Climate and 20th century establishment in alpine treeline ecotones of the western USA. Presented by Jeremy Littell at the "Perth II: Global Change and the World's Mountains" conference in Perth, Scotland in September 2010.

1. 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
High Five II Missoula, MT 27 Jun 2010 Climate Science in the Public Interest

2. Treeline
• Ecotone where forest shifts to some
Photo: Andrea Lloyd
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:
Absaroka-Beartooth wilderness, MT – Temperature: min. / max. / annual
– Growing season length: GDD / snow free
– Soil moisture
• Limiting mechanisms
– Physical --> life history
– Physical --> carbon fixation
– Physical --> carbon storage
Boulder Valley, SW MT

3. 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

4. Establishment is a
Demographic Bottleneck
• In many years, there are
many germinants, but in most
years most germinants die.
Survivorship for three treeline species,
Tetons, Beartooths, and Snowy Range.
Maher & Germino, 2006. • In the first few years, there is
very high mortality.
• Successful establishment – is
increasingly confined to
favorable microsites with time
Larix lyallii

5. 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.

6. Sky Exposure
• Affects radiative cooling at
night and thus probability
of frost in microsites
• Affects daytime radiation
• Photoinhibition

7. TN
SN
•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
Littell, Germino,Pederson, Graumlich in prep

8. 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)

9. Herb
2 None
5 4
Grass
11
Sedge
8 53
Rock
15 Conifer
sapling
16 Tree
Wood
n ~ 2000 dates, including
~ 400 established trees

10. Successful establishment appears episodic within
sites, sometimes synchronous across sites
Establishment Count

11. Site climatologies
• Daily / hourly hobo
observations
• Monthly lapse with
valley stations
• Extrapolate for
period of record:
– Tmax, Tmin
– Degree days
– Snowpack
duration

12. Establishment, climate, and…what
else…?
• Years of high establishment • AND – most establishment
qualitatively relate to still within 10m – 40m of
favorable climate…but many existing treeline
ways to fail

13. • Hydrologic
model (VIC)
to calculate
SWE (but at
6km averaged
over site)
• Anomalies are
good, but
mean is
probably cold.
• Trends
evident

15. Role of extreme events

17. jlittell@u.washington.edu