Climate and 20th century establishment

in alpine treeline ecotones of the

western USA

Jeremy Littell1, Greg Pederson2,

Matt Germino3, L. J. Graumlich4

1Climate Impacts Group, University of Washington 2USGS Northern Rocky Mountain Science Center;

3Department of Biological Sciences, Idaho State University; 4University of Washington College of the Environment

Climate Science in the Public Interest High Five II Missoula, MT 27 Jun 2010

Treeline

• Ecotone where forest shifts to some other vegetation

– Boreal treeline / forest to tundra

– Alpine treeline / forest to tundra or meadow

– Lower treeline / forest to steppe~grassland

• Limiting factors often related to climate:

– Temperature: min. / max. / annual

– Growing season length: GDD / snow free

– Soil moisture

• Limiting mechanisms

– Physical --> life history

– Physical --> carbon fixation

– Physical --> carbon storage

– Physical --> carbon utilization

– Biotic: competition / facilitation

Photo: Andrea Lloyd

Absaroka-Beartooth wilderness, MT

Boulder Valley, SW MT

What is the role of climate in seedling

establishment in the alpine treeline ecotone?

• Climatic correlates of tree

germination, seedling

survival, growth, mortality:

mere hints

• Ecophysiologically, the

mechanisms related to

establishment affected by

climate are often more

complex

• Not stationary in time or

place

Establishment is a

Demographic Bottleneck

• In many years, there are

many germinants, but in most

years most germinants die.

• In the first few years, there is

very high mortality.

• Successful establishment – is

increasingly confined to

favorable microsites with time

Survivorship for three treeline species,

Tetons, Beartooths, and Snowy Range.

Maher & Germino, 2006.

Larix lyallii

What is a favorable

microsite?

Range of sky exposures occupied by emergent (top panel)

or older (seedlings and juveniles; bottom panel) individuals of

subalpine fir, Engelmann spruce, and whitebark pine in the Teton,

Beartooth and Medicine Bow Mountains. Maher & Germino, 2006.

Sky Exposure

• Affects radiative cooling at

night and thus probability

of frost in microsites

• Affects daytime radiation

• Photoinhibition

•Climatic gradient in winter snowpack

•9 sites 1950m – 3300m

•Seedling census – 6 x 50m transects

•Established tree ages (cores)

•Seedling and sapling ages (sections)

•1 year air and surface temperature

TN

SN

Littell, Germino,Pederson, Graumlich in prep

Site climate

• Hobos placed year prior; 1 or 4

hrly – Two air temperature, two surface

temperature per site

• Gives monthly lapse rate (AT) and

also snow pack duration (ST)

n ~ 2000 dates, including

~ 400 established trees

53

16

15

8

11

5 4

2

Herb

None

Grass

Sedge

Rock

Conifersapling

Tree

Wood

Successful establishment appears episodic within

sites, sometimes synchronous across sites E

sta

blis

hm

ent

Count

Site climatologies

• Daily / hourly hobo

observations

• Monthly lapse with

valley stations

• Extrapolate for

period of record:

– Tmax, Tmin

– Degree days

– Snowpack

duration

Establishment, climate, and…what

else…? • Years of high establishment

qualitatively relate to

favorable climate…but many

ways to fail

• AND – most establishment

still within 10m – 40m of

existing treeline

• Hydrologic

model (VIC)

to calculate

SWE (but at

6km averaged

over site)

• Anomalies are

good, but

mean is

probably cold.

• Trends

evident

Role of extreme events

jlittell@u.washington.edu