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Massive Forest Dieback

ALLEN, CRAIG D.
U.S. Geological Survey, Jemez Mountains Field Station, Los Alamos, NM 87544

Presented August 9, 2007 at joint meeting of Ecological Society of America and Society for Ecological Restoration

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 presecribed
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.