[MPWG] FYI massive forest diebacks Fwd: [conservation-psychology] Will forest-based biofuels really be sustainable?

[cafesombra at aol.com]

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Subject: [conservation-psychology] Will forest-based biofuels really be sustainable?



Calculations of forests' potential as sustainable biofuel assume that
orests will survive expected climate changes. But will they?
-----------------------------------------------
ASSIVE FOREST DIEBACK 

LLEN, CRAIG D.  
.S. Geological Survey, Jemez Mountains Field Station, Los Alamos, NM
7544
Presented August 9, 2007 at joint meeting of Ecological Society of America
nd Society for Ecological Restoration

n coming decades, climate changes are expected to produce large shifts in
egetation distributions, largely due to mortality.  However, most field
tudies and model-based assessments of vegetation responses to climate
ave focused on changes associated with natality and growth, which are
nherently slow processes for woody plants-even though the most rapid
hanges in vegetation are caused by mortality rather than natality.  This
alk reviews the sensitivity of western montane forests to massive
ieback, including drought-induced tree mortality and related insect
utbreaks.  This overview illustrates the potential for widespread and
apid forest dieback, and associated ecosystem effects, due to anticipated
lobal climate change.
Climate is a key determinant of vegetation patterns at landscape and
egional spatial scales.  Precipitation variability, including recurrent
rought conditions, has typified the climate of the Mountain West for at
east thousands of years (Sheppard et al. 2002).
Dendrochronological studies and historical reports show that past droughts
ave caused extensive vegetation mortality across this region, e.g., as
ocumented in the American Southwest for severe droughts in the 1580s,
890s to early 1900s, 1950s, and the current drought since 1996 (Swetnam
nd Betancourt 1998, Allen and Breshears 1998 and in press).  Drought
tress is documented to lead to dieback in many woody plant species in the
est, including spruce (Picea spp.), fir (Abies spp.), Douglas-fir
Pseudotsuga menziesii.), pines (Pinus spp.), junipers (Juniperus spp.),
aks (Quercus spp.), mesquite (Prosopis spp.), manzanitas (Arctostaphylos
pp.), and paloverdes (Cercidium spp.).
 
rought-induced tree mortality exhibits a variety of nonlinear ecological
ynamics.  Tree mortality occurs when drought conditions cause threshold
evels of plant water stress to be exceeded, which can result in tree
eath by loss of within-stem hydraulic conductivity (Allen and Breshears -
n press).  Also, herbivorous insect populations can rapidly build up to
utbreak levels in response to increased food availability from
rought-weakened host trees, such as the various bark beetle species (e.g.
endroctonus, Ips, and Scolytus spp.) that attack forest trees (Furniss
nd Carolin 1977).  As bark beetle populations build up they become
ncreasingly successful in killing drought-weakened trees through mass
ttacks (Figure 1), with positive feedbacks for further explosive growth
n beetle numbers which can result in nonlinear ecological interactions
nd complex spatial dynamics (cf. Logan and Powell 2001, Bjornstad et al.
002).  Bark beetles also selectively kill larger and low-vigor trees,
runcating the size and age distributions of host species (Swetnam and
etancourt 1998).
 
he temporal and spatial patterns of drought-induced tree mortality also
eflect non-linear dynamics.  Through time mortality is usually at lower
ackground levels, punctuated by large pulses of high tree death when
hreshold drought conditions are exceeded (Swetnam and Betancourt 1998,
llen and Breshears - in press).  The spatial pattern of drought-induced
ieback often reveals preferential  mortality along the drier, lower
ringes of tree species distributions in western mountain ranges.  For
xample, the 1950s drought caused a rapid, drought-induced ecotone shift
n the east flank of the Jemez Mountains in northern New Mexico, USA
Allen and Breshears 1998).  A time sequence of aerial photographs shows
hat the ecotone between semiarid ponderosa pine forest and piñon-juniper
oodland shifted upslope extensively (2 km or more) and rapidly (< 5 years) due 
o 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 
ittle ponderosa pine reestablishment has occurred in the ecotone shift zone.

evere droughts also markedly reduce the productivity and cover of
erbaceous plants like grasses.  Such reductions in ground cover can
rigger nonlinear increases in erosion rates once bare soil cover exceeds
ritical threshold values (Davenport et al. 1998, Wilcox et al. 2003). 
or example, in concert with historic land use practices (livestock
razing and fire suppression), the 1950s drought apparently initiated
ersistent increases in soil erosion in piñon-juniper woodland sites in
he eastern Jemez Mountains that require management intervention to
everse (Sydoriak et al. 2000).  Thus, a short- duration climatic event
pparently brought about persistent changes in multiple ecosystem
roperties.  Over the past decade, many portions of the Western US have
een subject to significant drought, with associated increases in tree
ortality evident.  GIS compilations of US Forest Service aerial surveys
f insect-related forest dieback since 1997 show widespread mortality in
any areas.  For example the cumulative effect of multi-year drought since
996 in the Southwest has resulted in the emergence of extensive bark
eetle outbreaks and tree mortality across the region.  In the Four
orners area piñon (Pinus edulis) has been particularly hard hit since
002, with mortality exceeding 90% of mature individuals across broad
reas (Figure 1), shifting stand compositions strongly toward juniper
ominance.  Across the montane forests of the West substantial dieback has
een recently observed in many tree species, including Engelmann spruce
Picea engelmanni), Douglas-fir, lodgepole pine (Pinus contorta),
onderosa pine, piñon, junipers, and even aspen (Populus tremuloides).

 number of major scientific uncertainties are associated with forest
ieback phenomena.  Quantitative knowledge of the thresholds of mortality
or various tree species is a key knowledge gap - we basically don't know
ow much climatic stress forests can withstand before massive dieback
icks in.  Thus the scientific community currently cannot accurately model
orest dieback in response to projected climate changes, nor assess
ssociated ecological and societal effects.  More research is needed to
etermine if warm minimum temperatures over the past decade+ are
xacerbating the effects of droughts and insects on tree mortality, as: 
) warmer temperatures result in greater plant water stress for a given
mount of water availability;  and 2) relaxation of low temperature
onstraints on insect population distributions and generation times may be
llowing more extensive and rapid buildup of outbreak population levels. 
t is thought that substantial and widespread increases in tree densities
n many forests and woodlands as a result of more than 100 years of fire
uppression also contributes to current patterns of mortality, due to
ompetitive increases in tree water stress and susceptibility to beetle
ttacks;  however, more research is needed on the effectiveness of
echanical thinning and presecribed burning
s protective management approaches. 
Substantial uncertainties exist about the relationship between massive
orest dieback and fire behavior.  Although severe (crown) fire activity
as apparently increased in some overdense forest types in the West, in
ome areas forest dieback is reducing the vertical and horizontal
ontinuity of a key crown fire fuel component (live needles in tree
rowns) as needles drop from dead tress, and that reductions in the
patial extent of uncontrollable crown fires may result.  Feedbacks
etween forest dieback and fire activity (ignition probabilities, rate of
pread, severity, controllability) need more work.
Recent examples of massive forest dieback illustrate that even relatively
rief climatic events (e.g., droughts) associated with natural climate
ariability can have profound and persistent ecosystem effects.  The
nprecedentedly rapid climate changes expected in coming decades could
roduce rapid and extensive contractions in the geographic distributions
f long-lived woody species in association with changes in patterns of
isturbance (fire, insect outbreaks, soil erosion) (IPCC 2001, Allen and
reshears 1998).  Because regional droughts of even greater magnitude and
onger duration than the 1950s drought  are expected as global warming
rogresses (Easterling et al. 2001, IPCC 2001), the scale of forest
ieback associated with global climate change (Figure 3) could become even
reater than what has been observed in recent years (National Research
ouncil 2001).  Since mortality-induced vegetation shifts take place more
apidly than do natality-induced shifts associated with plant
stablishment and migration
Allen and Breshears - in review), dieback could easily outpace new forest
rowth for a period of years to decades in many areas.  Further, as woody
egetation contains the bulk of the world's terrestrial carbon, an
mproved understanding of mortality-induced responses of woody vegetation
o climate is essential for addressing some key environmental and policy
mplications of climate variability and global change (Breshears and Allen
002).  Thus it is important to more accurately incorporate
limate-induced vegetation mortality and the complexity of associated
cosystem responses (e.g., insect outbreaks, fires, soil erosion, and
hanges in carbon pools) into models that predict vegetation dynamics.

eferences Cited 

llen, C.D., and D.D. Breshears.  1998.  Drought-induced shift of a
orest/woodland ecotone: rapid landscape response to climate variation. 
roceedings of the National Academy of Sciences of the United States of
merica 95:14839-14842.

llen, C.D., and D.D. Breshears.  (In press).  Drought, tree mortality,
nd landscape change in the Southwestern United States: Historical
ynamics, plant-water relations, and global change implications.  In J.L.
etancourt and H.F. Diaz (eds.), The 1950's Drought in the American
outhwest: Hydrological, Ecological, and Socioeconomic Impacts. 
niversity of Arizona Press, Tucson.

jornstad, O.N., M. Peltonen, A.M. Liebhold, and W. Baltensweiler. 2002.
aves of larch budmoth outbreaks in the European Alps. Science
98:1020-1023.

reshears, D.D., and C.D. Allen. 2002.  The importance of rapid,
isturbance-induced losses in carbon management and sequestration.  Global
cology and Biogeography Letters 11:1-15.

avenport, D.W., D.D. Breshears, B.P. Wilcox, and C.D. Allen.1998.
iewpoint: Sustainability of piñon- juniper ecosystems - A unifying
erspective of soil erosion thresholds. J. Range Management
1(2):229-238. 

asterling, D.R., G.A. Meehl, C. Parmesan, S.A. Changnon, T.R. Karl, and
.O. Mearns. 2000. Climate extremes: observations, modeling, and impacts.
cience, 289, 2068-2074.

urniss, R.L., and V.M. Carolin. 1980. Western Forest Insects. USDA For.
erv. Misc. Publ. No. 1339. Government Printing Office, Washington, D.C.

PCC 2001-a. Climate Change 2001: Synthesis Report. A Contribution of
orking Groups I, II, and III to the Third Assessment Report of the
ntergovernmental Panel on Climate Change [Watson, R.R. and the Core
riting Team (eds.)]. Cambridge University Press, Cambridge, UK. 398 pp.

ogan, J. A., and J. A. Powell. 2001. Ghost forests, global warming, and
he mountain pine beetle. American Entomologist. 47: 160-173

ational Research Council.  2001. Chapter 5 - Economic and Ecological
mpacts of Abrupt Climate Change, pp. 90-117 In:  Abrupt Climate Change:
nevitable Surprises.  Committee on Abrupt Climate Change, Ocean Studies
oard, Polar Research Board, Board on Atmospheric Sciences and Climate,
ational Research Council. Washington, D.C.

heppard, P.R., A.C. Comrie, G.C. Packin, K Angersbach, and M.K. Hughes.
002. The climate of the US Southwest. Climate Research 21:219-238.

wetnam, T.W. and J.L. Betancourt.  1998.  Mesoscale disturbance and
cological response to decadal climatic variability in the American
outhwest.  Journal of Climate 11: 3128-3147.

ydoriak, C.A., C.D. Allen, and B.F. Jacobs. 2000.  Would ecological
andscape restoration make the Bandelier Wilderness more or less of a
ilderness?  Pp. 209-215 In: D.N. Cole, S.F. McCool, W.T. Borrie, and F.
'Loughlin (comps.).  Proceedings: Wilderness Science in a Time of Change
onference-Volume 5: Wilderness Ecosystems, Threats, and Management; 1999
ay 23-27; Missoula, MT. USDA  Forest Service, Rocky Mountain Research
tation, Proceedings RMRS-P-15-VOL-5. Ogden, UT.

ilcox, B.P., D.D. Breshears, and C.D. Allen. 2003. Ecohydrology of a
esource-conserving semiarid woodland: Temporal and spatial scaling and
isturbance. Ecological Monographs 73(2):223-239.
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