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As the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization.
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Temperature as a potent driver of regional forest
drought stress and tree mortality
A. Park Williams1*, Craig D. Allen2, Alison K. Macalady3,4, Daniel Griffin3,4, Connie A. Woodhouse3,4,
David M. Meko4, Thomas W. Swetnam4, Sara A. Rauscher5, Richard Seager6,
Henri D. Grissino-Mayer7, Jeffrey S. Dean4, Edward R. Cook6, Chandana Gangodagamage1,
Michael Cai8and Nate G. McDowell1
As the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the
relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress
index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000–2007. The
FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and
cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures
of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the
vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will
exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century
changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps
water-limited forests globally, towards distributions unfamiliar to modern civilization.
Recent declines in forest productivity and tree survival have
been documented at many sites globally and attributed to
water limitation1,2. Forest declines may be accelerating in
many regions because warming has amplified water limitation3–10.
We describe forest declines due to water limitation as drought-
induced declines, whether by lack of precipitation or by increased
evaporative demand. In the southwestern United States (SWUS;
Supplementary Fig. S1), drought impacts on forests have been
relatively severe since the late 1990s (refs 5,11). This period has
been punctuated by the highest temperatures in the observed
record, positively influencing atmospheric moisture demand12. The
ongoing heat-driven drought, in combination with a multitude of
ecophysiological data documenting regional forest processes, makes
the SWUS an ideal natural experimental target13 for examining
the influence of heat-induced drought on productivity and tree
mortality in drought-sensitive forests.
Climate modelling experiments collectively project the SWUS to
become warmer and more arid as greenhouse-gas concentrations
rise and the sub-tropical high-pressure zones expand and shift
polewards14,15. Water balance has long been known to substantially
limit tree growth and influence forest disturbances in the SWUS;
however, the relative importance of temperature and precipitation
requires clarification16. Temperature should be expected to influ-
ence water balance because it exponentially influences atmospheric
evaporative demand (Supplementary Fig. S2). Uncertainty regard-
ing the relative roles of evaporative demand versus precipitation in
dictating forest drought-stress and tree mortality has limited our
1Earth & Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA, 2US Geological Survey, Fort Collins
Science Center, Jemez Mountains Field Station, Los Alamos, New Mexico 87544, USA, 3School of Geography and Development, University of Arizona,
Tucson, Arizona 85721, USA, 4Laboratory of Tree-ring Research, University of Arizona, Tucson, Arizona 85721, USA, 5Theoretical Division, Los Alamos
National Laboratory, Los Alamos, New Mexico 87545, USA, 6Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA,
7Laboratory of Tree-Ring Science, Department of Geography, The University of Tennessee, Knoxville, Tennessee 37996, USA, 8Space Data Systems, Los
Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. *e-mail:
ability to anticipate future impacts to forests, which might otherwise
be estimated through global climate model output. Here we de-
velop a tree-ring-based index of forest drought-stress that explicitly
resolves the contributions of the vapour-pressure deficit (VPD;
difference between the actual- and saturation-vapour pressure,
largely controlled by temperature) and precipitation. We link this
index to the most comprehensive set of regional forest productivity
and disturbance records assembled so far. We use an ensemble
of climate-model projections of VPD and precipitation to project
future levels of forest drought-stress and place them in the context
of historic events known to have caused widespread tree mortality.
Forest drought-stress and climate
Annual tree-ring widths reflect variability in the environmental
stressors that limit growth17. We used tree-ring records to develop
an index of annual forest stress for dominant southwestern tree
species, ad 1000–2007. The index is based on 335 collections of site-
specific tree-ring width measurements (13,147 specimens) from
throughout the SWUS and surrounding regions (Supplementary
Fig. S1; see Supplementary Information for all methods and data
sources). Nearly all chronologies represent the three most abundant
conifer species in the SWUS: piñon (Pinus edulis), ponderosa pine
(P. ponderosa) and Douglas-fir (Pseudotsuga menziesii). Sites reflect
a range of elevations along landscape moisture gradients.
We call the first-principal-component time series of ring-width
chronologies the SWUS FDSI (Fig. 1a) because it is highly
representative of interannual variability in ring width across
© 2012 Macmillan Publishers Limited. All rights reserved.
Predictive contribution of VPD (%)
Predictive contribution of precipitation (%)
Correlation (r)
Less drought stress
More drought stress
r = 0.91
1900 1920 1940 1960 1980 2000
100 90 80 70 60 50 40 30 20 10 0
Figure 1 |Correlation between the FDSI and climate. a, The annual FDSI
derived from tree ring-width index records (red, 1896–2007) and
estimated with climate data using equation (1) (black, 1896–2012). See
Supplementary Fig. S3 for estimated confidence ranges in the FDSI values.
b, Curves show correlation between the estimated and the actual FDSI,
allowing predictive contributions of the warm-season VPD and cold-season
precipitation to vary from 0 to 100% and 100 to 0%, respectively. The
straight lines connect optimal correlations with axes. Grey areas represent
95% confidence intervals.
space and species (Supplementary Fig. S1) and because the index
correlates well with observed drought-related climate variables.
See Supplementary Fig. S3 for sample size and error. The FDSI
is unique because it explicitly represents regionally coherent
tree-growth variability calculated from all available ring-width
records from the three main conifer species in the SWUS. In
contrast, climate-reconstruction approaches (for example, ref. 18)
are tuned to represent a specific drought variable during a specific
season, using only ring-width records that correlate well with the
drought variable of interest.
Considering observed climate data during 1896–2007, the FDSI
most strongly correlates with the log of cold-season precipitation
(positive relationship) and the warm-season VPD (a combination
of the previous August–October and growing-season May–July
VPD, negative relationship). The FDSI can be estimated using the
following formula:
Estimated FDSI =0.44[zscore(ln(Pndjfm))]
0.56[zscore((VPDaso +VPDmjj)/2)](1)
where the subscripts are the initials of months, underlined initials
indicate months of the previous year, and zscore indicates time-
series standardization so values during 1896–2007 have a mean of
zero and a standard deviation of one (Supplementary Information).
Coefficients in equation (1) indicate that the warm-season VPD
and cold-season precipitation (P) account for 56% and 44% of the
predictive power, respectively (derived visually in Fig. 1b). Com-
bined, these variables account for 82% of the tree-ring-derived FDSI
variability (p<0.0001, 95% confidence: 0.74 R20.88). Relative
contributions of the warm-season VPD and cold-season precipita-
tion were stable throughout the instrumental period (Fig. 1b).
The seasons when precipitation and the VPD are best
correlated with the FDSI are consistent with our knowledge
of plant physiology. Stemwood growth in the SWUS is most
strongly dependent on soil–water recharge from cold-season
precipitation13,19,20. Warming in spring and summer causes the VPD
to increase exponentially (Supplementary Fig. S2) and soil moisture
decreases through evapotranspiration. Increased VPD coupled with
limited soil moisture increases the potential for hydraulic failure
(collapse of water columns within xylem cells) and can force
prolonged stomatal closure, decreasing photosynthesis, growth rate
and carbohydrate reserves16,21,22. Correlation between growth and
the VPD does not extend into August, consistent with observations
of a mid- to late-summer shutdown of radial growth among SWUS
trees (for example, refs 17,19). Instead, the FDSI is negatively
correlated with the August–October VPD of the previous year.
Although these months are not entirely within the warm season, the
mechanisms by which the previous autumn VPD limits tree growth
are similar to those of the warm season. When conditions allow,
photosynthesis continues during August–October after cambial
shutdown, allowing allocation of carbohydrates to reserves that
influence radial growth during the following growing season17. The
VPD during this period can also influence soil-moisture recharge
by subsequent cold-season precipitation17.
The FDSI reflects variability in regional-scale vegetation growth,
as supported by the strong correlation between the FDSI and
the satellite-derived regional normalized difference vegetation
index (NDVI) during 1981–2012 (Fig. 2a). The FDSI and NDVI
both capture ongoing decline in productivity following the mid-
1990s as well as a minimum in 2002. Turn-of-the-century
drought conditions reduced productivity and ecosystem uptake of
atmospheric carbon throughout western North America10.
Tree mortality
Ongoing drought-driven decline in regional productivity is associ-
ated with widespread tree mortality in the SWUS. Considering the
three focal species in this study, FDSI and NDVI minima in 2002
were followed by an approximate doubling of the proportion of
dead individuals in the SWUS (Fig. 2b). Other studies5,23,24 docu-
ment widespread post-2002 mortality for piñon, which occupies the
most arid habitat of the three focal species, but the same is true for
ponderosa pine and Douglas-fir.
Much of the conifer mortality in the 2000s was caused by
factors influenced by bark-beetle attack and wildfire9,11,13,25–28.
Bark-beetle populations seem to grow during relatively warm
periods and specifically target drought-stressed trees with weakened
defences9,25. Considering aerial survey data for 1997–2011, we find
a negative temporal correspondence between the 2-year running
FDSI and the SWUS area that experienced bark-beetle-induced
mortality of at least 10 trees per acre (Fig. 2c). Although a longer
record is needed to confirm the relationship between the FDSI and
beetle-induced tree mortality, the multi-year relationship between
the area impacted by bark beetles and the FDSI makes sense
because bark-beetle populations grow and decline over multiple
seasons25. Importantly, the logarithmic scale of the left-hand yaxis
in Fig. 2c suggests that incremental increases in drought conditions
promote exponential increases in the potential for beetle-induced
tree mortality. SWUS bark-beetle outbreaks may also be influenced
by forest-stand density, which has generally increased in the past
century owing to forest management practices13,29–32.
Our results also indicate a strong exponential correspondence
between forest drought-stress and satellite measurements of
forest and woodland area burned by wildfire (Fig. 2d). This
is consistent with regional analyses of fire scars left on tree
rings before observed records (for example, refs 13,26,33).
Using SWUS fire-scar data from a network of sites more than
four times as dense as those used by previous studies (for
example, ref. 13), we find that the probability of an extensive
wildfire year in the SWUS was exponentially related to the FDSI
during ad 1650–1899 (Fig. 2d inset). These analyses indicate that
© 2012 Macmillan Publishers Limited. All rights reserved.
r = 0.83 ¬2
1980 1990 2000
Wildfire area (km2)
r = ¬0.84
r = ¬0.82
Bark-beetle area (km2)
Ponderosa pine
1¬1 0
NDVIPercentage dead
Figure 2 |Measurements of forest productivity and mortality overlaid on
the FDSI (red, right yaxis). a, The annual average late-June to
early-August NDVI calculated from satellite (1981–1999: AVHRR,
2000–2012: MODIS) imagery. b, Annual forest inventory and analysis
measurements of the percentage of standing dead trees in the SWUS for
the three most common conifer species. Error bars represent standard
deviation of the percentage dead when each year’s forest inventory and
analysis measurements are randomly resampled 1,000 times
(Supplementary Information). c, Aerial-survey-derived estimates of the
area where 10 trees per acre were killed by bark-beetle attack.
d, Satellite-derived moderately and severely burned forest and woodland
area in the SWUS. See Supplementary Information and Supplementary
Fig. S4 for methods to calculate burned area. The inset shows the
percentage of years within a given FDSI class that were top-10% fire-scar
years during AD 1650–1899 (the horizontal line is at the expected
frequency of 10%, bins are 0.25 FDSI units wide). In all panels, the FDSI
values for 2008–2012 (open red squares) were estimated by applying
climate data to equation (1). Note the inverted yaxes for the FDSI in bd.
drought has been, and remains, a primary driver of widespread
wildfires in the SWUS.
Given the exponential relationships established between the
FDSI and tree mortality, severe drought events before the observed
record probably coincided with widespread tree mortality. As
observed climate and mortality data are unavailable for much of
the past millennium, we use the FDSI record to identify other
severe drought events likely to have caused widespread mortality
since ad 1000 (Fig. 3). A drought event is defined as any period
greater than three years when the mean FDSI is negative, the FDSI
is not positive for two consecutive years34, and the FDSI is less
than two standard deviation units below the 1896–2007 mean for
at least one year. Drought-event strength is the sum of the FDSI
values during the event. Updating the FDSI for 2008–2012 with
the FDSI values estimated from equation (1), three drought events
have occurred within the observed climate record: the present
drought (2000–2012, the fifth strongest since ad 1000), 1945–1964
(the sixth strongest) and 1899–1904 (the seventeenth strongest;
Fig. 3). The prolonged 1945–1964 event was indeed associated with
extensive tree mortality in the SWUS as indicated by documentation
1000 1200 1400 1600
1800 2000
Figure 3 |Eleven-year smoothed FDSI for AD1000–2012. Black area: 95%
confidence range of the FDSI, representing the range of FDSI values
expected if all 335 chronologies were available. Vertical grey areas highlight
drought events.
of bark-beetle outbreaks30,35, anomalously large wildfires31,32 and
widespread die-off of conifers30,31,35. The 1899–1904 drought was
also associated with forest declines36, although little documented.
Before the 1900s, the 1572–1587 event was the most recent
event exceeding the severity of the present event (Fig. 3). This
megadrought event37,38 ranks as the fourth most severe since
ad 1000 and the most severe since 1300. Although direct mortality
observations are not available for the 1500s event, studies of forest
age structure document a scarcity of trees on today’s landscape
that began growing before the late 1500s (refs 13,31). As lifespans
of SWUS conifers often greatly exceed 400 years, the scarcity of
trees preceding the 1500s event indicates that intense drought
conditions probably led to deaths of a large proportion of trees
living at the time. Before the late 1500s, the correspondence between
records of conifer pollen buried at archaeological sites and tree-ring
widths39 suggests that widespread tree mortality (indicated by
pollen) co-occurred with massive droughts in the 1200s (indicated
by tree rings). Notably, ancient Puebloan populations and land-use
practices were in great flux during this time, confounding the
attribution of a dominant cause of the 1200s forest decline40.
Future forest drought-stress
The ongoing VPD-dominated drought event (Fig. 4a) is consistent
with climate-model projections (phase 3 of the Coupled Model
Intercomparison Project (CMIP3)) of increasing warm-season
VPD (3.6% decade1) throughout the rest of the twenty-first
century in response to business-as-usual greenhouse-gas emissions
scenarios41 (SRES A2; Fig. 4a and Supplementary Fig. S6 for
alternative emissions scenarios: SRES A1B and B1). Dynamically
downscaled (0.5geographic resolution) model projections indicate
similar increases in the VPD (Fig. 4a and Supplementary Infor-
mation). Furthermore, most models project a slight decrease in
cold-season precipitation during the second half of this century
(∼−1.25% decade1, Fig. 4c). Applying model projections to
equation (1), all models indicate negative FDSI trends throughout
the twenty-first century (Fig. 4d). By 2050, the ensemble mean FDSI
is consistently more severe than that of any megadrought since at
least ad 1000 (megadrought conditions are surpassed by 2070 in the
most optimistic B1 emissions scenario, Supplementary Fig. S6d).
Notably, projections of the FDSI are more severe than projections
of gross water balance (precipitation–evaporation) because water-
balance projections are influenced more by decreased cold-season
precipitation than by increased warm-season VPD (ref. 15).
FDSI projections suggest that SWUS forest drought-stress is
entering a new era where natural oscillations such as those apparent
in Fig. 3 are superimposed on, and overwhelmed by, a trend
towards more intense drought stress. As the VPD diverges from the
range of observed variability, nonlinear effects may alter drought
impacts on forests (for example, Fig. 5 in ref. 42). During the
observed record, equation (1) was a better predictor of the FDSI
© 2012 Macmillan Publishers Limited. All rights reserved.
s.d. anomaly
VPD (kPa)
Tmax (°C)
Cold-season P (mm)
1900 1950 2000 2050
Figure 4 |Observed and modelled climate and forest drought-stress.
ad, The warm-season VPD (a), warm-season Tmax (b), cold-season
precipitation (c) and the FDSI (d). Black: observed records. Coloured bold
lines: CMIP3 ensemble mean values. Shading around time series: inner
50% of CMIP3 values. Green time series: 2042–2069 dynamically
downscaled NARCCAP ensemble mean values. Horizontal brown line and
shading in dshow the mean and 95% confidence FDSI values of the most
severe 50% of years during the 1572–1587 megadrought. The horizontal
grey lines show the anomaly in standard deviations from the observed
1896–2007 mean (right yaxis). See Supplementary Figs S5 and S6 for
individual model projections and alternative emissions scenarios.
in years of relatively high drought-stress (Supplementary Fig. S7).
This may indicate that forest sensitivity intensifies as drought
intensifies, consistent with exponential relationships between the
FDSI and tree mortality (Fig. 2c,d). Interestingly, the observed
intensification of drought sensitivity during high drought-stress
years was mainly due to heightened sensitivity to variability in
cold-season precipitation (Supplementary Fig. S7a,b,g). This may
mean that cold-season precipitation will gain relative importance as
drought intensifies in the coming decades. To account for this and
other possible nonlinear effects, we estimated the future FDSI where
the relative predictive contributions of cold-season precipitation
and the warm-season VPD are forced to vary. Reducing the future
predictive contribution of the warm-season VPD to 25%—less than
half the observed contribution—and increasing the contribution
of cold-season precipitation to 75%, the ensemble mean FDSI is
still estimated to equal or exceed 1500s megadrought levels by the
2060s in a business-as-usual emissions scenario (Supplementary
Fig. S8a). In the hypothetical case that climate models have over-
predicted VPD trends by a factor of two, possibly influenced
by model misrepresentation of SWUS monsoon characteristics,
megadrought levels are still surpassed during the late twenty-first
century (Supplementary Fig. S8d).
Importantly, forest-decline events are particularly sensitive to
extreme conditions (Fig. 2c,d). As widespread forest decline seems
to have occurred during the 1572–1587 megadrought, we treat the
mean FDSI during the most extreme half of the years during this
period as a forest-stress benchmark signifying a level of extreme
¬5 ¬4 ¬3 ¬2
¬1 0 1
1000 1200 1400 1600 1800
Figure 5 |Extreme drought stress. a, Cumulative distribution functions of
tree-ring derived FDSI during AD 1000–2007 (black) and model-projected
FDSI during AD 2000–2100 for the A2 (red), A1B (green) and B1 (blue)
emissions scenarios. Brown line: mean FDSI during the most extreme half
of the 1572–1587 megadrought. b, Fifty-year running frequency of annual
FDSI values more negative than the mean FDSI during the most negative
half of the years during the 1572–1587 megadrought. The colours in b
represent the same as in a. Shaded areas: 95% confidence ranges for
tree-ring-derived values and inner-quartile values for model ensemble
drought-stress likely to correspond with widespread forest decline
(benchmark FDSI =1.41). Although the 1200s megadrought
was longer, the 1500s megadrought was more extreme. During
ad 1000–2007, 4.8% of the FDSI values were more negative than the
1500s benchmark, and the highest 50-year frequency of benchmark
years was 18% during 1247–1296. During the present drought
event, 4 of the 13 years (31%) qualify as benchmark years. On the
basis of ensemble mean projections, 59% of years will be benchmark
years during 2000–2100 assuming the A2 emissions scenario
(Fig. 5a), and the frequency of benchmark years is projected to reach
approximately 80% during the second half of this century (Fig. 5b).
Assuming the most optimistic emissions scenario (B1), this value
is 53%. Very extreme FDSI values of less then 3 (unprecedented
during 1000–2012) are projected to occur with a frequency of ap-
proximately 20% in the twenty-first century (A2 scenario, Fig. 5b).
Projections of increased forest drought-stress and tree mortality
are relevant throughout the SWUS, not only to the most drought-
prone sites. Recent bark-beetle- and wildfire-induced tree mortality
has occurred approximately uniformly among a wide variety of
SWUS sites ranging from very drought prone to less drought prone
than average (Fig. 6a,b; see Supplementary Methods). Mortality
has been less common within the 20% of forest area least
prone to drought (high cold-season precipitation, low warm-
season VPD). Observed exponential relationships between the
FDSI and forest-decline processes suggest that less drought-prone
SWUS forests may become progressively more vulnerable to forest
decline processes and mortality as the warm-season VPD increases.
Furthermore, regeneration of common conifer species in the SWUS
generally occurs in pulses linked to wet/cool conditions13, and
often requires the presence of parent tree sources32. Loss of mature
trees across increasingly large areas due to high-severity fires
and bark-beetle-induced mortality (Fig. 2c,d and Supplementary
Fig. S9b), coupled with ongoing and projected increases in drought
stress due to climate change (Fig. 4), means that species-specific
tree regeneration needs are increasingly less likely to be met after
disturbance43. This increases the risks of long-term forest structural
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Figure 6 |Where have trees died? The xaxis represents a long-term
drought-stress gradient among SWUS forest grid cells. The grid cells with
the most severe long-term drought-stress are on the left side of plots.
a, The percentage of grid cells in each drought-stress class with 10 trees
per acre killed by bark beetles during 1997–2011. b,The percentage of grid
cells in each drought-stress class where moderate or severe wildfire
occurred during 1984–2011. Horizontal dotted black lines in aand b
indicate expected percentages if these mortality processes were spatially
uniform. c, Probability distribution function of the average FDSI during
1896–2007 among SWUS forest grid cells. The site-specific FDSI (xaxis)
estimated using equation (1). The methods are described in the
Supplementary Information. Grey and white shading is intended to assist
with interpretation.
and compositional changes and type conversions from forests to
shrublands or grasslands (for example, refs 32,44).
The warm-season VPD was at least as important as cold-season
precipitation in dictating SWUS forest drought-stress during
1896–2007. The warm-season VPD has been particularly high since
2000 and is the primary driver of an ongoing drought-stress event
that is more severe than any event since the late 1500s megadrought.
The present event has been associated with regional-scale declines in
canopy greenness and tree survival, due in part to large bark-beetle
outbreaks and increasingly large wildfires. On the basis of an
ensemble of climate-model projections, continued increases in the
warm-season VPD will by the mid-twenty-first century force mean
annual SWUS forest drought-stress levels to exceed the severity of
the strongest megadroughts since at least ad 1000. Importantly,
the warm-season VPD is largely driven by the maximum daily
temperature (Tmax) in the SWUS. As such, warm-season Tmax
is nearly as effective as the warm-season VPD at predicting the
FDSI (Supplementary Fig. S10). The importance of the VPD and
temperature in dictating the FDSI, combined with the relatively
high confidence in the projections of continued warming in the
SWUS (ref. 45), translates into a high confidence in projections
of intensified forest drought-stress. The strong correspondence
between forest drought-stress and tree mortality suggests that
intensified drought-stress will be accompanied by increased forest
decline. Importantly, human forest-management practices have
profoundly influenced the regional wildfire regime over the past
century46 and future practices will continue to influence the impacts
of drought on wildfire behaviour. Furthermore, there are many
complex interactions not accounted for in this study, including in-
teractions between disturbance processes29. We therefore constrain
our quantitative projections to the FDSI and do not forecast abso-
lute magnitudes of forest area impacted by bark beetles or wildfire.
The implications of this study extend beyond the well-studied
SWUS region. Given that the ongoing SWUS drought event is prob-
ably a product of both natural and anthropogenic forcing47, it serves
as a natural experiment where the recent forest response to drought
may serve as a harbinger of how drought-sensitive forests globally
may respond to warming1, with implications regarding terrestrial
carbon budgets10. Model projections48,49 and observations50 of a
poleward expansion of the subtropics indicate that forests near the
poleward edges of the subtropics may be particularly vulnerable
to enhanced drought-induced tree mortality. This study indicates
that if warming continues, increasing VPD and drought stress will
continue to cause twenty-first-century readjustments to the SWUS
forest structure, composition and distribution through amplified
disturbance processes that have become increasingly evident
regionally in recent decades (for example, Supplementary Fig. S9).
Given the reproductive and dispersal limitations of dominant native
tree species, climate-driven amplification of forest drought-stress
and associated disturbance processes can be expected to force
many landscapes in the SWUS and probably elsewhere towards
vegetation-type conversions, with species distributions quite
different from those familiar to modern civilization.
Received 21 March 2012; accepted 28 August 2012;
published online 30 September 2012
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The work was supported by LANL-LDRD and DOE-BER. We acknowledge contributors
to the International Tree-Ring Databank and funding by the NSF (grant 0823090) for
tree-ring data. We thank contributors of fire-scar data to the FACS database, accessed
with assistance from E. Bigio. Unpublished fire-scar data donated by C. Aoki, P. Brown,
E. Heyerdahl, P. Iniguez, M. Kaib and R. Wu. J. Paschke provided access to USFS
FHTET data. M. Brown provided access to GIMMS AVHRR NDVI data. Dynamically
downscaled model climate data came from NARCCAP, funded by NSF, DOE, NOAA
and EPA. We appreciate constructive comments from P. Brown, K. Cavanaugh,
M. Crimmins, P. Fulé, S. Garrity, J. Grahame, D. Gutzler, J. Hicke, X. Jiang, S. Leavitt,
M. Massenkoff, A. Meddens, J. Michaelsen, C. Millar, B. Osborn, H. Powers, T. Rahn,
N. Stephenson, C. Still, C. Tague and C. Xu.
Author contributions
A.P.W., C.D.A., A.K.M., D.G., C.A.W., D.M.M., T.W.S., S.A.R., R.S., M.C. and N.G.M.
conceived and designed the experiments. A.P.W. performed the experiments. A.P.W.
and E.R.C. analysed the data. A.K.M., D.G., C.A.W., C.G., D.M.M., T.W.S., S.A.R.,
H.D.G-M., J.S.D. and E.R.C. contributed data. A.P.W., C.D.A., A.K.M., D.G., C.A.W.,
D.M.M., T.W.S., S.A.R., R.S., H.D.G-M. and N.G.M. wrote the paper.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at
Correspondence and requests for materials should be addressed to A.P.W.
Competing financial interests
The authors declare no competing financial interests.
© 2012 Macmillan Publishers Limited. All rights reserved.
... These mortality events have corresponded with periods of severe heat, leading to the hypothesis that rising temperature exacerbates impacts associated with elevated hypoxia and salinity (Allen et al., 2021;Lovelock et al., 2017). This is consistent with observations and predictions from upland forests, which suggest that rising temperature, and in particular rising vapor pressure deficit (VPD), is driving increasing tree mortality Williams et al., 2013). ...
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Observations of woody plant mortality in coastal ecosystems are globally widespread, but the overarching processes and underlying mechanisms are poorly understood. This knowledge deficiency, combined with rapidly changing water levels, storm surges, atmospheric CO 2 , and vapor pressure deficit, creates large predictive uncertainty regarding how coastal ecosystems will respond to global change. Here, we synthesize the literature on the mechanisms that underlie coastal woody-plant mortality, with the goal of producing a testable hypothesis framework. The key emergent mechanisms underlying mortality include hypoxic, osmotic, and ionic-driven reductions in whole-plant hydraulic conductance and photosynthesis that ultimately drive the coupled processes of hydraulic failure and carbon starvation. The relative importance of these processes in driving mortality, their order of progression, and their degree of coupling depends on the characteristics of the anomalous water exposure, on topo-graphic effects, and on taxa-specific variation in traits and trait acclimation. Greater inundation exposure could accelerate mortality globally; however, the interaction of changing inundation exposure with elevated CO 2 , drought, and rising vapor pressure deficit could influence mortality likelihood. Models of coastal forests that incorporate the frequency and duration of inundation, the role of climatic drivers, and the processes of hydraulic failure and carbon starvation can yield improved estimates of inundation-induced woody-plant mortality. K E Y W O R D S carbon starvation, coastal, climate change, hydraulic failure, hypoxia, mortality, salinity, sea level rise This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
... There is also evidence to suggest some climate changes that are expected to support tree growth, for example through increased CO 2 fertilization and light exposure, can actually lead to mortality, for example, when increased growth leads to greater competition for resources (Huete et al., 2006;McDowell et al., 2018;Saleska et al., 2007). Climatic effects and contributions to tree mortality remain far less understood than effects on other forest processes, such as productivity and growth (Neumann et al., 2017;Park Williams et al., 2013). ...
Tropical forests in India have declined at an alarming rate over the past century, with extensive literature focusing on the high contributions of agricultural expansions to deforestation, while the effects of climate change have largely been overlooked. Climate change effects, such as increasing temperatures, drought and flooding, have already occurred, and are projected to worsen. Climate velocity, a metric that accounts for spatial heterogeneity in climate, can help identify contiguous areas under greater climate stress and potential climate refuges in addition to traditional temporal trends. Here, we examined the relative contribution of climate changes to forest loss within an area of India, Jammu, Kashmir, and Ladakh during the period 2001-2018, at two spatial (regional and study area) and two temporal (seasonal and annual) scales. This includes, for the first time, a characterization of climate velocity in the area. Our findings show that annual forest loss increased substantially over the 17-year period examined (2001-2018), with the majority of forest loss occurring in the Northeast region. Decreases in temporal trends of temperature and precipitation were most associated with forest losses, but there was large spatial and seasonal variation in the relationship. In every region except the Northeast, forest losses were correlated with faster velocities of at least one climate variable but overlapping areas of high velocities were rare. Our findings indicate that climate changes have played an important role in past forest loss within the study area but likely remain secondary to other factors at present. We stress concern for climates velocities recorded reaching 97 km year −1 , and highlight that understanding the different regional and seasonal relationships between climatic conditions and forest distributions will be key to effective protection of the area's remaining forests as climate change accelerates. K E Y W O R D S climate velocity, forest loss, precipitation, regional, seasonal variation, temperature
... Drought is one of the most deleterious abiotic stresses in agricultural production, posing a threat to the global food supply (Gamalero and Glick, 2012;Vurukonda et al., 2016;Gupta et al., 2022;Singhal et al., 2022). If this condition continues, it is anticipated that 30% of the world's water supplies will be depleted, and the number of regions experiencing drought will more than double by the year 2050 (Van Dijk et al., 2013;Williams et al., 2013;Van Loon et al., 2016). According to the available literature, arid and semi-arid regions would experience frequent hot droughts, which will have an impact on agricultural productivity (Yandigeri et al., 2012;Hessl et al., 2018). ...
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Globally, climate change could hinder future food security that concurrently implies the importance of investigating drought stress and genotype screening under stressed environments. Hence, the current study was performed to screen 45 diverse maize inbred lines for 18 studied traits comprising phenological, physiological, morphological, and yield characters under optimum and water stress conditions for two successive growing seasons (2018 and 2019). The results showed that growing seasons and water regimes significantly influenced (p < 0.01) most of the studied traits, while inbred lines had a significant effect (p < 0.01) on all of the studied traits. The findings also showed a significant increase in all studied characters under normal conditions compared to drought conditions, except chlorophyll content, transpiration rate, and proline content which exhibited higher levels under water stress conditions. Furthermore, the results of the principal component analysis indicated a notable distinction between the performance of the 45 maize inbred lines under normal and drought conditions. In terms of grain yield, the drought tolerance index (DTI) showed that Nub60 (1.56), followed by Nub32 (1.46), Nub66 (1.45), and GZ603 (1.44) were the highest drought-tolerant inbred lines, whereas Nub46 (0.38) was the lowest drought-tolerant inbred line. These drought-tolerant inbred lines were able to maintain a relatively high grain yield under normal and stress conditions, whereas those Frontiers in Plant Science 01 Balbaa et al. 10.3389/fpls.2022.959203 drought-sensitive inbred lines showed a decline in grain yield when exposed to drought conditions. The hierarchical clustering analysis based on DTI classified the forty-five maize inbred lines and eighteen measured traits into three column-and row-clusters, as inbred lines in cluster-3 followed by those in cluster-2 exhibited greater drought tolerance in most of the studied traits. Utilizing the multi-trait stability index (MTSI) criterion in this study identified nine inbred lines, including GZ603, as stable genotypes in terms of the eighteen studied traits across four environments. The findings of the current investigation motivate plant breeders to explore the genetic potential of the current maize germplasm, especially in water-stressed environments.
... The increased tension on the water column creates a risk of xylem conduit embolism or implosion that intensifies with drought severity and ultimately may lead to tree dieback after starving the canopy of water (Choat et al., 2018). At the ecosystem level, drought stressed forests have already seen severe tree mortality (Park Williams et al., 2013). Moreover, mass tree mortality events have not been restricted to arid ecosystems and are cause for global concern (Carnicer et al., 2011;Anderegg et al., 2012;Asner et al., 2016;Moore et al., 2016;Duke et al., 2017). ...
Red oaks (Quercus sect. Lobatae) are a taxonomic group of hardwood trees which occur in swamp forests, subtropical chaparral, and savannahs from Columbia to Canada. They cover a wide range of ecological niches, and many species are thought to be able to cope with current trends in climate change. Genus Quercus encompasses ca. 500 species, of which ca. 80 make up sect. Lobatae. Species diversity is greatest within the southeastern United States and within the northern and eastern regions of Mexico. This review discusses the weak reproductive barriers between species of red oaks and the effects this has on speciation and niche range. Distribution and diversity have been shaped by drought adaptations common to the species of sect. Lobatae, which enable them to fill various xeric niches across the continent. Drought adaptive traits of this taxonomic group include deciduousness, deep tap roots, ring-porous xylem, regenerative stump sprouting, greater leaf thickness, and smaller stomata. The complex interplay between these anatomical and morphological traits have given red oaks features of drought tolerance and avoidance. Here, we discuss physiological and genetic components of these adaptations to address how many species of sect. Lobatae reside within xeric sites and/or sustain normal metabolic function during drought. Although extensive drought adaptation appears to give sect. Lobatae a resilience to climate change, aging tree stands, oak life history traits, and the current genetic structures places many red oak species at risk. Furthermore, oak decline, a complex interaction between abiotic and biotic agents, has severe effects on red oaks, and is likely to accelerate species decline and fragmentation. We suggest that assisted migration can be used to avoid species fragmentation and increase climate change resilience of sect. Lobatae.
... With this variation, the tipping point for the natural regeneration of some species could be passed, something that may already have happened in some locations. Projections in the US show that if the VPD continues to increase as projected by climate models, forest drought stress in the 2050s will exceed that of the most severe droughts in the last 1000 years [48]. The species most affected by this increase in summer temperatures are those located at their ecological limit [9,30]. ...
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The Mediterranean climate has dry and hot summers, which is harsh for plants, especially seedlings. During the 1950s and 1960s, most reforestations carried out in Central Spain, a Mediterranean climate area, were successful, but in recent decades an increasing difficulty in forest regeneration has been observed, often attributed to increased summer drought. This study analyses changes in climatic parameters related to forest regeneration through statistical treatment of meteorological data series from the mid-twentieth century to the present. Simple and multiple regressions and ANOVAs were performed for five parameters, considering annual, summer and extended summer values. Rainfall reduction and prolongation of the summer drought period were not statistically significant. The change that better explains regeneration problems is the increase in temperature, especially in July and August, which was mostly significant between 2002 and 2021. Raising temperatures increase the vapor pressure deficit, exacerbating drought effects and plant mortality. Climate change scenarios point to an increase in temperatures until 2100; thus, the tipping point for natural regeneration of some species could be passed. The most affected species are those at their ecological limit. It is necessary to facilitate the adaptation of these forests to climate change, since their future will depend on the actions carried out today.
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Climate change is altering the distribution of woody plants by influencing demographic processes and modifying disturbance regimes. Trailing-edge forests may be particularly vulnerable to these effects because they exist at warm, dry margins of tree distributions. To better understand recent climate-driven changes in trailing-edge forests, we used Landsat time series and 1558 field reference plots to develop annual land cover maps from 1985 to 2020 in two large, biodiverse landscapes in central Arizona, USA. We then combined annual land cover maps with tree ring records and spatial data describing interannual climate, terrain, bark beetle (Curculionidae: Scolytinae) activity, wildfire, and harvest to quantify drivers of forest change. Throughout the two landscapes, forest extent declined by 0.3 % and 0.8 % from 1985 to 2020. However, considerable variation occurred within the study period, with abrupt (ca. 1–2 years) declines in forest extent followed by gradual (ca. 10 years) recovery on each landscape. Pinyon-juniper (Pinus edulis, Pinus monophylla, and/or Juniperus spp.) cover increased from 1985 to ca. 2000 but declined after 2000, a period of extreme drought and regional tree die-off. In contrast, pine-oak (Pinus ponderosa and Quercus spp.) cover increased from 2000 to 2020, primarily due to declines in ponderosa pine and mixed conifer cover over the same period. Wildfire was a key driver of transitions from forest to non-forest cover in our study area, with the occurrence of multiple compounded drought years playing an important role in unburned areas. By driving transitions to alternative forest types or non-forest cover, disturbance and drought will increasingly shape forest dynamics and ecosystem transformations throughout the southwestern US.
Grassland, the most widespread vegetation type in China, has been greening recently.However, the extent to which the greenness has been translated into productivity and the underlying mechanism of the decoupled grassland greenness and productivity remains unclear. In this study, we detected the trend of normalized difference vegetation index (NDVI) and gross primary productivity (GPP) of grassland in China from 2000 to 2019 and analyzed the driving mechanism of the inconsistency between them. It was found that the relative increase rate of productivity (27.27%, p < 0.05) was much greater than that of greenness (14.54%, p < 0.05) across grasslands in China from 2000 to 2019, especially in temperate regions. The temperature and precipitation were the main factors influencing the grassland growth change, and the impact of temperature and shortwave radiation on productivity was greater than on greenness. However, the increase of grassland greenness was not fully translated into productivity in subtropical and tropical grass as well as shrub. This study revealed the dominance of climatic factors in the translation process from ecosystem structure to function, which highlighted the challenge in enhancing carbon uptake capacity of terrestrial ecosystem facing accelerated climate change.
Background Mountain pine beetle (MPB) is a native disturbance agent across most pine forests in the western US. Climate changes will directly and indirectly impact frequencies and severities of MPB outbreaks, which can then alter fuel characteristics and wildland fire dynamics via changes in stand structure and composition. To investigate the importance of MPB to past and future landscape dynamics, we used the mechanistic, spatially explicit ecosystem process model FireBGCv2 to quantify interactions among climate, MPB, wildfire, fire suppression, and fuel management under historical and projected future climates for three western US landscapes. We compared simulated FireBGCv2 output from three MPB modules (none, simple empirical, and complex mechanistic) using three focus variables and six exploratory variables to evaluate the importance of MPB to landscape dynamics. Results We found that inclusion of MPB (empirical or mechanistic) in the simulations significantly changed past and future landscape dynamics and that the mechanistic MPB module had more cross-scale interactions that increased variability, and perhaps realism, of simulation results. We also evaluated impacts of fire and fuel management on MPB dynamics and found that fire suppression influenced fuel loadings more than MPB disturbance, but at a landscape scale, most fuel treatment programs did little to change fuel loadings, MPB dynamics, and burned area, except under high fire suppression. Conclusions Synergistic interactions of climate, MPB, and wildfire catalyzed landscape-scale changes in vegetation distributions, fuels, and fire regimes in FireBGCv2 simulations. Models that simulate climate change on pine-dominated landscapes may be improved by including mechanistic MPB simulations to account for potentially important ecological interactions.
Worldwide forests are being impacted by several components of global change such as climate warming, increased aridification, land-use changes modifying regimes of disturbances, increased carbon and nitrogen availability, and new pests which are often invasive species. In many cases, forests are responding to global change by showing a loss of vitality manifested as increased canopy defoliation and dieback, rising mortality rates, productivity loss, and growth decline. Forest health is a multidimensional and complex concept difficult to quantify but existing tree defoliation and mortality networks offer an opportunity to define and characterize dieback hotspots: sites or stands in which tree mortality surpasses historical background mortality rates. We present a study case on how drought-triggered forest dieback hotspots could be detected by using defoliation and mortality in European stands of Scots pine. The proposed framework allows deepening into the understanding of forest health by disentangling the spatiotemporal components of dieback hotspots.
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The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming. Analyses of observational and reanalysis data suggest that the strength of the western North American carbon sink declined by 30-298[thinsp]Tg carbon per year during the drought at the turn of the century.
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Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 × 1015 to 1.6 × 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.
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Integrating the potential for climate change impacts into policy and planning decisions requires quantification of the emergence of sub-regional climate changes that could occur in response to transient changes in global radiative forcing. Here we report results from a high-resolution, century-scale, ensemble simulation of climate in the United States, forced by atmospheric constituent concentrations from the Special Report on Emissions Scenarios (SRES) A1B scenario. We find that 21(st) century summer warming permanently emerges beyond the baseline decadal-scale variability prior to 2020 over most areas of the continental U.S. Permanent emergence beyond the baseline annual-scale variability shows much greater spatial heterogeneity, with emergence occurring prior to 2030 over areas of the southwestern U.S., but not prior to the end of the 21(st) century over much of the southcentral and southeastern U.S. The pattern of emergence of robust summer warming contrasts with the pattern of summer warming magnitude, which is greatest over the central U.S. and smallest over the western U.S. In addition to stronger warming, the central U.S. also exhibits stronger coupling of changes in surface air temperature, precipitation, and moisture and energy fluxes, along with changes in atmospheric circulation towards increased anticylonic anomalies in the mid-troposphere and a poleward shift in the mid-latitude jet aloft. However, as a fraction of the baseline variability, the transient warming over the central U.S. is smaller than the warming over the southwestern or northeastern U.S., delaying the emergence of the warming signal over the central U.S. Our comparisons with observations and the Coupled Model Intercomparison Project Phase 3 (CMIP3) ensemble of global climate model experiments suggest that near-term global warming is likely to cause robust sub-regional-scale warming over areas that exhibit relatively little baseline variability. In contrast, where there is greater variability in the baseline climate dynamics, there can be greater variability in the response to elevated greenhouse forcing, decreasing the robustness of the transient warming signal.
To preserve multi-centennial length variability in annual tree-ring chronologies, the Regional Curve Standardization (RCS) method calculates anomalies from a regionally common, non-climatic age-trend function. The influence of various factors on the estimation of the regional curve (RC) and resulting RCS-chronology is discussed. These factors are: the method of calculating anomalies from the age-trend function, estimation of the true pith offset, the number of series used, species composition, and site characteristics. By applying RCS to a collection of millennium-length tree-ring data sets, the potential and limitations of the RCS method are investigated. RCS is found to be reasonably robust with respect to tested factors, suggesting the method is a suitable tool for preserving low-frequency variance in long tree-ring chronologies.
Cross-validation is a statistical procedure that produces an estimate of forecast skill which is less biased than the usual hindcast skill estimates. The cross-validation method systematically deletes one or more cases in a dataset, derives a forecast model from the remaining cases, and tests it on the deleted case or cases. The procedure is nonparametric and can be applied to any automated model building technique. It can also provide important diagnostic information about influential cases in the dataset and the stability of the model. Two experiments were conducted using cross-validation to estimate forecast skill in different predictive models of North Pacific sea surface temperatures (SSTs). The results indicate that bias, or artificial predictability (defined here as the difference between the usual hindcast skill and the forecast skill estimated by cross-validation), increases with each decision-either screening of potential predictors or fixing the value of a coefficient-drawn from the data. Bias introduced by variable screening depends on the size of the pool of potential predictors, while bias produced by fitting coefficients depends on the number of coefficients. The results also indicate that winter SSTs are predictable with a skill of about 20%-25%. Several models were compared. More flexible ones which allow the data to guide the selection of variables generally show poorer skill than the relatively inflexible models where a priori variable selection is used. The cross-validation estimates of artificial skill were compared with estimates derived from other methods. Davis and Chelton's method showed close agreement with the cross-validation results for a priori models. However, Monte Carlo estimates and cross-validation estimates do not agree well in the case of predictor screening models. The results of this study indicate that the amount of artificial skill depends on the amount of true skill, so Monte Carlo techniques which assume no true skill cannot be expected to perform well when there is some true skill.
1 To whom correspondence should be addressed. E-mail: {at}; For Ecol Man 254:390–406. CrossRef. ↵: Lawler ,; et al. ,; Huettman F,; Moritz C,; Peterson AT. (2004) New developments in museum-based informatics and applications in biodiversity analysis.