Pervasive climatemediated changes
in western forests
N. Stephenson, J. Littell,
P. van Mantgem, D. Peterson,
and D. McKenzie
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
DEFINING THE ISSUES
AND SETTING THE STAGE
Climate
Fire
Forest
Climate
Fire
Forest
Climate
Fire
Forest
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)
Credit: USFS
Credit: BC Ministry of Forests and Range
But what’s happening – and likely to happen –
in the bulk of mountain forests in the West,
which have not experienced extensive die-back?
Credit: Nate Stephenson
Why care?
Recent studies of
“healthy” forests in the
American tropics show
that substantial
directional changes are
in progress, with
potentially profound
consequences.
Credit: Nate Stephenson
Tropical forest COMPOSITION is changing
(e.g., lianas [woody vines] are increasing)
Credit: Yadvinder Malhi
Phillips et al., Nature, 2002
Tropical forest DYNAMICS are changing
(e.g., recruitment, growth, and mortality rates are increasing)
Recruitment
Mortality
Phillips et al., Phil. Trans. B, 2004
Tropical forest STRUCTURE AND FUNCTION are changing
(e.g., aboveground biomass, hence C storage,
may be increasing)
Basal area gain
Basal area loss
Difference
Lewis et al., Phil. Trans. B, 2004
Are similar changes underway in our
Western forests?
Are similar changes underway in our
Western forests?
We simply don’t know.
No one has been doing the
necessary systematic analyses.
Our questions:
In the bulk of mountain forests in the West (which
have not experienced extensive die-back):
• Are climatically-driven changes in progress?
• What might we expect for the future?
Our questions:
In the bulk of mountain forests in the West (which
have 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 determines NUMBERS of trees
(birth, natality, recruitment & death, mortality)
GROWTH determines SIZES of trees
TOGETHER, demography and growth rates give us a forest
(structure, composition, productivity, and dynamics)
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.
Possible cause:
Maximum snow water content
Summers are getting
longer and drier.
Snowpack has been
decreasing over most of
the West in recent
decades …
Mote et al., BAMS, 2005
… and spring
streamflow has been
arriving earlier.
Spring-pulse dates
Stewart et al., 2004
Evidence from California’s Sierra Nevada:
Mortality rate (% yr-1)
2.5
280
2.0
1.5
240
1.0
200
0.5
160
0.0
1985
1990
1995
2000
Summer water deficit (mm)
(3-yr running mean)
• Summer drought (water deficit) is increasing, due to
increasing temperature (not decreasing precipitation).
• Increasing tree mortality rates are being driven by
increasing deaths due to insects, pathogens, and stress.
2005
Year
van Mantgem & Stephenson, in prep.
DEMOGRAPHY:
possible futures
Some water-limited forests may be primed for
a southwestern-style die-back
Credit: Craig Allen & NSF
Annual temperature (˚C)
Annual precipitation (mm)
The recent southwestern drought was not exceptional (it was
wetter than the1950s drought), but the temperature was higher
Year
Breshears et al., PNAS, 2005
But what about forests that are not primed for a
similar die-back? Specifically,
• Forests that are currently temperature-limited, not waterlimited (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
turnover rates (mortality and recruitment) ...
• ... at least partly because environments that favor
tree growth also favor the organisms that kill trees.
5
-1
3
2
1
Tropical
(Amazonia)
Gy
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os
pe
xe
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rm
0
Mi
0
4
e rm
1
27
27
Temperate
only
sp
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T
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rs
o il
s
PToem
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oitle
s
4
158 50
An
Forest turnover (% yr )
Forest turnover (% yr-1)
5
Stephenson & van Mantgem, Ecol. Lett., 2005
Temperate
(global)
Consequences?
3
y = 2.76 - 0.00066 x
-1
Forest turnover (% yr )
In the coniferous forests of the Sierra Nevada:
a 4○ C increase in temperature is associated with a
0.5 %yr -1 increase in population turnover rate,
potentially reducing average tree age by one third.
r 2 = 0.49, P < 0.001
2
1
0
1500
2000
2500
3000
3500
Elevation (m)
Stephenson & van Mantgem, Ecol. Lett., 2005
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 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?
Tree and Forest Growth
•
Compared to biogeography, we know relatively little
about long-term, broad-scale climatic controls on lifehistory processes of trees
•
Especially true in non-plantation, mountain
ecosystem settings
•
Growth is an indicator of environmental factors
influencing species and may be a surrogate for
establishment.
GROWTH: Ongoing Changes, 1850-1980
Factor 1
Factor 1= 27 Chronologies w/ high loadings
Factor 2
Factor 2 = 12 Chronologies w/ high loadings
185 Tree-ring chronologies, traditional detrending, factor analysis
Factor 1 = Drought-sensitive tree-ring collections
Factor 2 = High elevation, maritime, high-latitude tree-ring collections
~ 146 chronologies don’t load highly on either continental pattern
McKenzie et al. 2001. Can. J. For. Res. 31: 526–538. Recent growth of conifer
species of western North America: assessing spatial patterns of radial
growth trends
GROWTH: Ongoing Changes Are Location and
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
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:
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:
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:
Within a watershed, for the same species,
elevation affects growth-climate relationships
Low elevation vs. Max. Sum. T
High elevation vs. Prior PDO
Case and Peterson. 2005. Can. J. For. Res. 35. Fine-scale variability in
growth–climate relationships of Douglas-fir, North Cascade Range,
Washington.
Growth-limiting factors are not really elevation,
latitude, physiography, or even biotic.
These are all surrogates for different scales of
climatic (water or energy) limitation, and point to the
need for
MULTI-SCALE, GRADIENT-BASED
studies of climatic limitation of growth
411 cm = ANN PPT
Quinault North (ONP)
219cm
Thornton North (NCNP)
72 cm
Robinson South (IPNF)
170cm
Belly River South (GNP)
Highest
Elevation
Climate Change
Local climate
Climate Variability Physiography
North
Topography
Lowest Elevation
South
Climate Dimensions of the PSME Transect
Mean Climate Data: DAYMET 1981-1997
Climatic Niche Dimensions: Thompson et al. 2001
ONP
Standard Chronology (mod. Z index)
NCNP
IPNF
GNP
Within each park, the
variability in treegrowth is similar
across low, middle,
and high elevations.
Climate-Growth Correlations:
Temperature
VIC Climate
Climate-Growth Relationships:
Hydrological Variables
Divisional Climate
Summary: Growth-Climate Relationships
•
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.
Bonsai PSME, Saint Mary, Glacier National Park
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 the
environment.
GROWTH: Possible Futures
Depends on species, climate regime, and
changes in water vs. energy. If we had
results for most western conifers, we could
estimate responses. But we don’t. Yet.
McCabe and Wolock. 2002.
Clim. Res. 20: 19–29, 2002
A CALL TO ACTION
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
two primary goals:
(2) Developing a
mechanistic
understanding
(otherwise we
are lost)
Mortality rate
(1) Change detection
(complementary to
remote sensing)
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Stephenson et al., in prep.
Such a network is taking shape in CORFOR
(the Cordillera Forest Dynamics Network)
http://mri.scnatweb.ch/content/view/88/30/
PLEASE JOIN US!
(... if you have the right kind of data.)