Pervasive Global Warming Changes in Western Forests

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.)

Pervasive Global Warming Changes in Western Forests

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