finding 7.1 : increased-forest-vulnerability

Climate change is increasing the vulnerability of many forests to ecosystem changes and tree mortality through fire, insect infestations, drought, and disease outbreaks.

This finding is from chapter 7 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: A central component of the process was a workshop held in July 2011 by the U.S. Department of Agriculture Forest Service to guide the development of the technical input report (TIR). This session, along with numerous teleconferences, led to the foundational TIR, “Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector.”78f2cbd8-d8f2-4d99-abbd-017bad4d52f1 The chapter authors engaged in multiple technical discussions via teleconference between January and June 2012, which included careful review of the foundational TIR and of 58 additional technical inputs provided by the public, as well as other published literature and professional judgment. Discussions were followed by expert deliberation of draft key messages by the authors and targeted consultation with additional experts by the lead author of each message.

Description of evidence base: The key message and supporting text summarizes extensive evidence documented in the TIR, “Effects of Climatic Variability and Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector.”78f2cbd8-d8f2-4d99-abbd-017bad4d52f1 Technical input reports (58) on a wide range of topics were also received and reviewed as part of the Federal Register Notice solicitation for public input. Dale et al.c3002370-0cf9-4544-a128-fbc42f3ab137 addressed a number of climate change factors that will affect U.S. forests and how they are managed. This is supported by additional publications focused on effects of drought and by more large-scale tree die-off events, 878be8a3-989e-497d-af88-5417df6ab074 e9b2f293-1699-4bee-99b7-0d2fd93d267d wildfire,5ba92ba4-eb88-480e-91de-a442b293e649 1639ecca-a0ca-48f4-b64c-a447fe137284 517021ad-7515-4d23-8b25-735f52448514 391560e0-40c1-4f9d-b063-e87d18c87e02 b95e9226-076c-4eb5-9367-472499624084 insects and pathogens. 878be8a3-989e-497d-af88-5417df6ab074 e9b2f293-1699-4bee-99b7-0d2fd93d267d Other studies support the negative impact of climate change by examining the tree mortality rate due to rising temperatures,b71bc9a2-2cf3-4544-8cf7-1dde524b5655 878be8a3-989e-497d-af88-5417df6ab074 0346508c-1b13-4e3e-a95d-33acaac2b2c1 9c23a870-58cf-49f6-9c6f-01cb94e4bb5a 5ba92ba4-eb88-480e-91de-a442b293e649 298cdb3f-64e7-4ac9-814c-f8deefbf964b 19710f0a-4207-4830-aa73-4a7104ec1536 e9b2f293-1699-4bee-99b7-0d2fd93d267d which is projected to increase in some regions.e9b2f293-1699-4bee-99b7-0d2fd93d267d Although it is difficult to detect a trend in disturbances because they are inherently infrequent and it is impossible to attribute an individual disturbance event to changing climate, there is nonetheless much that past events, including recent ones, reveal about expected forest changes due to future climate. Observational298cdb3f-64e7-4ac9-814c-f8deefbf964b and experimentale9b2f293-1699-4bee-99b7-0d2fd93d267d studies show strong associations between forest disturbance and extreme climatic events and/or modifications in atmospheric evaporative demand related to warmer temperature. Regarding eastern forests, there are fewer observational or experimental studies, with Dietz and Moorcroftc74b9c1e-ff67-4e76-8b15-825986629c8c being the most comprehensive. Pollution and stand age are the most important factors in mortality. Tree survival increases with increased temperature in some groups. However, for other tree groups survival decreases with increased temperature.c74b9c1e-ff67-4e76-8b15-825986629c8c In addition, this studyc74b9c1e-ff67-4e76-8b15-825986629c8c needs to be considered in the context that there have been fewer severe droughts in this region. However, physiological relationships suggest that trees will generally be more susceptible to mortality under an extreme drought, especially if it is accompanied by warmer temperatures.a68fee7a-d804-4ce3-8b4c-f7def043d687 9a761c4d-25f3-4257-ba8f-5a83730c6630 Consequently, it is misleading to assume that, because eastern forests have not yet experienced the types of large-scale die-off seen in the western forests, they are not vulnerable to such events if an extreme enough drought occurs. Although the effect of temperature on the rate of mortality during drought has only been shown for one species,e9b2f293-1699-4bee-99b7-0d2fd93d267d the basic physiological relationships for trees suggest that warmer temperatures will exacerbate mortality for other species as well.a68fee7a-d804-4ce3-8b4c-f7def043d687 9a761c4d-25f3-4257-ba8f-5a83730c6630 Figure 7.1: This figure uses a figure from Goetz et al. 2012088ac1ea-02ab-48cf-b9e2-c8a1a0501e16 which uses the MODIS Global Disturbance Index (MGDI) results from 2005 to 2009 to illustrate the geographic distribution of major ecosystem disturbance types across North America (based on Mildrexler et al. 2007, 2009f4e0a73e-f6c2-451f-9af1-07aa1cffdc05 d5cd84b4-14ed-49cb-a9e5-665190abee75). The MGDI uses remotely sensed information to assess the intensity of the disturbance. Following the occurrence of a major disturbance, there will be a reduction in Enhanced Vegetation Index (EVI) because of vegetation damage; in contrast, Land Surface Temperature (LST) will increase because more absorbed solar radiation will be converted into sensible heat as a result of the reduction in evapotranspiration from less vegetation density. MGDI takes advantage of the contrast changes in EVI and LST following a disturbance to enhance the signal to effectively detect the location and intensity of disturbances ( Moderate severity disturbance is mapped in orange and represents a 65%-100% divergence of the current-year MODIS Global Disturbance Index value from the range of natural variability, High severity disturbance (in red) signals a divergence of over 100%.088ac1ea-02ab-48cf-b9e2-c8a1a0501e16

New information and remaining uncertainties: Forest disturbances have large ecosystem effects, but high interannual variability in regional fire and insect activity makes detection of trends more difficult than for changes in mean conditions.41269e24-05f2-4c4b-9425-5555d7fcf694 089d8050-f4c8-4d07-bc35-25bf61691be3 291c90fe-6766-40c2-9580-aea56178ecaa Therefore, there is generally less confidence in assessment of future projections of disturbance events than for mean conditions (for example, growth under slightly warmer conditions).089d8050-f4c8-4d07-bc35-25bf61691be3 There are insufficient data on trends in windthrow, ice storms, hurricanes, and landslide-inducing storms to infer that these types of disturbance events are changing. Factors affecting tree death, such as drought, warmer temperatures, and/or pests and pathogens are often interrelated, which means that isolating a single cause of mortality is rare.878be8a3-989e-497d-af88-5417df6ab074 e7407665-0935-44f1-bd53-90048c76787c a68fee7a-d804-4ce3-8b4c-f7def043d687 298cdb3f-64e7-4ac9-814c-f8deefbf964b e9b2f293-1699-4bee-99b7-0d2fd93d267d 9a761c4d-25f3-4257-ba8f-5a83730c6630

Assessment of confidence based on evidence: Very High. There is very high confidence that under projected climate changes there is high risk (high risk = high probability and high consequence) that western forests in the United States will be affected increasingly by large and intense fires that occur more frequently.5ba92ba4-eb88-480e-91de-a442b293e649 1639ecca-a0ca-48f4-b64c-a447fe137284 517021ad-7515-4d23-8b25-735f52448514 391560e0-40c1-4f9d-b063-e87d18c87e02 b95e9226-076c-4eb5-9367-472499624084 This is based on the strong relationships between climate and forest response, shown observationally298cdb3f-64e7-4ac9-814c-f8deefbf964b and experimentally.e9b2f293-1699-4bee-99b7-0d2fd93d267d Expected responses will increase substantially to warming and also in conjunction with other changes such as an increase in the frequency and/or severity of drought and amplification of pest and pathogen impacts. Eastern forests are less likely to experience immediate increases in wildfire unless/until a point is reached at which warmer temperatures, concurrent with seasonal dry periods or more protracted drought, trigger wildfires.

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