finding 6.1 : key-message-6-1

It is very 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 (high confidence). It is also likely that other changes, resulting from gradual climate change and less severe disturbances, will alter forest productivity and health and the distribution and abundance of species at longer timescales (decades to centuries; medium confidence). 

This finding is from chapter 6 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

Lead authors, chapter authors, and technical contributors engaged in multiple technical discussions via teleconference between September 2016 and March 2018, which included a review of technical inputs provided by the public and a broad range of published literature as well as professional judgment. Discussions were followed by expert deliberation on draft Key Messages by the authors and targeted consultation with additional experts by the authors and technical contributors. A public engagement webinar on May 11, 2017, solicited additional feedback on the report outline. Webinar attendees provided comments and suggestions online and through follow-up emails. Strong emphasis was placed on recent findings reported in the scientific literature and relevance to specific applications in the management of forest resources. 

Description of evidence base:

Many ecological responses to climate change in U.S. forests are mediated though disturbance, because the occurrence and magnitude of most major forest disturbances are sensitive to subtle changes in climate.212b019e-f046-40a4-bc19-5e752527fb1c Published literature since the Third National Climate Assessment (NCA3) continues to show an increase in the frequency of large (thousands to hundreds of thousands of acres) ecological disturbances in forests across the United States. There is strong evidence that these changes, in combination with accumulated fuels, have resulted in larger wildfires in recent years (the past 10 to 20 years),df19cf82-b4eb-4281-a379-2f1863e7142f,9e081680-2097-4818-ad74-114c7ab2025d,ca5c4b38-9aa8-4edc-9aea-42f1625cc45b making them harder to suppress and increasing human health and safety concerns for nearby communities61aece8e-581d-4b3a-a2ca-c0ccafe04db3 and wildland firefighters.6aa821e9-b41a-448c-98c7-64a3ec3dee70 Fire suppression costs continue to increase in response to larger fires and an expanding wildland–urban interface.

Although the increasing size and costs of fighting wildfires are known with high certainty,75a9f6ad-7530-4f06-83ba-9db2cfb2e7c2 short- and long-term effects on forests vary according to the ability of tree species to survive or regenerate after wildfire.1192f0ee-6948-433f-91e5-166267541d52 Future fire regimes and their impacts on U.S. forests will be governed by climate as well as topography, ecosystem productivity, and vegetation adaptations to fire. For example, altered distribution and abundance of dominant plant species may affect the frequency and extent of future wildfires (Ch. 29: Mitigation). The potential of an area to reburn (that is, burn again after experiencing a previous fire) will depend on how the previous fire was suppressed, the severity of that fire, how rapidly fuel accumulated after the fire, and postfire management activities.f82cbbbd-e33b-418a-8f5b-9a39e06f3fa8 These variables create uncertainty in predicting the spatial distribution, number, and sizes of wildfires in future decades.

The published literature contains strong evidence that insects are causing rapid changes in forest structure and function across large landscapes. Causal factors are primarily elevated temperatures, droughts, and water stress, which exert indirect effects mediated through host tree species and direct effects on insects. For example, in western North America, several species of bark beetles have had notable outbreaks over the past 30 years, and some have exceeded the spatial extent of what has been previously documented, affecting ecosystem services at broad spatial scales.74435108-c954-43b3-a05c-5717d9a53620 The spatial extent of recent outbreaks of mountain pine beetles represents an area larger than the 11 smallest U.S. states combined, and insect outbreak models project increased probabilities of mountain pine beetle population success in the future.703f4c0b-a9f3-4393-ad55-e26a62fa5a95 In addition, evidence suggests that climate change is expanding the range of bark beetles in both the western and eastern United States,98e8338c-3c49-49f7-9334-d4c28a901ad0,171606c5-ddd3-4a2f-a0b0-e7af34ab8548,eb3aabd6-6983-43de-acfd-b83484404c39 caused by higher minimum temperatures associated with climate change. For example, whitebark pine is expected to suffer significant mortality in future decades due to the combined effects of white pine blister rust, mountain pine beetles, and climate change.f6b77c84-f7f8-49b5-a9ba-7bededfd5ad5

The magnitude and direction of defoliator responses to climate change vary, limiting our ability to project the effects of climate change68d16e4e-dc4a-4bb6-8592-c8ff6cea4937 and preventing generalizations about climate-related effects on defoliators, despite their importance throughout the United States. Fungal pathogens that depend on stressed plant hosts for colonization are expected to perform better and have greater impacts on forests.29ccd0a0-9e94-4f1d-9f91-bca006e3a975,b3ba546e-9bbf-47c2-a9da-3ddc4252561c,37dde2a6-0bc1-4f4b-90a8-4358a8edd797 In contrast, some pathogens directly affected by moisture availability (for example, needle blights) are expected to have reduced impact.b3ba546e-9bbf-47c2-a9da-3ddc4252561c

Mounting evidence suggests that some bird and insect populations show changes in distribution that align with temperature increases in recent decades (Ch. 7: Ecosystems).ab72ca84-1290-4fcc-9167-7e98600795c3,79d015c8-8eb4-4abf-86c5-b734f936b6d3,62d405d6-7a55-4d80-b324-e3cbabcdae2b,9a0b4626-d670-4071-a94e-d1eddad25e01 These species groups are characterized by short generation times, high mobility, or both. Some evidence suggests that the rate of climate change is outpacing the capacity of trees and forests to adjust, placing long-lived tree populations at risk. Species distribution models concur that climate change can affect suitable habitat,b7106aed-b1b9-4c9d-b3df-f8bd84c4106c although it is unclear if these effects are translating into species range shifts. Some studies report shifts in elevation ranges,3ce6e5b7-f100-4297-afb8-406dc87acf9d,4cdea44a-a22c-4793-bb06-bdb938c82a20 whereas others do not.cccc1ac5-69a9-4bfb-b465-9b44f6ab390f,b2fdc89d-2103-478b-9379-14536265c022,47f8f3a1-d289-4b7a-9960-587f0b9f4f9e In summary, evidence indicates substantial effects of climate change on forest health but varied capacity for tree species to relocate as conditions change.

Understanding and predicting the effects of climate change on forests are obscured by the slow response times of long-lived trees.79707634-1d52-4c53-ac9e-2ec3a4856854 Increasing evidence suggests that climate-related stresses weaken trees, predisposing them to additional stresses that take many years to be observed,401a75ed-2a6e-492e-8088-0fe197e50676 and that growth reductions following drought can persist for years.abe49f4d-90c4-40e2-a4b9-a58158c00560,7dc6e8e9-a248-4bc5-bee4-ec4c83127223,d8731246-25c1-4f5f-95ed-26eb14d03840 For species in which seed crops depend on resources stored over several growing seasons, it is likely that reproductive responses will lag behind climate variation.16273839-b574-4a23-bef6-5b6d56b0a711 Recent studies in the eastern United States suggest that changes in tree species composition (such as an increased proportion of mesophytes) over the past few decades in some forests are contributing to lower streamflowf3c9d456-8919-4504-9d2e-ba2edd7f3409 and increased vulnerability of forests to drought.8b370b07-12b3-4307-878a-a8f98c1fd798 Warming temperatures and changing precipitation are altering leaf phenology (for example, earlier spring leaf-out and later leaf fall) in some areas, which is likely to affect forest carbon and water cycling.34f6ed3b-0fbd-4fef-8332-903a23216341,30006c1a-2e5e-4c0e-86b3-5bf51bcc64f5

New information and remaining uncertainties:

Although wildfire frequency and extent are very likely to increase in a warmer climate, spatial and temporal patterns of fire are difficult to project, especially at smaller than regional scales. The effects of a warmer climate are well known for some insect species (such as bark beetles), but the effects of long-term thermal changes on most insect species and their community associates are uncertain. Scientific information on the effects of climate change on fungal pathogens is sparse, making projections of forest diseases uncertain. It is possible to project that some tree species will have decreased growth and others increased growth, but the magnitude of growth changes is uncertain. Finally, species distribution and abundance are likely to change in a warmer climate, but the magnitude, geographic specificity, and rate of future changes are uncertain.

Assessment of confidence based on evidence:

Published literature and model projections imply high confidence that more frequent extreme weather events will increase the frequency and extent of large ecological disturbances, driving rapid (months to years) and often persistent changes in forest structure and function across large landscapes. Forests are long-lived and inherently resilient to climatic variability, so long-term monitoring (of, for example, growth and productivity, structure, regeneration, and species distribution and abundance) will be needed to confirm the direct effects of incremental changes in temperature. As a result, there is medium confidence that changes resulting from direct (but gradual) climate change and less severe disturbances will occur in the context of altered forest productivity, health, and species distribution and abundance that occur at longer timescales (decades to centuries).

This finding was derived from scenario rcp_4_5
This finding was derived from scenario rcp_8_5

Regions Covered

Related NASA GCMD keywords

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