Susan Frankel, Ecosystem Function and Health Program, USFS Pacific Southwest Research Station
Jennifer Juzwik, Biological and Environmental Influences on Forest Health and Productivity, USFS Northern Research Station
Frank Koch, Eastern Forest Environmental Threat Assessment Center,USFS Southern Research Station
An archived version of this topic paper is available.
Tree diseases occur everywhere that forest trees grow. Infectious diseases caused by biotic pathogens develop over time from interaction of these pathogens with a favorable environment and susceptible host plants (1). Environmental factors that cause plant stress, especially from moisture deficit caused by drought, commonly predispose trees to forest pathogen attack. Some diseases are species-specific, while others affect multiple host species. Pathogens that incite tree diseases include fungi, bacteria, viruses, parasitic plants, nematodes and other microorganisms. Insects can play a major role in disease development by serving as vectors, providing wounds that allow pathogen ingress, and other functions (2). Non-infectious forest diseases are caused by abiotic factors that are directly damaging to tree health, such as freezing temperatures and air pollutants (3).
Anticipated changes in climatic conditions may impact the prevalence and severity of these non-infectious diseases. With respect to infectious diseases, altered climatic conditions may dramatically affect the outcome of pathogen - host - insect interactions in forest environments. The cascade of multiple changes will influence a forest’s ability to sustain goods and services at existing levels (4,5). Direct damage to host tissues occurs in affected trees and can lead to tree mortality.
Understanding the relationship between climate and tree disease is essential for addressing issues associated with changing climate  at different spatial scales. Several relationships where greater understanding is required are:
- Effects of biotic and abiotic disease factors on tree survival and growth, forest structure, and species composition (6).
- Effects of pathogenic and decay fungi on the forest carbon cycle which impact forest carbon stocks and fluxes (7).
- Development and impact of tree pathogen and associated insect - host interactions under varying temperature and precipitation regimes.
- Documentation of beneficial changes due to changing climate effects on tree diseases and tree susceptibility in relation to forest species diversity and forest structure (6).
Introduced invasive pathogens, such as the chestnut blight fungus and white pine blister rust, caused extensive damage to U.S. forests in the past century (8). Thus, studies of both native and exotic invasive pathogens are needed. Understanding how the severity and distribution of tree diseases are affected by seasonal changes in temperature, moisture conditions (precipitation, relative humidity, and soil water availability), tree phenology, and tree physiological stress is also important in forecasting the direction of change expected under predicted climate scenarios.
The effects of climate change on forest tree diseases will vary by spatial scale, and will depend on the trajectory of change. Projections and prediction maps of where drier and wetter conditions will likely occur in the U.S. have been and will continue to be published (9).
Predictions of changes in impact caused by forest diseases have been made under both "warmer and wetter" and "warmer and drier" scenarios. For diseases where temperature and moisture more directly affect host susceptibility to infection and disease development, "warmer and drier" climate will favor disease increase. In general, root and canker diseases fall within this category. As a specific example, Armillaria root disease in the western states is predicted to increase in severity and impact under this climate scenario (10). Climate warming accompanied by increasing drought events (warmer/drier) also can lead to increases in "decline disease" frequency and severity (11). Such diseases typically involve sequential and additive effects of site conditions, drought, and insect and pathogen build-up on stressed hosts (12). With warming temperatures, some forest tree diseases may be able to occur further north and/or at higher elevations. Opportunistic pathogens may also be favored by such changes.
Diseases where temperature and moisture directly affect the causal organism’s reproduction, spread, infection and survival are predicted to increase under warmer/wetter conditions. In general, foliar and rust diseases fall within this category. Predictions based on warmer/wetter projections have been made for specific diseases such as sudden oak death, Phytophthora root rot and Swiss needle cast (10).
Seasonal shifts in precipitation pattern alone can lead to increasingly severe occurrences of tree diseases. For example, documented climate changes over one or more decades that affected timing of rainfall were associated with severe outbreaks of red band needle blight on lodgepole pine in British Columbia (13) and bur oak blight in Iowa (14).
The complexities of climatic effects on community interactions in which diseases occur make it extremely difficult to assign probabilities or predict the trends of climate change impact on forest tree diseases in the future (15). Changes in precipitation timing, intensity and form within a region or state will vary, further contributing to the complexity of disease prediction. Even microclimate differences can profoundly affect disease severity. However, some general principles can be helpful in predicting responses of forest tree diseases to changing climatic conditions:
- Prediction of disease outbreaks will be more difficult in periods of rapidly changing climate and unstable weather, because altered reproduction and spread of forest tree pathogens will influence changes in initiation, development and severity of diseases.
- Host resistance to pathogens may be overcome as trees become stressed or as pathogens' evolution accelerates more quickly than their long-lived hosts.
- Warmer winters will contribute to greater overwintering success of pathogens and/or associated insects, leading to increasing disease occurrence and severity.
The rate and pattern of wood decay in forests changes due to influence of changing moisture and temperature regimes on decay fungi. Thus, carbon cyling rates may increase or decrease depending on the direction of future climate changes.
Options for Management
Forest managers, whether working at the local or the landscape scale, should be aware of current and historic forest health conditions in their jurisdictions, and then integrate that knowledge with climate change projections. Monitoring, forecasting, planning and mitigation strategies are needed to prevent and to adaptively manage tree diseases at various geographic scales (16).
Monitoring - Early detection of tree diseases can increase the potential for successful disease management. Thus, continued and improved surveillance of forests for tree health problems is required. Integrated data from state, federal (such as the USFS Forest Inventory and Analysis program and the USFS Forest Health Protection programs), and private monitoring are useful for detecting deviations from historical baseline conditions. Follow-up investigations of identified problems and "at-risk" forests are also required.
Forecasting - Climate change scenarios have been used to estimate future risks for several tree diseases (17, 18, 19). However, uncertainties generated by forecasting must be sufficiently characterized and additional species need to be addressed. Forecasting may be achieved using models, risk analysis and risk rating systems. Models may 1) project potential forest disease impacts, 2) provide insights on the magnitude and direction of change, 3) help focus monitoring activities, and 4) aid in the evaluation of management strategies (20). Risk analysis and risk rating systems are developed through evaluation of available evidence and anticipated behavior of a plant pathogen to estimate its impact in a new environment and to determine a management response.
Planning and Mitigation Strategies - Increasing the capacity of an ecosystem to absorb disturbance without shifting to a qualitatively different state (ecological resilience) is required to mitigate effects of climate change (21), using proactive options such as
- Increasing species and age class diversity to promote growth and resilience to mortality,
- Using appropriate silvicultural interventions to increase tree vigor and lower pathogen and insect pest impacts under predicted climate scenarios,
- Carefully and judiciously using facilitated tree species migration, and
- Increasing tolerance and resistance to pathogens as part of breeding programs designed to increase species tolerance to environmental stressors.
Frankel, S.; Juzwik, J.; Koch, F. (October, 2012). Forest Tree Diseases and Climate Change. U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. www.fs.usda.gov/ccrc/topics/forest-disease
The following documents have been recommended by the authors of the synthesis paper and by the CCRC Production team.
Allen, C.D.; Macalady, A.K.; Chenchouni, H.; Bachelet, D.; McDowell, N.; Venntier, M.; Kitzberger, T.; Rigling, A.; Breshears, D.D.; Hogg, E.H.; Gonzalez, P.; Fensham, R.; Zhang, Z.; Castro, J.; Demidova, N.; Lim, J.H.; Allard, G.; Running, S.W.; Semerici, A.; Cobb, N. 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 259: 660-84.
Boland, G.J., Melzer, M.S.; Hopkin, A.; Higgins, V.; Nassuth, A. 2004. Climate change and plant diseases in Ontario. Canadian Journal of Plant Pathology. 26: 335-50.
Breshears, D.D.; Cobb, N.S.; Rich, P.M.; Price, K.P.; Allen, C.D.; Balice, R.G.; Romme, W.H.; Kasten, J.H.; Floyd, M.L.; Belnap, J.; Anderson, J.J.; Meyers, O.B.; Meyer, C.W. 2005. Regional vegetation die-off in response to global-change-type drought. Proceedings of the National Academy of Sciences, USA. 102: 15144-8.
Desprez-Loustau, M-L.; Marcais, B.; Nageleisen, L.M.; Ouiym, D.; Vannini, A. 2006. Interactive effects of drought and pathogens in forest trees. Annals of Forest Science. 63: 597-612.
Dukes, J.S.; Pontius, J.; Orwig, D.; Garnas, J.R.; Fodgers, V.L.; Brazee, N.; Cooke, B.; Theoharides, K.A.; Stange, E.E.; Harrington, R.; Ehrenfeld, J.; Gurevitch, J.; Lerdau, M.; Stinson, K.; Wick, R.; Ayers, M. 2009. Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: what can we predict? Canadian Journal of Forest Research. 39: 231-48.
Hennon, P.E.; D'Amore, D.V.; Schaberg, P.G.; Wittwer, D.T.; Shanley, C.S. 2012. Shifting climate, altered niche, and a dynamic conservation strategy for yellow-cedar in the North Pacific coastal rainforest. BioScience. 62: 147-158.
Hicke, J.A.; Allen, C.D.; Desai, A.R.; Dietze, M.C.; Hall, R.J.; Hogg, E.H.; Kashian, D.M.; Moore, D.; Raffa, K.F.; Sturrock, R.N.; Vogelmann, J. 2012. Effects of biotic disturbances on forest carbon cycling in the United States and Canada. Journal of Global Change Biology. 18:7-34.
Kliejunas, J.T. 2011. A risk assessment of climate change and the impact of forest diseases on forest ecosystems in the Western United States and Canada. Gen. Tech. Rep. PSW-GTR-236. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 70 p.
Sturrock, R.N.; Frankel, S.J.; Brown, A.V.; Hennon, P.E.; Kliejunas, J.T.; Lewis, K.J.; Worrall, J.J.; Woods, A.J. 2011. Climate change and forest diseases. Plant Pathology. 60: 133-49.
Woods, A.J.; Heppner, D.; Kope, H.; Burleigh, J.; Maclauchlan, L. 2010. Forest health and climate change: a British Columbia perspective. Forestry Chronicle. 86:412-422.
These summaries represent Forest Service research related to tree diseases and climate change. More examples will be added as our Research Roundup is updated.
American Chestnut Restoration
The American chestnut is a tree species of unique ecological and economic value that was virtually eliminated following a blight caused by a fungal pathogen, Cryphonectria parasitica. In order to restore this economically and ecologically valuable species, multiple approaches to decrease the virulence of the pathogen or increase the resistance of the tree have been evaluated. Climate change presents new implications for the recovery of the species, especially at its historic northern range limits.
Contact: Paul Schaberg
Forest tree genetic risk assessment system: a tool for conservation decision-making in changing times
The Forest Tree Genetic Risk Assessment System (ForGRAS) is a framework that allows managers to assess the relative risk of genetic degradation to forest trees affected by multiple threats.
Contact: Kevin Potter
Technology development to support a national early warning system for environmental threats
Scientists and collaborators have launched the ForWarn tool, the strategic research component of the national early warning system, to help natural resource managers rapidly detect, identify, and respond to unexpected changes in the nation’s forests. ForWarn produces maps showing potential forest disturbance across the conterminous United States at 231-meter resolution every 8 days, based on images obtained over the preceding 24-day analysis window.
Contact: William Hargrove
Tropical Forest Mycology
The Center for Forest Mycology Research (CFMR), part of the Northern Research Station, leads critical research on the biology of tropical fungi native to Hawaii, US territories in the Caribbean and to other countries in the Caribbean Basin. The primary goals of this research are to: (1) recognize emerging tropical forest diseases, especially those with the potential to spread to the continental US and (2) identify the effects of environmental change on the distributions of beneficial and harmful forest fungi.
Contact: D. Jean Lodge
Yellow Birch, acid deposition, and climate change risk
Yellow birch is affected by low soil calcium due to acid rain and vulnerable to climate change.
Contact: Ken Stolte
Synthesis of tree responses to climate change - Pacific Northwest
Several decades of research exist on the potential responses of trees and forests to climate-related stresses. Researchers synthesized more than 400 research articles addressing physiological and ecological responses of trees and forests to variations in climate and associated stresses and disturbance agents. Although based on an international body of research, the synthesis highlights potential climate changes and responses from species and ecosystems in the Pacific Northwest. It is organized around key themes: elevated levels of atmospheric carbon dioxide, temperature, precipitation, fire, pests, and their interactions, and discusses vulnerabilities and risks from a forestry management perspective. The authors identify options for silvicultural and genetic approaches to managing for forest adaptation.
Contact: Paul D. Anderson
Yellow cedar continues uphill retreat
Continuing research on yellow-cedar populations in southeast Alaska found many dead trees at lower elevations and live trees most common at mid elevations. Regeneration peaked at higher elevations. These trends are consistent with the understanding that the presence of spring snow is a primary factor in the health and successful regeneration of yellowcedar. This knowledge is guiding decisions about where to favor this valuable tree through planting and thinning.
Contact: Paul Hennon
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- Allen, C.D.; Macalady, A.K.; Chenchouni, H.; Bachelet, D.; McDowell, N.; Venntier, M.; Kitzberger, T.; Rigling, A.; Breshears, D.D.; Hogg, E.H.; Gonzalez, P.; Fensham, R.; Zhang, Z.; Castro, J.; Demidova, N.; Lim, J.H.; Allard, G.; Running, S.W.; Semerici, A.; Cobb, N. 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.
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- Woods A.; Coates, K.D.; Hamann, A. 2005. Is an unprecedented dothistroma needle blight epidemic related to climate change? BioScience. 55: 761-769.
- Harrington, T.C.; McNew, D.; Yun, H.Y. 2012. Bur oak blight, a new disease on Quercus macrocarpa caused by Tubakia iowensis sp. nov. Mycologia. 104:79-92.
- Dukes, J.S.; Pontius, J.; Orwig, D.; Garnas, J.R.; Fodgers, V.L.; Brazee, N.; Cooke, B.; Theoharides, K.A.; Stange, E.E.; Harrington, R.; Ehrenfeld, J.; Gurevitch, J.; Lerdau, M.; Stinson, K.; Wick, R.; Ayers, M. 2009. Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: what can we predict? Canadian Journal of Forest Research. 39: 231-248.
- Sturrock, R.N.; Frankel, S.J.; Brown, A.V.; Hennon, P.E.; Kliejunas, J.T.; Lewis, K.J.; Worrall, J.J.; Woods, A.J. 2011. Climatechange and forest diseases. Plant Pathology 60: 133-149.
- Bergot, M.; Cloppet, E.; Perarnaud, V.; Deque, M.; MarÃ§ais, B.; Desprez-Loustau, M.-L. 2004. Simulation of potential range expansion of oak disease caused by Phytophthora cinnamomi under climate change. Global Change Biology. 10: 1539-1552.
- Desprez-Loustau, M.-L.; Robin, C.; Reynaud, G.; Deque, M.; Badeau, V.; Piou, D.; Husson, C.; Marcais, B. et al. 2007. Simulating the effects of a climate-change scenario n the geographical range and activity of forest-pathogenic fungi. Canadian Journal of Plant Pathology 29:101-120.
- Rehfeldt, G.E.; Ferguson, D.E.; Crookston, N.L. 2009. Aspen, climate and sudden decline in western USA. Forest Ecology and Management. 258:2353-2364.
- Woods, A.J.; Heppner, D.; Kope, H.; Burleigh, J.; Maclauchlan, L. 2010. Forest health and climate change: a British Columbia perspective. Forestry Chronicle. 86:412-422.
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