. The mesophication process shifts forest composition from one that is more tolerant of drought to one that is less drought tolerant. For example, many forests historically dominated by oak (Quercus spp.) have shifted to become dominated by maple (Acer spp.). How this observed change in tree species composition will affect the vulnerability of the region’s forests to drought is an active area of research. However, there is some evidence that maple-dominated forests are more stressed by late-season drought than oak-dominated forests, and late-season droughts can have strong “legacy effects” by suppressing subsequent years’ growth (An et al. 2020).

Wind

Wind disturbances that uproot trees (blowdowns) can transfer aboveground forest biomass carbon to dead woody carbon pools that will then decompose and cause carbon losses. However, blowdowns can increase forest structural diversity, late-successional forest structure, and carbon storage if tree regeneration is successful (Meigs and Keeton 2018). Hurricanes, which are of higher intensity compared to blowdowns, more significantly reduce aboveground carbon storage and carbon uptake immediately following the disturbance. The overall effect of a hurricane on the carbon balance of a forest depends on if the downed woody debris is left onsite or harvested. If left on site, the downed woody debris can continue to store carbon in the ecosystem for decades as it slowly decomposes. If the biomass is harvested, carbon may be lost quickly to the atmosphere if the resulting wood products are short-lived, like pulp, or the carbon may remain out of the atmosphere for decades if stored in long-lived wood products (Tumber-Davila et al. 2024).

 

Key Terms:

  • Biodiversity
  • Biomass
  • Blowdowns
  • Carbon emissions
  • Carbon pool
  • Carbon storage
  • Carbon uptake
  • Disturbance
  • Forest management
  • Forest stand
  • Harvested wood products
  • Mesophication

For more terms and definitions, see the Carbon Terminology page.

References

Au, T. F.; Maxwell, J. T.; Novick, K. A.; Robeson, S. M.; Warner, S. M.; Lockwood, B. R.; Phillips, R. P.; Harley, G. L.; Telewski, F. W.; Therrell, M. D.; Pederson, N. 2020. Demographic shifts in eastern US forests increase the impact of late‐season drought on forest growth. Ecography. 43: 1475–1486. https://doi.org/10.1111/ecog.05055.

 

Coble, A. P.; Vadeboncoeur, M. A.; Berry, Z. C.; Jennings, K. A.; McIntire, C. D.; Campbell, J. L.; Rustad, L. E.; Templer, P. H.; Asbjornsen, H. 2017. Are Northeastern U.S. forests vulnerable to extreme drought? Ecological Processes. 6(1): 34. https://doi.org/10.1186/s13717-017-0100-x.

 

Dey, D. C.; Knapp, B. O.; Battaglia, M. A.; Deal, R. L.; Hart, J. L.; O’Hara, K. L.; Schweitzer, C. J.; Schuler, T.M. 2019. Barriers to natural regeneration in temperate forests across the USA. New Forests. 50: 11–40. https://doi.org/10.1007/s11056-018-09694-6.

 

Dye, A. W.; Houtman, R. M.; Gao, P.; Anderegg, W. R. L.; Fettig, C. J.; Hicke, J. A.; Kim, J. B.; Still, C. J.; Young, K.; Riley, K.L. 2024. Carbon, climate, and natural disturbance: a review of mechanisms, challenges, and tools for understanding forest carbon stability in an uncertain future. Carbon Balance and Management. 19: 35. https://doi.org/10.1186/s13021-024-00282-0.

 

Keller, A.B.; Handler, S. 2024. Effects of fire on ecosystem carbon in the Midwest and Eastern United States. Technology Transfer. Houghton, MI: U.S. Department of Agriculture, Northern Forests Climate Hub. 8 p. http://doi.org/10.32747/2024.8633530.ch.

 

Kolb, T. E.; Fettig, C. J.; Ayres, M. P.; Bentz, B. J.; Hicke, J. A.; Mathiasen, R.; Stewart, J. E.; Weed, A.S. 2016. Observed and anticipated impacts of drought on forest insects and diseases in the United States. Forest Ecology and Management. 380: 321–334. https://doi.org/10.1016/j.foreco.2016.04.051.

 

Meigs, G. W.; Keeton, W.S. 2018. Intermediate‐severity wind disturbance in mature temperate forests: legacy structure, carbon storage, and stand dynamics. Ecological Applications. 28: 798–815. https://doi.org/10.1002/eap.1691.

 

Ontl, T.A.; Swanston, C.W.; Janowiak, M.K.; Daley, J. 2020. Practitioner’s menu of adaptation strategies and approaches for forest carbon management. In: Forest management for carbon sequestration and climate adaptation [Ontl, T.A; Janowiak, M.K.; Swanston, C.W.; Daley, J.; Handler, S.D.; Cornett, M.; Hagenbuch, S.; Handrick, C.; McCarthy, L.; Patch, N.]. Journal of Forestry. 118(1): 86-101. https://doi.org/10.1093/jofore/fvz062. 

 

Tumber‐Dávila, S. J.; Lucey, T.; Boose, E. R.; Laflower, D.; León‐Sáenz, A.; Wilson, B. T.; MacLean, M. G.; Thompson, J.R. 2024. Hurricanes pose a substantial risk to New England forest carbon stocks. Global Change Biology. 30: e17259. https://doi.org/10.1111/gcb.17259.

 

U.S. Department of Agriculture, Forest Service [USDA FS]. 2023. Future of America’s forest and rangelands: Forest Service 2020 Resources Planning Act Assessment. Gen. Tech. Rep. WO-102. Washington, DC. 348 p. https://doi.org/10.2737/WO-GTR-102.

 

U.S. Global Change Research Program [USGCRP]. 2023. Fifth National Climate Assessment. [Crimmins, A.R.; Avery, C.W.; Easterling, D.R.; Kunkel, K.E.; Stewart, B.C.; Maycock, T.K., eds.]. Washington, DC. 

 

About this Topic Page

This text was prepared by:

  • Adrienne Keller, Northern Institute of Applied Climate Science, Michigan Technological University.
  • Katie Frerker, Northern Institute of Applied Climate Science, USDA Forest Service Eastern Region.
  • Manashree Padiyath, formally Northern Institute of Applied Climate Science, USDA Forest Service Northern Research Station.
  • Kailey Marcinkowski, Northern Institute of Applied Climate Science, Michigan Technological University.

Graphics were adapted, designed, and produced by Kailey Marcinkowski, Northern Institute of Applied Climate Science, Michigan Technological University.

This topic page is part of a collection of resources related to understanding forest carbon. 

Browse the full set of resources