Managing Effects of Drought in the Midwest and Northeast United States

CHAPTER 8 Managing Effects of Drought in the Midwest and Northeast United States CO-LEAD AUTHORS Heidi Asbjornsen and John L. Campbell Anthony W. D’Amato, Jeff Garnas, John S. Gunn, Louis R. Iverson, Todd A. Ontl, Neil Pederson, Matthew P. Peters, and P. Danielle Shannon H. Asbjornsen is an Associate Professor J. Garnas is an Assistant Professor of N. Pederson is a Senior Ecologist, Harvard of Ecosystem Ecology and Ecohydrology, Forest Ecosystem Health, University of New University, Harvard Forest, Petersham, University of New Hampshire, Department of Hampshire, Department of Natural Resources MA 01366. Natural Resources and the Environment and and the Environment, Durham, NH 03824. M.P. Peters is an Ecologist, U.S. Department Earth Systems Research Center, Durham, J.S. Gunn is a Research Assistant Professor of Agriculture, Forest Service, Northern NH 03824. of Forest Management, University of New Research Station, Northern Institute of J.L. Campbell is a Research Ecologist, U.S. Hampshire, New Hampshire Agricultural Applied Climate Science, Delaware, OH Department of Agriculture, Forest Service, Experiment Station, Durham, 43015. Northern Research Station, Durham, NH NH 03824. P.D. Shannon is the Climate Hub Northern 03824. L.R. Iverson is a Landscape Ecologist, U.S. Forests Coordinator, U.S. Department A.W. D’Amato is a Professor of Silviculture Department of Agriculture, Forest Service, of Agriculture, Forest Service, Northern and Applied Forest Ecology and Forestry Northern Research Station, Northern Institute Research Station, Northern Institute of Program Director, University of Vermont, of Applied Climate Science, Delaware, OH Applied Climate Science, and Michigan Rubenstein School of Environment and 43015. Technological University, Houghton, Natural Resources, Burlington, VT 05405. T.A. Ontl is a Climate Hub Fellow, U.S. MI 49931. Department of Agriculture, Forest Service, Northern Research Station, Northern Institute of Applied Climate Science, Houghton, MI 49931. 166 CHAPTER 8 Managing Effects of Drought in the Midwest and Northeast United States BACKGROUND Midwest (e.g., oak forests in the Ozark Mountains of Missouri; Jenkins and Pallardy 1995). When drought Severe droughts are relatively rare in the Midwest triggers mortality, the affected trees have usually been and Northeast compared to other parts of the United predisposed to drought by other stressors. Some of States. This 20-State region, hereafter referred to as the these stressors are associated with the dense human Northern Region, is defned as the States bounded by population, such as air pollution and the prevalence of Maine, Minnesota, Missouri, and Maryland. Although pests, pathogens, and invasive species (Haavik et al. the Northern Region has a cool, wet climate and is 2015, Jenkins and Pallardy 1995, Pedersen 1998). generally considered to have an abundance of water, model projections suggest that droughts may become Despite these issues, the Northern Region has a more frequent and severe in the future. The Northern diversity of tree species, which may help enhance Region is densely populated (39 percent of the U.S. resistance and resilience to drought (Peters et al. 2015). population) (U.S. Census Bureau 2018), so changes This high biodiversity also increases management in precipitation may be especially disruptive. Impacts options by providing a broader selection of drought- will affect forest ecosystems and the services they tolerant tree species. However, how the region’s trees provide, including timber and nontimber products, water may fare in the future is diffcult to predict for several regulation and supply, erosion and pollution control, reasons: the unprecedented projected changes in biodiversity protection, and recreation. climate, interactions with multiple simultaneously changing drivers (e.g., atmospheric CO2, ozone, nitrogen Nearly 43 percent of the Northern Region is forested deposition), and the relative dearth of research on (Oswalt et al. 2014), so management of this key drought impacts on forests in the Northern Region. resource is central to maintaining the economy and Given these complexities and uncertainty in future quality of life. Unlike much of the Western United climate, drought poses a challenge to land managers States, the majority (74 percent) of forest land in the in the Northern Region and warrants consideration in Northern Region is privately owned, mostly as smaller management decisions. family forest holdings (Oswalt et al. 2014). This model of ownership is challenging from a management DROUGHT DEFINITIONS AND TRENDS perspective because of diffculties facilitating change at the landscape scale when so many individuals are Drought can be defned from many different involved. Unlike government agencies that can alter land perspectives, and each approach will lead to a different management practices more directly, making changes understanding of how drought is expressed on the in management of privately owned land is largely landscape across both temporal and spatial scales. In accomplished through education and using incentives this report, we consider three types of drought that are to achieve desired outcomes (e.g., cost-share payments especially relevant to forest managers—meteorological, for implementing specifc management practices). hydrological, and ecological drought—and describe past and projected future drought trends. Most forests in the Northern Region are not currently managed with drought in mind. Because drought Meteorological Drought has historically had less of an impact on forest health compared to other regions, drought management tools Meteorological drought is often defned solely and techniques are not well established. The increased by precipitation, based on the degree of dryness probability of future drought in the Northern Region has and duration of the dry period (Wilhite and Glantz created a need for information about both the impacts 1985). The thresholds for the duration and severity of drought on forests and the options for land managers of meteorological drought are site-specifc and are to cope with acute and chronic reductions in water identifed by evaluating deviations from normal (i.e., availability. average historical) climatic conditions. An extreme drought or wet spell can be quantifed statistically as the Forests in the Northern Region are typically energy- tails of the historical rainfall distribution (Smith 2011). limited rather than water-limited, and widespread Although this approach is limited by the availability of drought-induced diebacks are rare. However, drought reliable data, analysis of tree rings (which can serve as has caused widespread tree mortality in some historical proxies spanning centuries) and modeling (for ecosystems in this region, especially in the lower projecting future climate trends) can greatly expand the EFFECTS OF DROUGHT ON FORESTS AND RANGELANDS IN THE UNITED STATES 167 CHAPTER 8 Managing Effects of Drought in the Midwest and Northeast United States capacity to assess longer term drought trends. Several average, there is heightened concern about future indices (e.g., Palmer Drought Severity Index [PDSI] and drought effects on forests because of both projected Standardized Precipitation Evapotranspiration Index variability and extremes in precipitation and warming [SPEI]) have been developed to identify periods of due to warming temperatures. meteorological drought and are valuable for monitoring long-term trends (e.g., Donat et al. 2013, Palmer 1965). Future drought trends for the Northeast and Midwest However, these indices often require variables, such as areas of the Northern Region were also evaluated soil moisture, that are rarely available for long periods by modeling PDSI through the end of the century. across broad regions. A common issue with characterizing trends using drought and aridity indices (such as PDSI) is that they Within the Northern Region, tree-ring records indicate produce location-based, time series datasets that that severe meteorological droughts occurred before cannot be easily compared at broader spatial scales or the 20th century (Cook and Jacoby 1977, Pederson among time periods. To remedy this, PDSI time series et al. 2013, Stahle et al. 2007). There is evidence of datasets were aggregated into weighted values, such a megadrought in the 1500s (Stahle et al. 2000) and that the frequency of drought events is weighted by then a series of repeated severe droughts during the their intensity. Using this approach, a single cumulative middle of the 1600s (McEwan et al. 2011, Pederson value can represent the relative potential for drought of et al. 2014). Over the 20th century, the frequency and a location, (see chapter 2 for details of the Cumulative magnitude of droughts have declined. Conditions in Drought Severity Index [CDSI] calculations). Cumulative the early 21st century have been wetter (Pederson Drought Severity Index values were compared for et al. 2015), and although droughts still occur (e.g., two models each under two future greenhouse gas Sweet et al. 2017), they have not been as severe as the emissions scenarios—representative concentration megadroughts of the past. pathway (RCP) scenarios 4.5 and 8.5 (Moss et al. 2008)—and for three 30-year periods: 2010–2039, Average annual precipitation across States in the region 2040–2069, and 2070–2099. The 30-year period ranges from 178 to 330 inches (NCDC 2017). Although of 1980–2009 was used as a baseline. These four some areas of the United States, such as parts

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