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8/3/2019 Pervasive Global Warming Changes in Western Forests
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Pervasive climate-
mediated changes
in western forests
N. Stephenson, J. Littell,
P. van Mantgem, D. Peterson,
and D. McKenzie
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Part 1: Defining the issues and setting the stage
Part 2: FOREST DEMOGRAPHY:
Ongoing changes
Possible future changes
Part 3: FOREST GROWTH:
Ongoing changes Possible future changes
Part 4: A call to action
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DEFINING THE ISSUES
AND SETTING THE STAGE
Climate
Fire Forest
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Climate
Fire Forest
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Climate
Fire Forest
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Credit: USFS Credit: BC Ministry of Forests and Range
Already, climate has been linked to episodes of
broad-scale forest die-back in the mountainous West
Drought Warming(e.g., southern Calif.) (e.g., British Columbia)
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But whats happening and likely to happen
in the bulk of mountain forests in the West,
which have not experienced extensive die-back?
Credit: Nate Stephenson
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Why care?
Recent studies of
healthy forests in theAmerican tropics show
that substantial
directional changes arein progress, with
potentially profound
consequences.
Credit: Nate Stephenson
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Tropical forest COMPOSITION is changing
(e.g., lianas [woody vines] are increasing)
Credit: Yadvinder Malhi
Phillips et al., Nature, 2002
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Tropical forest DYNAMICS are changing
(e.g., recruitment, growth, and mortality rates are increasing)
Phillips et al., Phil. Trans. B, 2004
Recruitment
Mortality
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Tropical forest STRUCTURE AND FUNCTION are changing
(e.g., aboveground biomass, hence C storage,
may be increasing)
Lewis et al., Phil. Trans. B, 2004
Basal area gainBasal area loss
Difference
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Are similar changes underway in our
Western forests?
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Are similar changes underway in our
Western forests?
We simply dont know.
No one has been doing thenecessary systematic analyses.
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Our questions:
In the bulk of mountain forests in the West (whichhave not experienced extensive die-back):
Are climatically-driven changes in progress? What might we expect for the future?
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Our questions:
In the bulk of mountain forests in the West (whichhave not experienced extensive die-back):
Are climatically-driven changes in progress? What might we expect for the future?
Our approach:Here, we will address each question separately for:
Forest demography Forest growth
DEMOGRAPHY d t i NUMBERS f t
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DEMOGRAPHY determines NUMBERS of trees
(birth, natality, recruitment & death, mortality)
GROWTH determines SIZES of trees
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GROWTH determines SIZES of trees
TOGETHER demography and growth rates give us a forest
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TOGETHER, demography and growth rates give us a forest
(structure, composition, productivity, and dynamics)
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DEMOGRAPHY:
ongoing changes Data from dozens of permanent forest plots
show that over the last few decades, in the
otherwise undisturbed old-growth forests of
California, Oregon, and Washington,
tree mortality rates have been increasing. However, unlike the tropics,
recruitment rates have NOT been increasing.
Maximum snow water content
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Spring-pulse dates
Stewart et al., 2004
Possible cause:
Summers are getting
longer and drier.
Snowpack has been
decreasing over most ofthe West in recent
decades
and spring
streamflow has beenarriving earlier.
Maximum snow water content
Mote et al., BAMS, 2005
E id f C lif i Si N d
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Evidence from Californias Sierra Nevada: Summer drought (water deficit) is increasing, due to
increasing temperature (notdecreasing precipitation). Increasing tree mortality rates are being driven by
increasing deaths due to insects, pathogens, and stress.
Year
1985 1990 1995 2000 2005
Mo
rtalityrate(%yr-1)
0.0
0.5
1.0
1.5
2.0
2.5
Summ
erwaterdeficit(mm)
160
200
240
280
(3
-yrrunning
mean)
van Mantgem & Stephenson, in prep.
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DEMOGRAPHY:
possible futures
Some water-limited forests may be primed for
a southwestern-style die-back
Credit: Craig Allen & NSF
The recent southwestern drought was not exceptional (it was
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The recent southwestern drought was not exceptional (it was
wetter than the1950s drought), but the temperature was higher
Annual
precipitation(mm) A
nnualtempera
ture(C)
Year Breshears et al., PNAS, 2005
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But what about forests that are notprimed for asimilar die-back? Specifically,
Forests that are currently temperature-limited, not water-
limited (e.g., high-elevation forests, coastal rain
forests). Forests that may currently be water-limited, but that will
experience substantially increased precipitation.
We can get hints from natural productivity gradients.
Globally forests of productive environments have higher
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0
1
2
3
4
550 46 30 27 27
Riche
rsoils
Poorersoils
Angio
sperm
Gym
nosperm
Mix
ed
Tropical Temperate(Amazonia) (global)
Forestturnover(%y
r-1)
Forestturnover(%y
r-1)
0
1
2
3
4
5158 84
Tropical
Tem
perate
Temperate
only
Stephenson & van Mantgem, Ecol. Lett., 2005
Globally, forests of productive environments have higher
turnover rates (mortality and recruitment) ...
... at least partly because environments that favor
tree growth also favor the organisms that kill trees.
Consequences?
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Elevation (m)
1500 2000 2500 3000 3500
Forestturnov
er(%yr-1)
0
1
2
3
y= 2.76 - 0.00066x
r2
= 0.49, P< 0.001
Consequences?In the coniferous forests of the Sierra Nevada:
a 4
C increase in temperature is associated with a0.5 %yr-1 increase in population turnover rate,
potentially reducing average tree age by one third.
Stephenson & van Mantgem, Ecol. Lett., 2005
A ibl f i
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A possible future scenario ...
Benign climatic changes (e.g., warmer and wetter)
Increased forest turnover rates
Smaller, younger trees
Cascading effects on
wildlife and biodiversity
Reduced forest
carbon storage
A ibl f t i
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A possible future scenario ...
Benign climatic changes (e.g., warmer and wetter)
Increased forest turnover rates
Smaller, younger trees
Cascading effects on
wildlife and biodiversity
Reduced forest
carbon storage
... but what about changing growth rates?
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Tree and Forest Growth
Compared to biogeography, we know relatively little
about long-term, broad-scale climatic controls on life-history processes of trees
Especially true in non-plantation, mountainecosystem settings
Growth is an indicator of environmental factorsinfluencing species and may be a surrogate for
establishment.
GROWTH: Ongoing Changes 1850 1980
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GROWTH: Ongoing Changes, 1850-1980
Factor 1= 27 Chronologies w/ high loadings Factor 2 = 12 Chronologies w/ high loadings
185 Tree-ring chronologies, traditional detrending, factor analysis
Factor 1 Factor 2
Factor 1 = Drought-sensitive tree-ring collections
Factor 2 = High elevation, maritime, high-latitude tree-ring collections
~ 146 chronologies dont load highly on either continental pattern
McKenzie et al. 2001. Can. J. For. Res. 31: 526538. Recent growth of conifer
species of western North America: assessing spatial patterns of radial
growth trends
GROWTH: Ongoing Changes Are Location and
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Species Dependent
High Lat. &
Low Elev.
Low Lat. &
High Elev.
These patterns point to three kinds of growth limitation by climate:
Water limitation
Energy limitation
Some combination of water and energy
These patterns primarily represent
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sites dendroclimatologists would
choose.
What about the rest of the forests
in the West?
How do we go from
reconstruction-grade sites that tell
us about the most sensitive trees
to more mechanistic responses
that allow inferences for large
areas of forests?
GROWTH:
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GROWTH:
Within a species range across biogeographical
space, climate impacts depend on elevation
PDO
Peterson and Peterson. 2001. Ecology 82: 3330-3345. Mountain hemlock
growth responds to climatic variability at annual and decadal time scales.
GROWTH:
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GROWTH:
Across forest types and species within a mountain
range, climate impacts depend on physiography
Nakawatase and Peterson. 2006. Can. J. For. Res. 36: 77-91. Spatial
variability in forest growth climate relationships in the Olympic Mountains,
Washington.
GROWTH:
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GROWTH:
Within a watershed, for the same species,
elevation affects growth-climate relationships
Low elevation vs. Max. Sum. T
High elevation vs. Prior PDOCase and Peterson. 2005. Can. J. For. Res. 35. Fine-scale variability in
growthclimate relationships of Douglas-fir, North Cascade Range,
Washington.
Growth-limiting factors are not really elevation,
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g y ,
latitude, physiography, or even biotic.
These are all surrogates for different scales of
climatic (water or energy) limitation, and point to the
need forMULTI-SCALE, GRADIENT-BASED
studies of climatic limitation of growth
170cm219cm 72 cm411 cm = ANN PPT
Thornton North NCNP Bell River South GNPQuinault North ONP Robinson South IPNF
Climate Change
L l li t
Highest
Elevation
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Climate Variability
Local climate
North South
v
Lowest Elevation
TopographyPhysiography
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Climate Dimensions of the PSME Transect
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Climate Dimensions of the PSME Transect
Mean Climate Data: DAYMET 1981-1997Climatic Niche Dimensions: Thompson et al. 2001
ONP
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StandardChronology(mod.Z
index)
NCNP
IPNF
GNP
Within each park, the
variability in tree-growth is similar
across low, middle,
and high elevations.
Cli t G th C l ti
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Climate-Growth Correlations:
TemperatureVIC Climate
Climate-Growth Relationships:
H d l i l V i bl
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Hydrological Variables
Divisional Climate
Summary: Growth-Climate Relationships
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Bonsai PSME, Saint Mary, Glacier National Park
Most frequent patterns of
correlations point to
combined influence of(-)
temperature and (+)
precipitation during summer
Underscored by PDSI (+)
and water balance deficit (-),
esp. in IPNF and GNP.
Some cool season (+) temp.
and (-) snow relationships,
primarily in ONP and NCNP.
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The magnitude of the correlation between seasonal
hydrological variables and tree-growth depends on the
position of the plot along a gradient of surplus water in theenvironment.
GROWTH: Possible Futures
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Depends on species, climate regime, andchanges in water vs. energy. If we had
results for most western conifers, we could
estimate responses. But we dont. Yet.
McCabe and Wolock. 2002.
Clim. Res. 20: 1929, 2002
A CALL TO ACTION
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We do a pretty good job of monitoring weather, snow, and hydrology.
We need a complementary network of forest gauging stations.
This network of forest gauging stations will have
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two primary goals:
(1) Change detection
(complementary to
remote sensing)
(2) Developing a
mechanistic
understanding
(otherwise we
are lost)
Mo
rtalityrate
Sitep
roductivi
tyIndividualtre
e
growthrate
Low
High Low
High
Stephenson et al., in prep.
Such a network is taking shape in CORFOR
(the Cordillera Forest Dynamics Network)
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(the Cordillera Forest Dynamics Network)
http://mri.scnatweb.ch/content/view/88/30/
PLEASE JOIN US!(... if you have the right kind of data.)