James Vose, Center for Integrated Forest Science, Southern Research Station, USDA Forest Service
Additional editors of the original publication:
James Clark, Nicholas School of the Environment, Duke University
Charles Luce, Rock Mountain Research Station, USDA Forest Service
Toral Patel-Weynand, Sustainable Forest Management Research, USDA Forest Service
This topic page was adapted from the executive summary of the report Effects of Drought on Forests and Rangelands in the United States: a comprehensive synthesis (WO-GTR-93b)
Characterizing and Predicting Future Drought
In simple terms, drought is a lack of water over a given temporal and spatial scale. Drought can be a severe natural disaster with substantial social and economic consequences. Drought becomes most obvious when large-scale changes are observed (e.g., insect outbreaks or wildfires) or when water requirements for human or agricultural needs are not met; however, even moderate drought can have long-lasting impacts on the structure and function of forests and rangelands without these obvious large-scale changes. Droughts are generally identified as one of four types: meteorological, hydrologic, agricultural, or socioeconomic (1). Meteorological and hydrologic droughts relate water availability to a reference condition (e.g., long-term mean precipitation or streamflow); agricultural and socioeconomic droughts relate to impacts. In systems with perennial vegetation (both natural and agricultural systems), characterizing and assessing drought impacts is complex, as responses can vary in space, time, and among species. For forest and stream ecosystems, meteorological and hydrologic drought frameworks are useful for characterizing impacts of a given drought event. For example, meteorological or hydrologic drought may presage or correlate to fire events (2) or insect outbreaks (3).
Historical and paleoclimatic evidence shows that drought has always impacted the physical environment and will continue to do so (4). The direction of trends in recent history varies from region to region, with the Western United States showing a trend toward dry conditions while trends in the East are more variable and complex (5). Much of the variability in how drought is characterized depends on definitions for terms, reference conditions, and the methods used for developing drought indices. Predicting future changes in drought frequency and severity has proven difficult using General Circulation Models (GCMs) (6, 7, 8, 9) but recent trends are a growing global concern. Uncertainty arises primarily from limited capacity to predict future precipitation changes, particularly long-term lapses in precipitation. Despite this uncertainty, there is growing consensus that extreme precipitation events (e.g., lapses in precipitation and more intense storms) will increase in frequency, and warmer temperatures will exacerbate the impacts of drought on forests and rangelands in the future (4).
Understanding the Effects of Drought on Forests and Rangelands
Drought severity and drought-associated forest disturbances are expected to increase with climatic change. Drought affects forest and rangeland systems both directly and indirectly. In regions where seasonal droughts are common, forest and rangeland ecosystems respond through various physiological and morphological adaptations. In regions where drought is less common, responses can be substantial because ecosystems are not well adapted to drought conditions.
High evaporative demand, the combination of high temperature and low humidity, combines with low soil moisture to induce stress through closure of stomata, which can lead to carbon stress, loss of hydraulic function, and mortality (10). Species vary in their vulnerability to drought due to differences in their allocation to roots, mycorrhizal associations, and xylem anatomy (11). Large stand-level impacts of drought are already underway in the West (12) but all U.S. forests are vulnerable to drought (13). Changes in climate will continue to stress forests and alter suitable habitat (11). Combined field evidence and models suggest that climate change is causing relocation of habitats at rates much faster than populations of trees can migrate (14, 15). Reorganizations of stand structure and species composition are expected to lag behind shifts in habitat caused by increasing drought and temperature change (14).
Droughts are predicted to accelerate the pace of invasion by some nonnative plant species into rangelands and grasslands (16, 17, 18). Drought can also promote plant invasion indirectly by modifying the environment to favor nonnative species. For example, opportunities for invasion are created when drought kills native plants leaving open niches and bare ground (19). Drought is also an important contributor to the invasive annual grass-wildfire loop that threatens ecosystems not adapted to fire (e.g., cheatgrass’ positive feedback with fire in parts of western North America’s sagebrush biome) (20, 21, 22). In this self-perpetuating cheatgrass-fire loop, drought increases the frequency of wildfires, and nonnative plants (especially annual grasses) are likely to invade burned sites (23).
Drought alters ecosystem processes such as nutrient, carbon, and water cycling in ways that are not yet well understood. Drought tends to slow nutrient uptake by plants and reduce retranslocation of foliar nutrients with premature leaf senescence (24, 25). Dieback that results from combinations of drought and natural enemies can severely reduce carbon exchange between atmosphere and biosphere. Recent large diebacks have had global impacts on carbon cycles, including carbon release from biomass and reductions in carbon uptake from the atmosphere, although impacts may be offset by vegetation regrowth in some regions. Multi-year or severe droughts can have substantial impacts on hydrological and stream biogeochemical processes (26).
Indirect effects of drought on forests can be widespread and devastating. Notable recent examples include insect and pathogen outbreaks (3) and increased wildfire risk (2). Available evidence suggests a nonlinear relationship between drought intensity and bark beetle outbreaks; moderate drought reduces outbreaks whereas long, intense drought can increase it (27). As a consequence of long-term drought and warming in the Western United States, bark beetles are currently the most important biotic agent of tree mortality. Multiple large outbreaks have killed hundreds of millions of trees in recent decades. Host trees weakened by drought allow beetle populations to build. Warming facilitates northward range expansion. In contrast, there is little current evidence for a role of drought in bark beetle outbreaks in coniferous forests of the Eastern United States (28). Fungal pathogens are poorly understood, but available evidence suggest reduced pathogen performance and host impacts in response to drought for primary pathogens and pathogens whose lifecycle depends directly on moisture (27, 29, 30, 31). In comparison, secondary pathogens that depend on stressed hosts for colonization are anticipated to respond to drought with greater performance and host impacts.
Historical and pre-settlement relationships between drought and wildfire have been well documented in much of North America, with forest fire occurrence and area burned clearly increasing in response to drought. This body of evidence indicates that the role of drought in historical and likely future fire regimes is an important contingency that creates anomalously high potential for ignition, fire spread, and large fire events. However, drought is only one aspect of a broader set of controls on fire regimes, and by itself is insufficient to predict fire dynamics or effects. Whereas the relationships between fire occurrence or area burned and drought are well documented, the relationship between drought and fire severity can be complex. For example, north-facing slopes might offer some degree of local protection during mild droughts, but even they become dry under extreme conditions, reducing fine-scale heterogeneity in vegetation consequences (2).
Streamflow and groundwater recharge respond directly to drought through reductions in precipitation (rain and/or snowfall), and they respond indirectly via evapotranspiration responses to changing evaporative energy and water availability. Hydrologic responses to drought can be either mitigated or exacerbated by forest vegetation, depending on vegetation water use and how drought affects forest population dynamics (32). Drought affects water quality both directly and indirectly. Direct impacts are primarily physical, as reduced streamflow concentrates nutrients and sediment and warms more quickly. Indirect effects include a combination of terrestrial, riparian, and instream processes that impact sediment and nutrient concentrations and fluxes (32).
Economic Consequences of Drought
Drought has direct consequences to forest and rangeland production (33). Droughts can negatively impact forest inventories by increasing mortality and reducing growth. Drought in rangelands reduces forage and water available for livestock grazing. Reduced vegetative cover can lead to wind and water erosion. Drought-related disturbance, such as wildfire, can have protracted effects that include significant timber market losses.
Reduced water yield from forests and rangelands during extended meteorological drought can have substantial impacts on domestic and agricultural water supplies, which often results in water markets implementing quantity controls. Drought can also have nonmarket effects on forests and rangelands. For example, drought affects outdoor recreation, where low reservoir levels can reduce availability of fishing, recreational boating, swimming, and camping (although some net benefit can result from more precipitation-free days). Low winter snow cover reduces economic benefits from skiing and related activities.
Options for Management
Managing Forests and Rangelands To Increase Resiliency and Drought Adaptation
How can forest and rangeland practices adapt to changing drought regimes? Frequent low-severity drought may selectively favor more drought-tolerant species and create forests and rangelands better adapted to future conditions without the need for management intervention. By contrast, severe drought (especially in combination with insect outbreaks or fire), may threaten large-scale changes that warrant substantial management responses. Actions could range from reducing vulnerability, facilitating post-drought recovery, or facilitating a transition to a new condition.
Management actions can either mitigate or exacerbate the effects of drought. A first principal for increasing resilience and adaptation would be to avoid management actions that exacerbate the effects of current or future drought. Options can include altering structural or functional components of vegetation, minimizing drought-mediated disturbance such as wildfire or insect outbreaks, and managing for reliable flow of water. Managers can implement structural changes by thinning or density management of planted forests. Thinned stands require less water and may be less vulnerable to water stress and insect outbreaks. Reduced fuel loads in thinned stands can also reduce wildfire risk.
Managers can also implement functional changes by favoring or planting more drought-adapted species. Management for a diversity of species can reduce stand vulnerability to drought, as uncertainty in future climate can encourage management for mixtures of drought-tolerant species and genotypes. Species diversity can also reduce intensity of insect attacks. In some regions of the United States, planting or favoring more drought-tolerant species may conflict with management objectives that favor rapid accumulation of biomass, as fast-growing woody species often use more water and exacerbate drought impacts.
While harvesting increases annual water yield in some forest ecosystems, a large reduction of forest cover is needed to have an appreciable effect on water yield. Hence, potential increases in streamflow through forest cutting are limited by the amount of land that managers can harvest. In addition, streamflow responses are often short term due to rapid forest regrowth, and the aggrading postcut forest may actually have lower streamflow than the uncut forest. In contrast to management actions that are intended to augment streamflow, increasing drought stress in some forest ecosystems may warrant management strategies that retain water (and hence reduce streamflow) on the landscape in order to keep trees alive. Land managers may need to plan the timing of some management activities to ensure that ecosystems have optimal growing conditions and that these activities do not disturb streams during low-flow periods. Removal and alteration of riparian vegetation increases stream temperatures; therefore, maintaining or increasing shading from solar radiation through riparian buffer zone conservation and restoration may mitigate any changes in stream temperatures caused by drought.
Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. Effects of Drought on Forests and Rangelands. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. www.fs.usda.gov/ccrc/topics/drought
The U.S. Drought Monitor, established in 1999, is a weekly map of drought conditions that is produced jointly by the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture, and the National Drought Mitigation Center (NDMC) at the University of Nebraska-Lincoln. U.S. Drought Monitor maps come out every Thursday morning at 8:30 Eastern Time, based on data through 7 a.m. Eastern Standard Time (8 a.m. Eastern Daylight Time) the preceding Tuesday. The map is based on measurements of climatic, hydrologic and soil conditions as well as reported impacts and observations from more than 350 contributors around the country. Eleven climatologists from the partner organizations take turns serving as the lead author each week. The authors examine all the data and use their best judgment to reconcile any differences in what different sources are saying.
1. Wilhite, D.A.; Glantz, M.H. 1985. Understanding the drought phenomenon: the role of definitions. Water International. 10(3): 111-120.
2. Little, J.; Peterson, D.; Riley, K.; Liu, Y.; Luce, C. Fire and Drought. p 135-154. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
3. Kolb, T.; Fettig, C.; Bentz, B.; Stewart, J.; Weed, A.; Hicke, J.; Ayres, M. Forest insect and fungal pathogen responses to drought. p 113-133 In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
4. Luce, C.; Pederson, N.; Campbell, J.; Millar, C.; Kormos, P.; Vose, J.; Woods, R. Characterizing drought for forested landscapes and streams. p. 13-48. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
5. Melillo, J.M.; Richmond, T.; Yohe, G.W. 2014. Climate change impacts in the United States: the third national climate assessment: U.S. Global Change Research Program. 841 p. doi:10.7930/J0Z31WJ2. http://nca2014.globalchange.gov. [Date accessed: October 14, 2015].
6. Dai, A. 2013. Increasing drought under global warming in observations and models. Nature Climate Change. 3(1): 52-58.
7. Hoerling, M.P.; Eischeid, J.K.; Quan, X.-W. [and others]. 2012. Is a transition to semipermanent drought conditions imminent in the U.S. Great Plains? Journal of Climate. 25(24): 8380-8386.
8. Sheffield, J.; Wood, E.F. 2008b. Projected changes in drought occurrence under future global warming from multi-model, multiscenario, IPCC AR4 simulations. Climate Dynamics. 31(1): 79-105.
9. Trenberth, K.E.; Dai, A.; van der Schrier, G. [and others]. 2014. Global warming and changes in drought. Nature Climate Change. 4(1): 17-22.
10. Manzoni, S.; Katul, G.; Porporato, A. 2014. A dynamical system perspective on plant hydraulic failure. Water Resources Research. 50(6): 5170-5183.
11. McDowell, N.; Hanson, P.; Ibáñez, I.; Phillips, R.; Ryan, M. Physiological responses of forests to future drought. P. 49-58. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
12. Breshears, D.D.; Cobb, N.S.; Rich, P.M. [and others]. 2005. Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences. 102(42): 15,144-15,148.
13. Allen, C.D.; Macalady, A.K.; Chenchouni, H. [and others]. 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management. 259: 660-684.
14. Clark, J., Iverson, L., Woodall, C. Impacts of increasing drought on forest dynamics, structure, diversity, and management. p. 59-96. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
15. Zhu, K.; Woodall, C.W.; Ghosh, S. [and others]. 2014. Dual impacts of climate change: forest migration and turnover through life history. Global Change Biology. 20: 251-264.
16. Abatzoglou, J.T.; Kolden, C.A. 2011. Climate change in western U.S. deserts: potential for increased wildfire and invasive annual grasses. Rangeland Ecology & Management 64: 471-478.
17. Everard, K.; Seabloom, E.W.; Harpole, W.S.; de Mazancourt, C. 2010. Plant use affects competition for nitrogen: why drought favors invasive species in California. American Naturalist. 175:85–97.
18. Finch, D.M., ed. Climate change in grasslands, shrublands, and deserts of the interior American West: a review and needs assessment. Gen. Tech. Rep. RMRS-GTR-285. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 80-96.
19. Finch, D.; Pendleton, R.; Reeves, M.; Ott, J.; Kilkenny, F.; Butler, J.; Ott, J.; Pinto, J.; Ford, P.; Runyon, J.; Rumble, M.; Kitchen, S. Rangeland Drought: effects, restoration, and adaptation. p 155-194. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
20. Brooks, M.L.; D’antonio; C.M.; Richardson, D. M. [and others]. 2004. Effects of invasive alien plants on fire regimes. BioScience. 54(7): 677–688.
21. Westerling, A.L.; Hidalgo, H.G.; Cayan, D.R.; Swetnam, T.W. 2006. Warming and earlier spring increases western U.S. forest wildfire activity. Science. 313(5789): 940-943. doi:10.1126/ science.1128834. [Published online: July 6, 2006].
22. Whisenant, S.G. 1990. Changing fire frequencies on Idaho’s Snake River Plains: ecological and management implications. Gen. Tech. Rep. INT-276. In: McArthur, E.D.; Romney, E.M.; Smith, S.D.; Tueller, P.T., eds. Symposium on cheatgrass invasion, shrub die-off and other aspects of shrub biology and management. Ogden, UT: U.S. Department of Agriculture, Forest Service: 4–10.
23. Balch, J.K.; Bradley, B.A.; D’Antonio, C.M.; Gómez-Dans, J. 2013. Introduced annual grass increases regional fire activity across the arid western USA (1980–2009). Global Change Biology. 19: 173–183.
24. Killingbeck, K.T. 1996. Nutrients in senesced leaves: Keys to the search for potential resorption and resorption proficiency. Ecology. 77: 1716–1727.
25. Minoletti, M.L.; Boerner, R.E.J. 1994. Drought and site fertility effects on foliar nitrogen and phosphorus dynamics and nutrient resorption by the forest understory shrub Viburnum acerifolium L. American Midland Naturalist. 131: 109–119.
26. Schlesinger, W.; Dietze, M.; Jackson, R.; Phillips, R.; Rhoades, C.; Rustad, L.; Vose, J. Forest Biogeochemistry in response to drought. p 97-111. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
27. Jactel, H.; Petit, J.; Desperez-Loustau, M.-L. [and others]. 2012. Drought effects on damage by forest insects and pathogens: a meta-analysis. Global Change Biology. 18: 267–276.
28. Huberty, A.F.; Denno, R.F. 2004. Plant water stress and its consequences for herbivorous insects: a new synthesis. Ecology. 85: 1383–1398.
29. Desprez Loustau, M.-L.; Marcais, B.; Nageleisen, L.-M. [and others]. 2006. Interactive effects of drought and pathogens in forest trees. Annals of Forest Science. 63: 597–612.
30. Klopfenstein, N.B.; Kim, M.-S.; Hanna, J.W. [and others]. 2009. Approaches to predicting potential impacts of climate change on forest disease: an example with Armillaria root disease. Research Paper RMRS-RP-76. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 10 p.
31. Sturrock, R.N.; Frankel, S.J.; Brown, A.V. [and others]. 2011. Climate change and forest diseases. Plant Pathology. 60: 133–149.
32. Vose, J.; Miniat, C.; Luce, C. Ecohydrological implications of drought. p. 231-251. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p
33. Prestemon, J.; Kruger, L. Economics and Societal Considerations of Drought. p. 253-281. In: Vose, J.; Clark, J.; Luce, C.; Patel-Weynand, T. eds. 2016. Effects of drought on forests and rangelands in the United States: A comprehensive science synthesis: Gen. Tech. Rep. WO-93b. Washington, DC: U.S. Department of Agriculture, Forest Service, Washington Office. 289 p