Page 17 - Climate Change Impacts in the United States

7: FORESTS

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

Process for Developing Key Messages: Although it is difficult to detect a trend in disturbances because
A central component of the process was a workshop held in July they are inherently infrequent and it is impossible to attribute an
2011 by the U.S. Department of Agriculture Forest Service to individual disturbance event to changing climate, there is nonethe-
guide the development of the technical input report (TIR). This less much that past events, including recent ones, reveal about
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session, along with numerous teleconferences, led to the founda- expected forest changes due to future climate. Observational
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tional TIR, “Effects of Climatic Variability and Change on Forest and experimental studies show strong associations between for-
Ecosystems: A Comprehensive Science Synthesis for the U.S. For- est disturbance and extreme climatic events and/or modifications
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est Sector.” in atmospheric evaporative demand related to warmer tempera-
ture. Regarding eastern forests, there are fewer observational or
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The chapter authors engaged in multiple technical discussions via experimental studies, with Dietz and Moorcroft being the most
teleconference between January and June 2012, which included comprehensive.
careful review of the foundational TIR and of 58 additional tech-
nical inputs provided by the public, as well as other published Pollution and stand age are the most important factors in mortal-
literature and professional judgment. Discussions were followed ity. Tree survival increases with increased temperature in some
by expert deliberation of draft key messages by the authors and groups. However, for other tree groups survival decreases with
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targeted consultation with additional experts by the lead author of increased temperature. In addition, this study needs to be con-
each message. sidered in the context that there have been fewer severe droughts
in this region. However, physiological relationships suggest that
Key message #1 Traceable accounT trees will generally be more susceptible to mortality under an ex-
Climate change is increasing the vulnerabil- treme drought, especially if it is accompanied by warmer tempera-
ity of many forests to ecosystem changes and tures. 13,68 Consequently, it is misleading to assume that, because
tree mortality through fire, insect infestations, eastern forests have not yet experienced the types of large-scale
drought, and disease outbreaks. die-off seen in the western forests, they are not vulnerable to such
events if an extreme enough drought occurs. Although the effect
Description of evidence base of temperature on the rate of mortality during drought has only
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The key message and supporting text summarizes extensive evi- been shown for one species, the basic physiological relation-
dence documented in the TIR, “Effects of Climatic Variability and ships for trees suggest that warmer temperatures will exacerbate
Change on Forest Ecosystems: A Comprehensive Science Synthe- mortality for other species as well. 13,68
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sis for the U.S. Forest Sector.” Technical input reports (58) on a
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wide range of topics were also received and reviewed as part of the Figure 7.1: This figure uses a figure from Goetz et al. 2012 which
Federal Register Notice solicitation for public input. uses the MODIS Global Disturbance Index (MGDI) results from
2005 to 2009 to illustrate the geographic distribution of major
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Dale et al. addressed a number of climate change factors that will ecosystem disturbance types across North America (based on Mil-
affect U.S. forests and how they are managed. This is supported drexler et al. 2007, 2009 6,69 ). The MGDI uses remotely sensed in-
by additional publications focused on effects of drought and by formation to assess the intensity of the disturbance. Following the
more large-scale tree die-off events, 11,22 wildfire, 16,23,25 insects occurrence of a major disturbance, there will be a reduction in En-
and pathogens. 11,22 Other studies support the negative impact hanced Vegetation Index (EVI) because of vegetation damage; in
of climate change by examining the tree mortality rate due to ris- contrast, Land Surface Temperature (LST) will increase because
ing temperatures, 9,11,14,15,16,17,19,22 which is projected to increase in more absorbed solar radiation will be converted into sensible heat
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some regions. as a result of the reduction in evapotranspiration from less vegeta-
tion density. MGDI takes advantage of the contrast changes in
EVI and LST following a disturbance to enhance the signal to ef-

191 CLIMATE CHANGE IMPACTS IN THE UNITED STATES