Calculations of forests’ potential as sustainable
biofuel assume that forests will survive expected
climate changes. But will they?


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

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). Continue reading


“I think it’s well accepted that 2 degrees is
likely to be exceeded,” said Vicky Pope,
manager of the MetOffice Hadley Centre’s climate change research programme.

“Largely thanks to man-made greenhouse gas
emissions, temperatures worldwide rose some 0.7
degrees last century, and another 0.6 degrees is
locked in as the world’s oceans catch up with
quicker warming over land.”

Reuters News Service

World Likely to Pass Dangerous Warming Limits – Study

LONDON – The world will probably exceed a global
warming limit which the European Union calls
dangerous, scientists at Britain’s MetOffice
Hadley Centre said on Tuesday, presenting a new,
5-year research programme.

But not all scientists agree, demonstrating a
shift in debate from whether climate change is
happening — on which where there is near
consensus — to how bad it will get and what to
do about it. Continue reading


“Bees are such great environmental samplers. When they go out and forage, they go almost two miles away from the hive. That’s a very large area, about 2,500 acres, and the same size as the grid elements of a lot of climate ecosystem models,” Esaias said.

“If we’re headed into rougher weather, as it appears we are, we’ll have more difficulties with our bees,” Mussen said. “It won’t matter if you’re a backyard beekeeper or someone with 10,000 colonies.”

Washington Post
Monday, September 10, 2007; A05

Weather May Account for Reduced Honey Crop
By Jane Black

That the 2007 honey crop has been disappointing won’t surprise anyone who has picked up the newspaper in recent months. Since early spring, colony collapse disorder (CCD), a disease that causes honeybees to suddenly, mysteriously disappear from their hives, has made headlinesaround the world. Without honeybees to pollinate, experts warn that one-third of the food supply — from apples and peaches to cucumbers and squash — is at risk.

Continue reading

Climate Change: Faster Than Expected
Loss of Arctic ice leaves experts stunned
David Adam, environment correspondent
Guardian Unlimited
Tuesday September 4 2007

The Arctic ice cap has collapsed at an unprecedented rate this summer
and levels of sea ice in the region now stand at record lows,
scientists have announced.

Experts say they are “stunned” by the loss of ice, with an area
almost twice as big as the UK disappearing in the last week alone.

So much ice has melted this summer that the Northwest passage across
the top of Canada is fully navigable, and observers say the Northeast
passage along Russia’s Arctic coast could open later this month.

If the increased rate of melting continues, the summertime Arctic
could be totally free of ice by 2030. Continue reading