Calculations of forests’ potential as sustainable
biofuel assume that forests will survive expected
climate changes. But will they?
Lance
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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.
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Drought, tree mortality, and landscape change in
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