Complex Responses of
Subalpine Forests to
Climate Change
Connie Millar, Robert Westfall,
and Diane Delany
USDA Forest Service
Sierra Nevada Research Center
Albany & Lee Vining, CA
Two Lines of Active Research
1. Species & Communities Move Uphill in Response to Warming
Standard lapse-rates (e.g., -6.5°C/km) incorporated into regional
modeling & conservation assessments
Hayhoe et al. 2004 PNAS 101: 12422-12427
California Climate Change Center. 2006. Our
Changing Climate; Assessing the Risks to California
Projected losses of 70-95% ASF
A Classic Interpretation Also in Paleoecology
1967
Thompson 1988
Western North America
Vegetation History
K Cole
Dots = LGM
Lines = Current
2. Complex Mountain Climatology
Cold-air pooling & inversions are
common, and define widely varying
lapse rates from positive to negative
July
Atmospheric conditions for these conditions
are projected to increase in the future and may
overwhelm changes due to warming
Cascade Watersheds, Oregon
Daly 2008
Cold-Air Pooling Studies,
Lundquist 2008
Loch Vale Watershed, CO
Studies on temperature
inversions in the White Mtns,
Chris van de Ven, Stu Weiss, 2008
Uphill migration (shift in treeline) is not the only
or typical response to warming climates…
Species in the alpine and
subalpine zone respond to
climate complexly and
individualistically
I. FOREST DENSIFICATION
(without change in treeline elevation)
A. Treeline Zone Infilling
Whitebark pine
Kuna Crest
• Deadwood scattered within
live zone but not higher
• Abundance changes over time;
currently recruits are increasing
within upper deadwood zone
• Krummholz individuals persist
by vegetative layering
King & Graumlich 1999,
Rogers, Millar, Westfall, 1999
I. FOREST DENSIFICATION
(without change in treeline elevation)
B. General Subalpine Forest Infilling
1907
Gaylor Pk -- Tioga Pass
Vale & Vale 1984
1984
Vale & Vale 1994
I. FOREST DENSIFICATION
(without change in treeline elevation)
C. Colonization of Formerly Persistent Snowfields
Trees Colonizing Snowfields
Primary response is
to temperature
Mean & SD
T min
Millar et al. 2004
I. FOREST DENSIFICATION
(without change in treeline elevation)
D. Colonization of Subalpine Meadows
Trees in Mdws
Mean & SD
Correlation with
Negative PDO and
Low PDSI
PDO/
PDSI
Tmin
Precip/
Streamflow
Complex interactions
between temperature,
precipitation, & interannual
variability
Millar et al. 2004
Warren Bench, 2005
Meadow colonization
is readily reversible
Horse Mdws, 2006
Warren Bench, 2006
II. CHANGE IN FORM & GROWTH
(without change in treeline elevation)
krummholz
flags & skirts
Whitebark pine, a
sublimely plastic
subalpine species
upright trees
Flags
Mean & SD
PDO/
PDSI
Development of Flags
Responds to Temperature and PDO
As with meadow
colonization, flag
response is reversible
Tmin
Precip/
Streamflow
Millar et al. 2004
Stem Growth in Krummholz Whitebark Pine at Treeline
Doubled over the 20th Century
1907
Stem Growth
Mean & SD
1984
Parker Pass, Kuna Pk, YNP
Vale & Vale. 1994
Millar et al. 2004,
Rogers, Millar, Westfall 1999
III. CHANGE IN PATTERNS OF MORTALITY
(without change in treeline elevation)
A. Change in Drought and Insect & Disease Effects
Mortality event in subalpine
limber pine
Mt Warren, Mono Basin
1988-1994 drought
Death Dates
3 Sites
Tmin/
Tmax
+ elevated temperatures
+ mountain pine beetle
+ mistletoe infection
+ blue-stain fungus
= “Global-Warming Style
Drought” Mortality Event
Precip
Millar et al. 2007
III. CHANGE IN PATTERNS OF MORTALITY
(without change in treeline elevation)
B. Change in Genetic Diversity & Adaptation
Millar et al. 2007
Westfall & Millar 2006
Live trees remain
Typical sparse old-growth stand
Whitebark Pine Mortality Event 2008
…differential growth response as in limber pine?
Owens River Headwaters
Rock Creek
2008 field season
III. CHANGE IN PATTERNS OF MORTALITY
(without change in treeline elevation)
C. Change in Subalpine Fire Relationships
Cascade Crest Complex, OR, 2006
High elevation pine & fir forests of western NA
IV. CHANGE IN ASPECT
(without change in treeline elevation)
Limber pine, Great Basin
Sparse live stands now
only N/NE aspects
Mt Grant, Wassuk Range
Deadwood on all aspects
Ages of individual limber
pine tree-ring series,
Wassuk Range, NV
NE
N
Millar, Westfall, King, unpub
Forest Presence & Absence Correlated to
Wet vs Dry Periods;
Upper & Lower Forest-Border Elevations
Didn’t Change Significantly Over ~3600 yrs
Mean Elevation of Forest Borders
Live Forest: Upper 3222m Lower 2721m
Deadwood: Upper 3215m Lower 2715m
V. CHANGE IN ELEVATION
A. Differential Shifts in Elevation – Individualistic Response
Example 1: Whitewing Mtn and San Joaquin Mtn
Deadwood
Stem Dates
821-1350 CE
Millar et al. 2006
Deadwood Species
42 Whitebark Pine krumm
13 W White Pine ↓250 m
7 Lodgepole Pine ↓250 m
5 Jeffrey Pine ↓500 m
6 Mountain Hemlock ↓250 m
5 Sugar Pine ↓600 m & W SN
Differences Modeled between Whitewing Mtn
Paleo- and Current Climates
Medieval: Ann ppt
-24 mm
Ann min T +3.2°C
Ann max T +2.3°C
V. CHANGE IN ELEVATION
A. Differential Shifts in Elevation
Example 2: Leapfrog Recruitment of Limber Pine
vs Bristlecone Pine in the White Mtns, CA
3570 m
* 300 m above current limber-pine
treeline
* Dolomitic soils
* No or few live bristlecone pine
* Area of historic bristlecone pine
Millar et al., in prep
Highest Elevation Sites, High Densities Limber Pine, Episodic
Recruitment
Correlate with:
-- Annual Tmin, Tmax
-- May & June Tmin
-- Sept Precipitation
-- Atlantic Multidecadal Oscillation
Pulse
~ 1970-1991
V. CHANGE IN ELEVATION
B. Shifts Down in Elevation
with Warming and Drying
Limber Pine is recruiting
below its lower forest
border in north ravines &
riparian conditions
High-Elevation Forests Respond
Complexly to Climate
1. Increases in Forest Density
2. Changes in Growth & Form
3. Changes in Mortality & Genetic Diversity
4. Shifts in Aspect
5. Differential Shifts Up & Down in
Elevation
Simple movements uphill are
inadequate interpretations of
forest response to warming
Conservation & Adaptation
Lodgepole pine – really at risk?
Implications
Hannah 2006
* Future ranges – Moving or not?
Which direction?
* Refugia -- Where? How abundant?
* Individualistic responses – Novel
community compositions?
* Assisted migration?
* Germplasm diversity choices?
* Triage decisions?
* Corridors?
Millar & Brubaker 2006
Millar, Stephenson, & Stephens 2007
Science
2008