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Sent: Sat, 22 Sep 2007 12:28 pm<br>
Subject: [conservation-psychology] Will forest-based biofuels really be sustainable?<br>
<br>
<div id=AOLMsgPart_0_fe4e5dae-43d1-42ac-a3d2-d4f762328c40 style="FONT-SIZE: 12px; MARGIN: 0px; COLOR: #000; FONT-FAMILY: Tahoma, Verdana, Arial, Sans-Serif; BACKGROUND-COLOR: #fff"><PRE style="FONT-SIZE: 9pt"><TT>Calculations of forests' potential as sustainable biofuel assume that
forests will survive expected climate changes. But will they?
-----------------------------------------------
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.
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