MASSIVE FOREST DIEBACK
ALLEN, CRAIG D.
U.S. Geological Survey, Jemez Mountains Field Station, Los Alamos, NM 87544
In coming decades, climate changes are expected to produce large shifts in vegetation distributions, largely due to mortality. However, most field studies and model-based assessments of vegetation responses to climate have focused on changes associated with natality and growth, which are inherently slow processes for woody plants-even though the most rapid changes in vegetation are caused by mortality rather than natality. This talk reviews the sensitivity of western montane forests to massive dieback, including drought-induced tree mortality and related insect outbreaks. This overview illustrates the potential for widespread and rapid forest dieback, and associated ecosystem effects, due to anticipated global climate change.
Climate is a key determinant of vegetation patterns at landscape and regional spatial scales. Precipitation variability, including recurrent drought conditions, has typified the climate of the Mountain West for at least thousands of years (Sheppard et al. 2002).
Dendrochronological studies and historical reports show that past droughts have caused extensive vegetation mortality across this region, e.g., as documented in the American Southwest for severe droughts in the 1580s, 1890s to early 1900s, 1950s, and the current drought since 1996 (Swetnam and Betancourt 1998, Allen and Breshears 1998 and in press). Drought stress is documented to lead to dieback in many woody plant species in the West, including spruce (Picea spp.), fir (Abies spp.), Douglas-fir (Pseudotsuga menziesii.), pines (Pinus spp.), junipers (Juniperus spp.), oaks (Quercus spp.), mesquite (Prosopis spp.), manzanitas (Arctostaphylos spp.), and paloverdes (Cercidium spp.).
Drought-induced tree mortality exhibits a variety of nonlinear ecological dynamics. Tree mortality occurs when drought conditions cause threshold levels of plant water stress to be exceeded, which can result in tree death by loss of within-stem hydraulic conductivity (Allen and Breshears-in press). Also, herbivorous insect populations can rapidly build up to outbreak levels in response to increased food availability from drought-weakened host trees, such as the various bark beetle species (e.g. Dendroctonus, Ips, and Scolytus spp.) that attack forest trees (Furniss and Carolin 1977). As bark beetle populations build up they become increasingly successful in killing drought-weakened trees through mass attacks (Figure 1), with positive feedbacks for further explosive growth in beetle numbers which can result in nonlinear ecological interactions and complex spatial dynamics (cf. Logan and Powell 2001, Bjornstad et al. 2002). Bark beetles also selectively kill larger and low-vigor trees, truncating the size and age distributions of host species (Swetnam and Betancourt 1998).
The temporal and spatial patterns of drought-induced tree mortality also reflect non-linear dynamics. Through time mortality is usually at lower background levels, punctuated by large pulses of high tree death when threshold drought conditions are exceeded (Swetnam and Betancourt 1998, Allen and Breshears-in press). The spatial pattern of drought-induced dieback often reveals preferential mortality along the drier, lower fringes of tree species distributions in western mountain ranges. For example, the 1950s drought caused a rapid, drought-induced ecotone shift on the east flank of the Jemez Mountains in northern New Mexico, USA (Allen and Breshears 1998). A time sequence of aerial photographs shows that the ecotone between semiarid ponderosa pine forest and piñon-juniper woodland shifted upslope extensively (2 km or more) and rapidly (< 5 years) due to the death of most ponderosa pine across the lower fringes of that forest type (Figure 1). This vegetation shift has been persistent since the 1950s, as little ponderosa pine reestablishment has occurred in the ecotone shift zone. Severe droughts also markedly reduce the productivity and cover of herbaceous plants like grasses. Such reductions in ground cover can trigger nonlinear increases in erosion rates once bare soil cover exceeds critical threshold values (Davenport et al. 1998, Wilcox et al. 2003). For example, in concert with historic land use practices (livestock grazing and fire suppression), the 1950s drought apparently initiated persistent increases in soil erosion in piñon-juniper woodland sites in the eastern Jemez Mountains that require management intervention to reverse (Sydoriak et al. 2000). Thus, a short- duration climatic event apparently brought about persistent changes in multiple ecosystem properties. Over the past decade, many portions of the Western US have been subject to significant drought, with associated increases in tree mortality evident. GIS compilations of US Forest Service aerial surveys of insect-related forest dieback since 1997 show widespread mortality in many areas. For example the cumulative effect of multi-year drought since 1996 in the Southwest has resulted in the emergence of extensive bark beetle outbreaks and tree mortality across the region. In the Four Corners area piñon (Pinus edulis) has been particularly hard hit since 2002, with mortality exceeding 90% of mature individuals across broad areas (Figure 1), shifting stand compositions strongly toward juniper dominance. Across the montane forests of the West substantial dieback has been recently observed in many tree species, including Engelmann spruce (Picea engelmanni), Douglas-fir, lodgepole pine (Pinus contorta), ponderosa pine, piñon, junipers, and even aspen (Populus tremuloides). A number of major scientific uncertainties are associated with forest dieback phenomena. Quantitative knowledge of the thresholds of mortality for various tree species is a key knowledge gap-we basically don't know how much climatic stress forests can withstand before massive dieback kicks in. Thus the scientific community currently cannot accurately model forest dieback in response to projected climate changes, nor assess associated ecological and societal effects. More research is needed to determine if warm minimum temperatures over the past decade+ are exacerbating the effects of droughts and insects on tree mortality, as: 1) warmer temperatures result in greater plant water stress for a given amount of water availability; and 2) relaxation of low temperature constraints on insect population distributions and generation times may be allowing more extensive and rapid buildup of outbreak population levels. It is thought that substantial and widespread increases in tree densities in many forests and woodlands as a result of more than 100 years of fire suppression also contributes to current patterns of mortality, due to competitive increases in tree water stress and susceptibility to beetle attacks; however, more research is needed on the effectiveness of mechanical thinning and prescribed burning as protective management approaches. Substantial uncertainties exist about the relationship between massive forest dieback and fire behavior. Although severe (crown) fire activity has apparently increased in some overdense forest types in the West, in some areas forest dieback is reducing the vertical and horizontal continuity of a key crown fire fuel component (live needles in tree crowns) as needles drop from dead tress, and that reductions in the spatial extent of uncontrollable crown fires may result. Feedbacks between forest dieback and fire activity (ignition probabilities, rate of spread, severity, controllability) need more work. Recent examples of massive forest dieback illustrate that even relatively brief climatic events (e.g., droughts) associated with natural climate variability can have profound and persistent ecosystem effects. The unprecedentedly rapid climate changes expected in coming decades could produce rapid and extensive contractions in the geographic distributions of long-lived woody species in association with changes in patterns of disturbance (fire, insect outbreaks, soil erosion) (IPCC 2001, Allen and Breshears 1998). Because regional droughts of even greater magnitude and longer duration than the 1950s drought are expected as global warming progresses (Easterling et al. 2001, IPCC 2001), the scale of forest dieback associated with global climate change (Figure 3) could become even greater than what has been observed in recent years (National Research Council 2001). Since mortality-induced vegetation shifts take place more rapidly than do natality-induced shifts associated with plant establishment and migration (Allen and Breshears-in review), dieback could easily outpace new forest growth for a period of years to decades in many areas. Further, as woody vegetation contains the bulk of the world's terrestrial carbon, an improved understanding of mortality-induced responses of woody vegetation to climate is essential for addressing some key environmental and policy implications of climate variability and global change (Breshears and Allen 2002). Thus it is important to more accurately incorporate climate-induced vegetation mortality and the complexity of associated ecosystem responses (e.g., insect outbreaks, fires, soil erosion, and changes in carbon pools) into models that predict vegetation dynamics. References Cited Allen, C.D., and D.D. Breshears. 1998. Drought-induced shift of a forest/woodland ecotone: rapid landscape response to climate variation. Proceedings of the National Academy of Sciences of the United States of America 95:14839-14842. Allen, C.D., and D.D. Breshears. (In press). Drought, tree mortality, and landscape change in the Southwestern United States: Historical dynamics, plant-water relations, and global change implications. In J.L. Betancourt and H.F. Diaz (eds.), The 1950's Drought in the American Southwest: Hydrological, Ecological, and Socioeconomic Impacts. University of Arizona Press, Tucson. Bjornstad, O.N., M. Peltonen, A.M. Liebhold, and W. Baltensweiler. 2002. Waves of larch budmoth outbreaks in the European Alps. Science 298:1020-1023. Breshears, D.D., and C.D. Allen. 2002. The importance of rapid, disturbance-induced losses in carbon management and sequestration. Global Ecology and Biogeography Letters 11:1-15. Davenport, D.W., D.D. Breshears, B.P. Wilcox, and C.D. Allen.1998. Viewpoint: Sustainability of piñon- juniper ecosystems-A unifying perspective of soil erosion thresholds. J. Range Management 51(2):229-238. Easterling, D.R., G.A. Meehl, C. Parmesan, S.A. Changnon, T.R. Karl, and L.O. Mearns. 2000. Climate extremes: observations, modeling, and impacts. Science, 289, 2068-2074. Furniss, R.L., and V.M. Carolin. 1980. Western Forest Insects. USDA For. Serv. Misc. Publ. No. 1339. Government Printing Office, Washington, D.C. IPCC 2001-a. Climate Change 2001: Synthesis Report. A Contribution of Working Groups I, II, and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Watson, R.R. and the Core Writing Team (eds.)]. Cambridge University Press, Cambridge, UK. 398 pp. Logan, J. A., and J. A. Powell. 2001. Ghost forests, global warming, and the mountain pine beetle. American Entomologist. 47: 160-173 National Research Council. 2001. Chapter 5-Economic and Ecological Impacts of Abrupt Climate Change, pp. 90-117 In: Abrupt Climate Change: Inevitable Surprises. Committee on Abrupt Climate Change, Ocean Studies Board, Polar Research Board, Board on Atmospheric Sciences and Climate, National Research Council. Washington, D.C. Sheppard, P.R., A.C. Comrie, G.C. Packin, K Angersbach, and M.K. Hughes. 2002. The climate of the US Southwest. Climate Research 21:219-238. Swetnam, T.W. and J.L. Betancourt. 1998. Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate 11: 3128-3147. Sydoriak, C.A., C.D. Allen, and B.F. Jacobs. 2000. Would ecological landscape restoration make the Bandelier Wilderness more or less of a wilderness? Pp. 209-215 In: D.N. Cole, S.F. McCool, W.T. Borrie, and F. O'Loughlin (comps.). Proceedings: Wilderness Science in a Time of Change Conference-Volume 5: Wilderness Ecosystems, Threats, and Management; 1999 May 23-27; Missoula, MT. USDA Forest Service, Rocky Mountain Research Station, Proceedings RMRS-P-15-VOL-5. Ogden, UT. Wilcox, B.P., D.D. Breshears, and C.D. Allen. 2003. Ecohydrology of a resource-conserving semiarid woodland: Temporal and spatial scaling and disturbance. Ecological Monographs 73(2):223-239. ------------------------------------------------------------------------- 1) "The ability to move, at some stage in the life cycle, is fundamental to success in life." Andrew Sugden and Elizabeth Pennisi SCIENCE VOL 313 11 AUGUST 2006 2) "Animals have no choice but to move, since their survival is at stake. Studies of more than 1,000 species of plants, animals, and insects, found an average migration rate toward the North and South Poles of about four miles per decade in the second half of the 20th century. That is not fast enough. During the past 30 years the lines marking the regions in which a given average temperature prevails, or isotherms, have moved poleward at a rate of about 35 miles per decade. "As long as the total movement of isotherms toward the poles is much smaller than the size of the habitat, or the ranges in which the animals live, the effect on species is limited. But now the movement is inexorably toward the poles, totaling more than 100 miles in recent decades. If emissions of greenhouse gases continue to increase at the current rate -- "business as usual" -- then the rate of isotherm movement will double during this century to at least 70 miles per decade. If we continue on this path, a large fraction of the species on Earth, as many as 50 percent or more, may become extinct." James Hansen 19 October 2006 The Planet in Peril - Part I http://yaleglobal.yale.edu/display.article?id=8305 3) "Each 1 degree C of global warming will shift temperature zones by about 160 km (100 miles). In the northern hemisphere this means that if the climate warms 3Ã‚Â°C, species may have to shift northward as much as 500 km (300 miles) in order to find suitable habitat under the new climatic regime." "Global warming may make a mockery of our attempts in all nature reserves, including Glacier National Park, to preserve natural communities and rare, threatened, and endangered native species." "Perhaps many of Glacier's species will be able to survive farther north, in the Banff-Jasper area. Protection of corridors linking the Greater Yellowstone Ecosystem, the Crown of the Continent Ecosystem, and parks in the Canadian Rockies may provide critical avenues for species dispersal." Glacier National Park Biodiversity Paper #7 http://www.nps.gov/glac/resources/bio7.htm 4) In its "Managing Mountain Parks," the UN's Food and Agriculture Organization says, "The major challenges for the twenty-first century" include this one: "To link together the isolated existing mountain protected areas by conservation corridors along the mountain ranges. This not only increases effective size, but provides migration corridors for gene flow and species movement. As the climate changes, poleward migration corridors in north-south ranges (e.g. the Andes) will better accommodate temperature change, and migration along the east-west ranges (e.g. the Western Tien Shan) will be a response to rainfall changes. full FAO report at: http://www.fao.org/documents/show_cdr.asp?url_file=/docrep/x0963E/x0963e06.htm 5) The United Nations Environmental Programme stresses the same basic point: "Forest management responses to climate change should focus on maintaining species diversity on national or continental scales through facilitating the processes of species migration, rather than by solely preserving specific reserves." full UNEP report at: http://www.unep-wcmc.org/forest/flux/executive_summary.htm