Changes in sub-alpine tree distribution in western North America: A review of climatic and other causal factors

Abstract

Changes in the distribution of sub-alpine tree species in western North America have been attributed to climatic change and other environmental stresses. These changes include tree-line fluctuations throughout the Holocene and recent invasion of sub-alpine meadows by forest. Most palaeoecological studies suggest that the tree-line was higher during a period of warmer climate approximately 9000 to 5000 BP and lower during the last 5000 years, with short periods of local tree-line advance. Recent advances in sub-alpine tree distribution can be compared with weather records, allowing an examination of relationships between tree advance and climate at a finer resolution. In general, recent sub-alpine forest advances in western North America, based on studies representing three climatic zones (maritime, Mediterranean and continental), have been associated with climatic periods favouring tree germination and growth, although factors such as fire and grazing by domestic livestock have had an impact in some areas. Limitations to tree establishment (e.g., winter snow accumulation, summer drought) vary in relative importance within each climate zone, as do predicted consequences of anthropogenic climatic change. Recent increases in establishment of sub-alpine trees may continue if climatic change alleviates the limitations to tree establishment important in each climatic zone. However, factors such as topography and disturbance may modify tree establishment on a local scale.

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The Holocene
DOI: 10.1177/095968369400400112
1994; 4; 89 The Holocene
Regina M. Rochefort, Ronda L. Little, Andrea Woodward and David L. Peterson
causal factors
Changes in sub-alpine tree distribution in western North America: a review of climatic and other
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Changes
in
sub-alpine
tree
distribution
in
western
North
America:
a
review
of
climatic
and
other
causal
factors
Regina
M.
Rochefort,
Ronda
L.
Little,
Andrea
Woodward
and
David
L.
Peterson*
(National
Park
Service,
Cooperative
Park
Studies
Unit,
College
of
Forest
Resources,
University
of
Washington,
AR-10,
Seattle,
Washington
98195,
USA)
Received
4
November
1993;
revised
manuscript
accepted
3
May
1993
Abstract :
Changes
in
the
distribution
of
sub-alpine
tree
species
in
western
North
America
have
been
attributed
to
climatic
change
and
other
environmental
stresses.
These
changes
include
tree-line
fluctuations
throughout
the
Holocene
and
recent
invasion
of
sub-alpine
meadows
by
forest.
Most
palaeoecological
studies
suggest
that
the
tree-line
was
higher
during
a
period
of
warmer
climate
approximately
9000
to
5000
BP
and
lower
during
the
last
5000
years,
with
short
periods
of
local
tree-line
advance.
Recent
advances
in
sub-alpine
tree
distribution
can
be
compared
with
weather
records,
allowing
an
examination
of
relationships
between
tree
advance
and
climate
at
a
finer
resolution.
In
general,
recent
sub-alpine
forest
advances
in
western
North
America,
based
on
studies
representing
three
climatic
zones
(maritime,
Mediterranean
and
continental),
have
been
associated
with
climatic
periods
favouring
tree
germination
and
growth,
although
factors
such
as
fire
and
grazing
by
domestic
livestock
have
had
an
impact
in
some
areas.
Limitations
to tree
establishment
(e.g.,
winter
snow
accumulation,
summer
drought)
vary
in
relative
importance
within
each
climate
zone,
as
do
predicted
consequences
of
anthropogenic
climatic
change.
Recent
increases
in
establishment
of
sub-alpine
trees
may
continue
if
climatic
change
alleviates
the
limitations
to
tree
establishment
important
in
each
climatic
zone.
However,
factors
such
as
topography
and
disturbance
may
modify
tree
establishment
on
a
local
scale.
Key
words:
climatic
change,
meadow
invasion,
sub-alpine
forest,
tree
establishment,
tree-line,
Holo-
cene,
North
America.
Introduction
Climate
change,
sub-alpine
forests
and
ecotones
Most
general
circulation
models
predict
an
increase
in
mean
annual
temperature
of
1-5°C
during
the
next
century
as
the
result
of
increased
levels
of
atmospheric
C02
and
other
greenhouse
gases
(Schneider,
1989).
Climatological
evidence
indicates
that
mean
annual
air
temperature
at
the
earth’s
surface
has
already
increased
approximately
0.6°C
since
AD
1880
(Jones et
al.,
1986;
Hansen
and
Lebedeff,
1988).
Even
small
changes
in
temperature,
such
as
the
1-1.5°C
cooler
period
of
the
’Little
Ice
Age’
(c.
AD
1650
to
1850;
Porter,
1981),
can
cause
significant
changes
in
vegetation
(Lamb,
1982;
Brubaker,
1988;
Sprugel,
1991).
Future
precip-
itation
patterns
are
also
predicted
to
change,
but
existing
models
predict
changes
in
only
magnitude
and/or
seasonal
pattern
(e.g.,
wetter
winters
with
drier
summers -
Kellogg
and
Zhao,
1988;
Silver
and
DeFries,
1990).
Even
small
changes
in
global
temperature
or
precipitation
patterns
are
likely
to
alter
conditions
affecting
earth’s
ecosystems
(Houghton
and
Woodwell,
1989),
resulting
in
changes
in
vegetation
patterns.
Linking
the
response
of
vegetation
to
future
changes
in
climate
is
a
major
challenge
facing
ecologists.
Detecting
initial
effects
of
environmental
change
on
vegetation
may
be
diffi-
cult
due
to
the
high
variability
within
most
biological
systems.
One
promising
approach
is
to
focus
on
ecotones -
transitions
between
adjacent
communities.
Ecotones
resulting
from
envi-
ronmental
gradients
(as
opposed
to
disturbances
such
as
fire)
may
be
particularly
responsive
to
climatic
changes
because
organisms
are
already
at
some
limit
to
their
existence.
Eco-
tones
at
the
sub-alpine/alpine
boundary
may
be
particularly
relevant
to
studies
of
climatic
change
because
the
physi-
ognomy
of the
dominant
vegetation
changes
from
trees
to
shrubs
and
herbs
(Peterson,
1991;
Rochefort
and
Peterson,
1991;
Woodward et
al.,
1991).
Tree
distribution
at
high
elevation
is
at
least
partially
limited
by
temperature
and
* Author
to
whom
correspondence
should
be
addressed.
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90
snowpack
(Wardle,
1974;
Tranquillini,
1979;
Stevens
and
Fox,
1991),
factors
certain
to
change
if
temperatures
rise
in
the
future.
Palaeoecological
studies indicate that
altitudinal
tree-line
has
fluctuated
in
response
to
climate
throughout
the
Holo-
cene
(Pears,
1968;
Markgraf
and
Scott,
1981;
Luckman
and
Kearney,
1986;
COHMAP,
1988),
and
that
expanding
ranges
of
tree
species
are
often
associated
with
periods
of
warmer
climate
(LaMarche,
1973;
Kearney
and
Luckman,
1983;
Grace,
1989;
Kullman,
1991).
Recent
studies
provide
the
opportunity
to
discover
local
factors,
in
addition
to
climate,
responsible
for
enhancing
or
preventing
tree
establishment.
Studies
in
western
North
America
suggest
recent
expansions
in
the
spatial
distribution
of
sub-alpine
tree
species
at
many
locations.
Terminology
and
scope
Many
different
terms
are
used
to
describe
ecotonal
relation-
ships
of
high-altitude
tree
species
(Love, 1970;
Douglas,
1972;
Wardle,
1974;
Tranquillini,
1979).
We
use
the
following
terminology
in
this
discussion.
The
sub-alpine
park
land
is
the
highest-elevation
belt
that
contains
tree
species,
consisting
of
a
mosaic
of
individual
trees,
tree
clumps,
and
meadows
that
extend
from
closed
forest
to
alpine
(treeless)
vegetation
(Figure
1 -
Henderson,
1974;
Franklin
and
Dyrness,
1987).
The
lower
boundary
of
the
sub-alpine
park
land
is
indicated
by
the
upper
limit
of closed
contiguous
forest
or
forest
line.
Tree-line
indicates
the
highest
elevation
at
which
erect
trees
are
found
within
the
sub-alpine
park
land.
Tree-limit
indicates
the
highest
elevation
at
which
any
trees
are
found,
including
prostrate
or
krummholz
growth
forms.
This
paper
focuses
on
altitudinal
species
distribution,
although
’tree-line’
and
’tree-
limit’
can
also
be
used
to
describe
latitudinal,
edaphic
and
maritime
tree
species
limits
(Hustich,
1983;
Payette,
1983).
Information
is
available
on
worldwide
tree-line
dynamics
Figure
1
Diagrammatic
representation
of
sub-alpine
conifer
distribu-
tion.
A
mosaic
of
trees
and
meadows
dominates
the
ecotone
between
the
continuous
forest
below
and
the
treeless
alpine
above.
and
on
factors
limiting
sub-alpine
forest
distribution
(Dau-
benmire,
1954;
Zimina,
1973;
Wardle,
1977;
Gorchakovsky
and
Shiyatov,
1978;
Payette
and
Gagnon,
1979;
Tranquillini,
1979;
Kullman,
1986).
We
focus
on
western
North
America
because:
(1)
many
data
exist
on
recent
changes
in
sub-alpine
forest
distribution;
and
(2)
many
sub-alpine
mountain
loca-
tions
in
this
area
lie
within
protected
areas
relatively
free
of
human
influence
(Billings,
1969;
Holtmeier,
1989).
The
im-
pacts
of
woodcutting,
grazing
and
other
activities
more
com-
mon
in
eastern
North
America
and
Europe
make
it
difficult
to
interpret
the
influence
of
climate
and
other
’natural’
environmental
forces
on
sub-alpine
vegetation
(Holtmeier,
1973;
Ives
and
Hansen-Bristow,
1983;
Helms,
1987).
Climatic
regimes
in
western
North
America
are
highly
varied,
and
ecosystems
range
from
temperate
rainforest
to
desert.
However,
climatic
regimes
in
mountainous
areas
can
be
classified
by
their
dominant
weather
patterns
and
geo-
graphic
location
into:
maritime,
continental
and
Mediterra-
nean
zones
(Schroeder
and
Buck,
1970;
Trewartha,
1980;
Critchfield,
1983).
Figure
2
indicates
the
location
of the
climatic
zones
and
principal
mountain
ranges
discussed
in
this
paper.
Alaska
and
Mexico
are
not
included
because
we
have
found
no
published
reports
of
recent
altitudinal
changes
in
the
distribution
of
sub-alpine
forests
in
these
areas
(Griggs,
1937;
Beaman,
1962;
Lauer,
1978).
The
climatic
zones
are
not
intended
to
correspond
exactly
with
macroscale
or
synoptic
climatic
classifications.
Characteristics
of
the
three
climatic
regimes
differ
sub-
stantially.
Maritime
climate -
predominant
in
the
northwest-
ern
portion
of the
continental
United
States
and
southwestern
Canada -
is
characterized
by
high
coastal
rainfall,
a
long
winter
period
of
rain
and
snow,
and
extensive
cloud
cover.
Winter
temperatures
are
cool,
summer
tem-
peratures
are
mild,
and
summer
precipitation
is
low.
Con-
tinental
climate
prevails
in
the
western
interiors
of
the
continental
United
States
and
Canada.
Much
of
the
winter
precipitation
is
snow,
and
summer
precipitation
occurs
as
thunderstorms
in
the
central
and
southern
part
of the
range.
Winter
temperatures
are
cold;
summer
temperatures
vary
from
cool
in
the
north
to
warm
in
the
south.
Mediterranean
climate
is
predominant
in
the
far
southwestern
continental
United
States.
Annual
precipitation
is
low,
and
most
of
it
falls
during
winter.
Summers
are
hot
and
dry,
with
persistent
droughts
in
many
years,
and
winter
temperatures
are
mild.
Several
authors
have
reviewed
the
causes
of
tree-line
and
tree
species
limit
(Griggs,
1946;
Daubenmire,
1954;
Wardle,
,
1971;
Tranquillini,
1979;
Arno
and
Hammerly,
1984;
Innes,
1991;
Stevens
and
Fox,
1991);
however,
these
reviews
empha-
size
environmental
impacts
on
growth
and
survival
of
mature
trees
rather
than
on
seedling
establishment.
In
this
paper,
we
focus
on
the
effects
of
environmental
factors
on
sub-alpine
tree
establishment
and
initial
survival,
because
this
early
stage
of
forest
development
determines
tree-line
advance
and
meadow
invasion.
We
discuss
changes
in
tree
establishment
with
respect
to
fluctuations
in
altitudinal
tree-line
and
tree-
limit,
and
forest
advance
into
sub-alpine
meadows.
Although
most
of
the
literature
cited
in
this
paper
is
from
North
America,
general
concepts
may
be
applicable
to
sub-alpine
forests
in
other
areas
of
the
world,
especially
the
temperate
zone
of
Europe
(Troll,
1973).
In
order
to
evaluate the
effects
of
climate
on
the
distribution
of
sub-alpine
species,
we
discuss
how
weather
components
(e.g.,
snow,
wind
and
temperature),
as
well
as
natural
disturbance,
limit
the
distribution
of
sub-
alpine
trees.
We
then
review
palaeoecological
and
modern
evidence
for
sub-alpine
vegetation
changes
during
the
Holo-
cene,
and
we
infer
the
relative
importance
of
environmental
variables
associated
with
these
changes.
Finally,
we
project
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91
Figure
2
Location
of
climatic
zones
(maritime,
continental,
Mediterranean)
and
major
mountain
ranges
of
western
North
America.
how
potential
climatic
change
could
affect
sub-alpine
forests
in
the
future.
Environmental
influences
on
seedling
establishment
and
survival
Patterns
of
snow
accumulation
and
snowmelt
either
promote
or
hinder
seedling
establishment
and
survival.
Snowpack
shields
conifer
needles
from
the
desiccating
winter
winds
common
at
upper
elevations
(Wardle,
1968;
Lindsay,
1971;
Hadley
and
Smith,
1983;
Minnich,
1984)
and
insulates
seed-
ling
roots
and
foliage
from
lethal
cold
temperatures
(Kur-
amoto
and
Bliss,
1970).
Snowmelt
delays
summer
drought
and
provides
increased
soil
moisture
early
in
the
growing
season
when
tree
seeds
germinate
(Evans
and
Fonda,
1990).
Snowpack
may
be
especially
beneficial
to
seedling
establish-
ment
in
the
windswept
continental
regions
and
dry
Medi-
terranean
mountains.
On
the
other
hand,
heavy
snow
accumulation
that
remains
on
the
ground
until
midsummer
results
in
a
shorter
growing
season
and
reduced
seedling
establishment
(Brink,
1959;
Fonda
and
Bliss,
1969;
Kuramoto
and
Bliss,
1970;
Douglas,
1972;
Heikkinen,
1984;
Butler,
1986;
Armstrong et
al.,
1988;
Hansen-Bristow et
al.,
1988).
Pro-
longed
snow
cover
also
promotes
infection
by
brown
felt
fungi
(Herpotrichia
spp.),
which
can
damage
and
kill
foliage
(Donaubauer,
1963;
Simms,
1967;
Holtmeier,
1987).
Al-
though
some
tree
seeds
can
germinate
directly
on
snow,
they
generally
have
a
low
survival
rate
(Franklin
and
Krueger,
1968;
Brooke
et
al.,
1970),
and
even
established
seedlings
can
be
physically
crushed
or
uprooted
by
the
weight
and
move-
ment
of
snow
(Ives
and
Hansen-Bristow,
1983).
The
majority
of
these
negative
effects
of
snow
on
establishment
are
re-
ported
for
maritime
regions,
where
deep
snowpacks
develop.
The
effect
of
both
snowdrift
and
wind
patterns
on
tree
survival
in
the
Rocky
Mountains
is
demonstrated
by
the
creation
of
gradually
moving
ribbon
forests
aligned
perpen-
dicular
to
westerly
winds
(Billings,
1969;
Benedict,
1984;
Holtmeier, 1987).
Availability
of
soil
moisture
is
critical
for
seedling
germina-
tion
and
survival,
particularly
during
the
first
year
(Cui
and
Smith, 1991).
Soil
moisture
depends
on
rainfall
and
snowmelt,
as
modified
by
soil
properties,
aspect,
microtopography
and
evapotranspiration
(Fonda,
1976;
Sawyer
and
Kinraide,
1980).
Periods
of
above
average
summer
precipitation
can
enhance
tree
establishment
(Agee
and
Smith,
1984;
Allen,
1984;
Taylor,
1990),
and
increased
water
availability
from
a
melting
snowpack
can
be
important
for
seedling
survival
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92
during
summer
droughts
(Billings, 1969;
Lindsay,
1971;
Cana-
day
and
Fonda,
1974).
Seedling
establishment
can
also
occur
when
a
wet
period
provides
adequate
soil
moisture
for
tree
seedlings
following
a
series
of
dry
years
that
reduces
the
competitive
ability
of
meadow
vegetation
(Vale,
1981;
Butler,
1986;
Taylor,
1990).
Foliar
desiccation
can
occur
during
winter
when
the
transpirational
demand
of
foliage
exceeds
the
ability
of the
roots
to
provide
water.
This
problem
is
exacerbated
by
frozen
or
cold
soils,
where
water
absorption
by
roots
is
reduced.
Winter
drying
of
tissues
controls
tree
morphology
and
distribution
at
many
high-elevation
locations
(Marr,
1977;
Tranquillini,
1979;
Ives
and
Hansen-Bristow,
1983;
Bella
and
Navratil,
1987).
Adequate
soil
moisture
is
important
for
seedlings
in
all
climatic
zones
because
they
all
have
dry
periods;
however,
winter
desiccation
may
be
more
prevalent
in
the
continental
zone,
because
high
winds
can
remove
snow
and
expose
soil
to
freezing
temperatures.
Summer
moisture
stress
is
one
of
the
primary
limits
to
tree-
line
advance
in
Mediterranean
areas,
especially
when
com-
bined
with
warm
temperatures
(Klikoff,
1965;
LaMarche,
1973).
Extreme
soil
temperatures
have
a
negative
impact
on
the
survival
of
tree
seedlings
(Wardle,
1968;
Munn et
al.,
1978;
DeLucia
and
Smith,
1987).
High
soil
temperatures
during
early
summer
can
be
lethal
to
seedlings
(Baig,
1972;
Ballard,
1972;
Douglas,
1972),
although
warm
summers
generally
benefit
seedling
survival
(Kearney,
1982).
Years
with
warm
air
temperatures
can
promote
tree
establishment
in
meadows
(Brink,
1959;
Franklin,
Moir et
al.,
1971;
Lowery,
1972;
Kearney
and
Luckman,
1983;
Heikkinen,
1984;
Clague
and
Mathewes,
1989;
MacDonald,
1989),
while
periods
with
cold
air
temperatures
can
inhibit
tree
establishment
(Brink,
1964;
Daly
and
Shankman,
1985).
Seedling
establishment
can
also
be
limited
in
the
immediate
path
of
cold
air
drainage
and
melt
water
originating
from
snow
banks
(Wardle,
1968;
Fonda
and
Bliss,
1969;
Brooke et
al.,
1970;
Moore,
1991).
Freezing
soil
and
air
temperature
can
damage
tissues
directly
through
intercellular
freezing
and
indirectly
by
dehydration
resulting
from
extracellular
freezing
(Kramer
and
Kozlowski,
1979;
Tranquillini,
1979).
Early
autumn
freezes
and
late
spring
freezes
may
be
especially
deleterious
to
seedling
tissues
that
are
not
cold-hardened
(Lindsay,
1971;
Ives
and
Hansen-
Bristow,
1983).
Seedlings
are
more
sensitive
than
mature
trees
to
frost
heaving
caused
by
differential
freezing
of
soil
layers
because
their
shallow
roots
break
or are
exposed
to
desiccation
(Brink,
1964;
Kramer
and
Kozlowski,
1979).
In
general,
seedlings
in
continental
and
Mediterranean
regions
are
exposed
to
extreme
cold
temperatures,
while
seedlings
in
maritime
regions
are
protected
by
deep
snow.
In
continental
regions,
consistent
cold
temperatures
during
summer
in
com-
bination
with
desiccating
winds
may
result
in
inadequate
maturation
of
new
tissue,
which
is
subsequently
killed
during
winter
(Hadley
and
Smith,
1986).
Disturbances
such
as
avalanches
and
fire
strongly
affect
sub-alpine
vegetation
patterns.
Species
composition
after
a
physical
disturbance
often
depends
on
the
type,
extent
and
intensity
of
disturbance
(Oliver et
al.,
1985).
For
example,
a
snow
avalanche
can
produce
a
gradient
of
disturbance
condi-
tions
within
its
path,
allowing
various
regeneration
mecha-
nisms
to
occur,
such
as
advance
regeneration,
resprouting,
or
establishment
of
species
with
light
seeds
(Cushman,
1981).
Frequent
avalanches
(15-20
years
apart)
in
the
Alberta
Rocky
Mountains
cause
trees
to
be
replaced
by
shrubs
in
avalanche
paths
(Johnson,
1987).
The
role
of
natural
fires
in
maintaining
sub-alpine
meadow
communities
has
been
shown
in
many
studies
(Kuramoto
and
Bliss, 1970;
De-Benedetti
and
Parson,
1979;
Vale,
1981;
Butler,
1986;
Helms,
1987;
Shank-
man
and
Daly,
1988).
Both
natural
fires
and
those
caused
by
people,
especially
by
early
European
settlers
and
Native
Americans,
have
favoured
open
meadows
(Vankat
and
Ma-
jor,
1978).
Fires
near
the
tree-line
are
usually
small
but
can
result
in
new
snowdrift
patterns
and
long-lasting
replacement
by
meadow
vegetation
(Billings,
1969).
Establishment
of sub-
alpine
trees
following
fire
is
often
slow
because
of
snow
creep,
substrate
instability,
herbivory
or
unfavourable
climate
(Agee
and
Smith,
1984;
Little
and
Peterson,
1991).
Fire
is
the
most
important
disturbance
in
the
Rocky
Mountains,
and
periods
of
frequent
and
intense
fires
have
created
open
forests
in
the
sub-alpine
park
land
(MacDonald,
1989;
Moore,
1991).
The
tree-line in
Colorado
can
be
depressed
below
its
climatic
upper
limit
in
areas
frequented
by
fire
(Peet,
1981;
Shankman
and
Daly,
1988).
Fire
is
also
common
in
regions
with
Medi-
terranean
climate
but
less
frequent
in
maritime
climatic
zones.
Interpretations
of
other
factors
controlling
tree-line
and
tree-limit
must
carefully
examine
impacts
of
disturbance
on
sub-alpine
vegetation
patterns.
Changes
in
sub-alpine
forest
distribution
during
the
Holocene
Fluctuating
tree-lines
and
changing
climatic
regimes
during
the
Holocene
are
recorded
by
pollen
records,
wood
fragments
and
macrofossils.
The
majority
of
palaeoecological
studies
are
from
the
continental
zone,
while
studies
of
recent
tree
establishment
are
concentrated
in
the
maritime
zone.
The
causes
of
these
recent
changes
are
often
difficult
to
ascertain,
because
the
changes
coincide
with
the
end
of
the
’Little
Ice
Age’
and
the
expansion
of
European
settlement
in
western
North
America.
Palaeoecological
evidence
of
tree-line
dynamics
Palaeoecological
studies
generally
indicate
a
warmer
climate
during
the
early
to
mid-Holocene
(9000
to
5000
BP)
with
tree-line
advances
(Table
1 -
Andrews et
al.,
1975;
Petersen
and
Mehringer,
1976;
Carrara et
al.,
1984;
Luckman
and
Kearney,
1986;
Clague
and
Mathewes,
1989;
Clague et
al.,
1992).
Cooler
climates
since
5000
BP
have
generally
resulted
in
stable
or
lower-elevation
tree-lines.
Detailed
reconstruc-
tions
of
past
precipitation
patterns
and
temperature
regimes
are
based
on
pollen
analysis
(Maher,
1972;
Luckman
and
Kearney,1986;
Clague
and
Mathewes,
1989),
isotope
analysis
(Carrara et
al.,
1984;
Friedman et
al.,
1988;
Carrara et
al.,
1991)
and
tree-ring
growth
patterns
(LaMarche
and
Mooney,
1972;
Payette et
al.,
1989).
These
reconstructions
describe
a
warm/dry
early
Holocene
in
the
maritime
and
Mediterranean
areas
(LaMarche
and
Mooney, 1972;
LaMarche, 1973;
Clague
and
Mathewes,
1989),
but
a
warm/moist
early
period
in
the
continental
zone
(Markgraf
and
Scott,
1981;
Friedman
et
al.,
1988;
Carrara
et al. ,
1991).
Maritime
Tree-lines
in
southwestern
British
Columbia
9100-8200
BP
were
approximately
60-130
m
higher
than
today
(Clague
and
Mathewes,
1989;
Clague et
al.,
1992).
Using
adiabatic
lapse
rates
(6.5°C
per
1000 m),
the
authors
estimated
that
tem-
perature
was
0.4-0.8°C
warmer
at
this
time.
Wood
fragments
from
colluvial
and
alluvial
sediments
and
pollen
collected
from
a
bog
above
the
present
tree-line
were
identified
as
Pinus
albicaulis
and
Abies
lasiocarpa,
species
still
present
in
the
area
today.
These
fragments
had
wider
annual
rings
than
living
krummholz
from
the
same
site,
indicating
that
condi-
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tions
favourable
for
tree
growth
were
present
at
high-eleva-
tion
sites
during
the
early
Holocene.
Wood
fragments
from
additional
sites
in
southern
British
Columbia
and
Alberta
represent
two
periods
of
higher
tree-line:
9100-7600
BP,
and
6600-5100
BP
(Clague
and
Mathewes,
1989).
Wood
frag-
ments
from
the
earlier
period
indicate
the
early
Holocene
may
have
been
warmer
than
the
mid-Holocene.
Pollen
de-
posited
in
the
upper
part
of
the
core
contained
high
percen-
tages
of
Cyperaceae,
Salix
and
other
wet-habitat
species,
indicating
a
recent
change
to
wetter
conditions.
Continental
Studies
in
the
continental
zone
reveal
a
similar
but
more
detailed
record
of
tree-line
movement.
Analysis
of
pollen,
macrofossils
and
oxygen-isotope
data
from
tree-rings
from
alpine
bogs
in
the
Canadian
Rocky
Mountains
indicate
a
fluctuating
tree-line
over
the
last
8800
years
(Kearney
and
Luckman,
1983;
Luckman
and
Kearney,
1986).
Pollen
ratios
(Abies/Pinus)
and
macrofossils
indicate
two
periods
of
ele-
vated
tree-line:
8800-7500
BP
and
7200-5200
BP.
Wood
frag-
ments
from
a
site
100 km
south
of the
bogs
also
provide
evidence
of
a
higher
tree-line
8300-8000
BP
(Luckman,
1988).
The
climate
was
warmer
and
drier
during
these
peri-
ods,
and
the
tree-line
was
at
least
100 m
higher.
Oxygen-
isotope
analysis
of
buried
logs
suggests
temperatures
were
at
least
0.5°C
warmer
8770-8060
BP
and
1.2-1.6°C
warmer
6000-5300
BP
(Luckman
and
Kearney, 1986).
Tree-lines
have
been
similar
to
or
lower
than
present
levels
since
4500
BP,
with
minimum
levels
during
the
last
500
years.
An
additional
study
in
Yoho
National
Park,
British
Columbia,
used
pollen
and
macrofossil
data
to
show
that
the
tree-line
was
at
least
90 m
above
the
modern
tree-line
during
the
period
8500-3000
BP,
in
response
to
warmer
climatic
conditions
(Reasoner
and
Hickman,
1989).
Tree-lines
declined
to
cur-
rent
levels
during
cooler
climates
after
3000
BP.
Studies
in
southwestern
Colorado
indicate
tree-line
and
tree-limit
were
depressed
prior
to
9800
BP,
advanced
upward
at
least
twice
between
9800
and
5400
BP,
and
have
generally
been
lower
than
or
equal
to
present
elevations
since
3500
BP
(Petersen
and
Mehringer,
1976;
Carrara et
al.,
1984;
1991).
Petersen
and
Mehringer
(1976)
used
Pinus/Picea
pollen
ratios
and
Picea
macrofossils
to
identify
three
periods
of
tree-line
advance
in
the
La
Plata
Mountains:
8500,
6700
and
2500
BP.
Studies
in
the
San
Juan
Mountains
of
Colorado
show
patterns
similar
to
those
described
by
Petersen
and
Mehringer
(1976)
(Andrews
et al.,1975;
Carrara
et al.,1984;1991).
Pollen,
wood
fragments
and
macrofossils
indicate
three
periods
of
tree-line
and
tree-limit
advance
of
at
least
80 m:
9600-7800
BP,
6700-5400
BP,
and
3100 BP.
The
tree-line
may
have been
140
m
higher
than
at
present
c.
8000
BP.
Mid-Holocene
July
temperatures
were
estimated
at
0.5-0.9°C
higher
than
today.
The
tree-line
declined
to
present-day
elevations
5400-3500
BP
and
again
after
a
short
upward
advance around
3100
BP,
when
tree
species
may
have
been
70
m
higher
than
today
(Andrews et
al.,
1975;
Carrara et
al.,
1984;
1991).
Two
studies
from
the
Colorado
Rocky
Mountains
describe
contrasting
climates
in
the
early
to
mid-Holocene.
Maher
(1972)
used
PicealPinus
pollen
ratios
to
infer
that
climates
from
10000
to
7600
BP
and
from
6700
to
3000
BP
were
cooler
and/or
wetter
than
today
and
tree-lines
were
lower.
Warming
trends
from
7600
to
6700
BP,
and
since
300
BP,
resulted
in
higher
tree-lines.
Conversely,
Short
(1984)
used
pollen
ratios
to
show
that
the
tree-line
reached
a
maximum
altitude
6500-3000
BP
and
that
it
has
since
lowered
in
response
to
climatic
cooling.
Mediterranean
Studies
of
Pinus
longaeva
in
the
White
Mountains,
California
(LaMarche, 1973),
and
Snake
Mountains,
Nevada
(LaMarche
and
Mooney, 1972),
indicate
higher
tree-lines
during
the
early
to
mid-Holocene.
Wood
fragments
above
the
current
tree-
line
at
the
California
sites
dated
7400
BP
indicate
that
the
tree-line
was
100
to
200 m
above
modem
levels
until
4200-2000
BP.
This
was
a
relatively
warm
period,
with
esti-
mated
mean
temperatures
3.5°C
higher
than
today.
Tree-line
elevation
became
lower
870-470
BP.
Analysis
of
site
condi-
tions
and
tree
growth/climate
relationships
(in
tree-rings)
showed
that
the
tree-line
was
controlled
by
precipitation
as
well
as
temperature.
Current
P.
longaeva
distribution
in
the
Snake
Mountains,
Nevada,
shows
a
progressive
dwarfing
with
increasing
altitude
as
follows:
erect,
tall-dwarf,
short-dwarf
and
krummholz
(LaMarche
and
Mooney,
1972).
The
authors
suggest
that
the
past
forest
included
only
erect
and
tall-dwarf
trees.
The
upper
boundary
of
erect
trees
was
at
least
100
m
higher
than
today,
and
tall-dwarf
trees
extended
to
the
summit
(3559
m)
of
Mount
Washington.
Surveys
conducted
in
the
southern
Sierra
Nevada
re-
constructed
similar
Holocene
tree-line
fluctuations
for
Pinus
balfouriana
(Scuderi,
1987).
Relict
wood
samples
collected
68
m
above
the
present
tree-line
were
dated
to
6300
BP.
Wide
growth
rings
indicate
warm,
favourable
growing
conditions.
Samples
collected
at
65
m
above
the
present
tree-line
were
dated
to
3530
BP,
suggesting
that
the
tree-line
was
main-
tained
at
this
elevated
position
for
about
3000
years.
Tree
mortality
at
20
m
above
the
present
tree-line
and
decreased
growth
between
2500
and
2300
BP
indicate
a
colder
climate.
The
tree-line
declined
to
10 m
below
the
present
level
between
1400
and
1300
BP,
and
increased
to
the
present
level
between
950
anjd
850
BP;
a
few
trees
established
5
to
10
m
above
the
present
tree-line
about
100
years
ago.
Summary
of
tree-line
fluctuation
’
.
Palaeoecological
studies
provide
a
fairly
uniform
scenario
of
climatic
change
and
tree
response
to
a
changing
environment.
The
Holocene from
10 000
to
3500
BP
was
generally
a
warmer
period
with
several
periods
of
tree-line
advance,
and
it
was
followed
by
a
cooler
period
with
tree-lines
lower
than
or
equal
to
those
of
today.
The
climate
in
the
maritime
and
Mediterranean
zones
was
generally
warm
and
dry
during
the
early
Holocene
and
became
cooler
and
more
moist
c.
3500
BP
(LaMarche,
1973;
Clague
and
Mathewes,
1989).
Conversely,
precipitation
in
the
continental
zone
has
decreased
since
the
early
Holocene
(Andrews et
al.,
1975;
Petersen
and
Mehringer,
1976;
Markgraf
and
Scott,
1981;
Carrara et
al.,
1984).
In
addition,
precipitation
patterns
have
influenced
tree
growth
form,
as
well
as
the
magnitude
and
timing
of
tree-line
movement
at
some
locations
(LaMarche, 1973).
However,
the
generally
synchronous
reconstruction
of
past
tree-line
fluctu-
ations
indicates
that
temperature
is
the
predominant
force
determining
tree-line
location
on
a
broad
geographic
scale.
Recent
tree
establishment
in
sub-alpine
meadows
Increases
in
tree
establishment
in
sub-alpine
meadows
have
been
documented
in
mountainous
areas
throughout
western
North
America
(Table
2).
Most
areas
show
initial
forest-
margin
expansion
after
AD
1890
and
significant
establish-
ment
peaks
between
1920
and
1950.
Additional
peaks
of
establishment
have
also
been
identified
in
some
areas.
Most
studies
conclude
that
recent
increases
in
tree
establishment
are
the
result
of
a
warmer,
drier
climate
following
the
’Little
Ice
Age’
(Franklin
et
al.,
1971;
Kearney,
1982;
Heikkinen,
1984;
Butler,
1986).
Although
most
studies
conclude
that
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95
Table
2
Summary
of
recent
sub-alpine
tree
invasion
attributed
to
climate.
Studies
are
from
the
United
States
except
as
indicated.
:
these
increases
were
caused
by
warming
trends,
it
is
uncertain
whether
this
is
a
long-term
directional
change
or
a
short-term
variation
in
relatively
stable
ecotones.
Maritime
In
one
of
the
first
studies
on
recent
increases
of
tree
establish-
ment
in
sub-alpine
meadows,
Brink
(1959)
documented
es-
tablishment
of
Abies
lasiocarpa
and
Tsuga
mertensiana
between
1919
and
1939
in
heather
communities
(Phyllodoce
spp.lCassiope
mertensiana)
of
the
Coast
Range,
British
Co-
lumbia.
Heather
communities
were
on
topographic
con-
vexities,
with
earlier
snowmelt
and
a
longer
growing
season.
Soils
were
also
more
xeric
than
surrounding
depressions,
but
not
as
dry
as
south-facing
forb
meadows
without
trees.
Climatic
warming
during
this
period
is
supported
by
evidence
of
decreased
glacial
volumes.
Amo
(1970)
noted
massive
invasions
of
heather
commu-
nities
by
Larix
lyallii,
T.
mertensiana
and
A.
lasiocarpa
in
the
northern
Cascade
Mountains
of
Washington.
Most
of
the
establishment
occurred
from
1919
to
1937,
although
about
half
of the
L.
lyallii
had
invaded
the
area
between
1885
and
1910.
Douglas
(1972)
also
documented
meadow
invasion
by
T.
mertensiana
and
A.
amabilis
in
heather
communities
in
the
northern
Cascades.
He
attributed
establishment
to
several
consecutive
years
of
low
snow
accumulation,
moderate
spring
and
early
summer
temperatures,
and
the
black-body
effect
of
tree
clumps
that
caused
faster
snowmelt
around
trees.
Tree
establishment
had
been
occurring
for
120
years
in
his
study
area,
with
peak
establishment
from
1920
to
1940.
Franklin et
al.
(1971)
surveyed
six
areas
in
the
Cascade
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96
Mountains
of
Washington
and
Oregon
and
documented
meadow
invasion
by
A.
lasiocarpa,
T.
mertensiana
and
L.
lyal-
lii
(one
site
in
the
northern
Cascades).
Most
invasion
oc-
curred
between
1925
and
1950,
predominantly
by
A.
lasiocarpa.
Seedling
density
and
growth
rates
varied
with
vegetation
type,
with
highest
densities
in
heather
commu-
nities
and
lowest
densities
in
Festuca
communities.
Invasion
was
attributed
to
a
series
of
warm,
dry
years
with
low
snowpack
and
the
possible
influence
of
large
seed
crops.
Henderson
(1974)
suggested
that
periods
of
establishment
in
this
area
occurred
during
warm
periods
with
average
or
above
average
precipitation,
which
reduced
moisture
stress
on
seedlings.
Heikkinen
(1984)
found
that
tree
advance
in
the
northern
Cascades
followed
the
same
temporal
patterns
as
noted
by
Franklin
et al.
(1971)
and
Brink
(1959).
The
oldest
trees
dated
back
to
1886,
and
tree
establishment
continued
until
1944,
with
invasion
peaks
during
1925-1934
and
1940-1944.
Tsuga
mertensiana
predominated
at
the
highest
elevations,
A.
lasio-
carpa
at
mid-elevations,
and
A.
amabilis
at
low
elevations.
In
a
study
of
stand
dynamics
in
the
northern
Cascades,
Lowery
(1972)
found
that
invasion
throughout
the
meadows
started
about
1920,
coinciding
with
the
expansion
of
older
tree
clumps,
warmer
weather
and
glacial
retreat.
Abies
lasio-
carpa
was
the
most
abundant
species,
especially
at
xeric
sites
such
as
the
centre
of
heather-dominated
mounds.
Tsuga
mertensiana
grew
on
wet,
cool
sites,
while
A.
amabilis
and
Chamaecyparis
nootkatensis
established
on
moist,
lower-ele-
vation
sites
and
the
exterior
of
tree
clumps.
Tree
invasion
and
enlargement
of
tree
clumps
continued
until
the
1950s,
when
a
cooler,
wetter
climate
prevailed
and
glacial
recession
ceased.
Studies
in
the
Olympic
Mountains
of
Washington
docu-
mented
tree
invasions
in
meadow
basins
with
normally
heavy
winter
snowpack
(Fonda
and
Bliss, 1969;
Kuramoto
and
Bliss,
1970).
These
studies
inferred
that
tree
establishment
occurred
during
periods
of
warm,
dry
weather.
There
were
several
discrete
peaks
of
establishment:
1923-1933,
1943-1948
and
1953-1960.
These
studies
also
found
that
establishment
was
highest
in
heather
communities,
and
that
tree
height
in-
creased
with
distance
from
melting
snow.
Agee
and
Smith
(1984)
studied
tree
establishment
on
three
sub-alpine
fire
sites
in
the
Olympics.
Fire
removed
all
vegeta-
tion
and
therefore
reduced
protection
from
sun,
drought
and
browsing
ungulates,
resulting
in
a
time
lag
of
40-70
years
before
seedlings
could
germinate
and
survive.
Burned
sites
had
peak
establishment
during
wet
periods
(1950s),
while
unburned
heather
sites
had
establishment
peaks
during
both
dry
periods
(1920-1950)
and
wet
periods
(1950s).
The
authors
concluded
that
mesic
conditions
were
optimal
for
tree
estab-
lishment ;
therefore,
drier,
burned
sites
required
wet
periods
for
establishment,
and
wetter,
unburned
areas
required
warm,
dry
periods.
Continental
Kearney
(1982)
noted
invasions
of
Abies
lasiocarpa
and
Picea
engelmannii
at
three
locations
in
the
Rocky
Mountains
of
southern
Alberta.
Invasion
occurred
during
1940-1960
and
was
especially
prominent
from
1965
to
1973.
Both
invasion
periods
were
correlated
with
warm,
dry
summers.
In
addition,
growth
rates
of
trees
on
heather
hummocks
were
often
twice
those
of
trees
in
the
hollows
between
hummocks.
Tree
invasion
in
the
Lemhi
Mountains
of
Idaho
began
in
the
1890s
and
continued
until
the
1950s
(Butler,
1986).
Invasion
peaks
occurred
in
1905-1919
and
1925-1945,
corre-
sponding
with
warm,
dry
periods
with
decreased
snowpack.
Tree
establishment
patterns
varied
slightly
among
the
three
meadows
studied,
indicating
some
variation
in
the
causal
agent
(climate)
or
site
conditions.
Invasion
peaks
in
the
lowest-elevation
meadow
were
earlier
than
in
the
other
two
by
about
ten
years.
Charcoal
in
the lowest
meadow
suggests
that
frequent
fire
could
have
limited
tree
establishment.
Daly
and
Shankman
(1985)
found
high
densities
of
Picea
engelmannii
and
Pinus
flexilis
seedlings
(5-27
years
old)
above
the
tree-limit
in
the
Colorado
Rocky
Mountains.
The
high
ratio
of
seedlings
to
trees
was
interpreted
as
an
increase
in
seedling
establishment,
which
could
produce
an
advance
of
the
tree-limit.
Mediterranean
Establishment
above
the
tree-limit
was
also
documented
at
one
site
in
the
Sierra
Nevada
(Scuderi,
1987)
and
two
sites
in
the
White
Mountains
of
California
(LaMarche,
1973).
Small,
erect
seedlings
of
Pinus
balfouriana
have
established
5-10
m
above
the
present
tree-line
at
Cirque
Peak,
Sierra
Nevada,
over
the
last
100
years.
Large
numbers
of
P.
longaeva
have
established
above
the
tree
limit
in
the
White
Mountains
since
1850.
Few
trees
established
between
1450
and
1850
at
one
site,
and
no
trees
established
between
1250
and
1850
at
the
other
site.
In
summary,
the
literature
indicates
that
climatic
warming
since
1850
has
resulted
in
periods
of
increased
tree
establish-
ment
in
sub-alpine
park
lands.
Studies
of
recent
tree
estab-
lishment
are
concentrated
in
the
maritime
zone,
so
it
is
difficult
to
determine
the
geographic
extent
and
variation
of
tree
establishment
in
western
North
America.
Modern
studies
demonstrate
a
more
complex
climate/site
interaction
than
can
be
inferred
from
palaeoecological
studies
of
tree-line
fluctuations.
Effect
of
potential
global
climate
change
on
tree
establishment
Our
review
indicates
that
weather
components
frequently
limit
establishment
of
trees
in
sub-alpine
and
alpine
meadows
in
the
maritime,
Mediterranean
and
continental
climatic
zones
of
western
North
America.
A
comparison
between
climatic
variables
and
limits
to
tree
establishment
(Table
3)
shows
that
the
most
extreme
characteristic
of
each
zone
is
often
the
most
limiting
factor,
specifically:
(1)
winter
precip-
itation
(snowpack)
in
maritime
climates,
(2)
cold
soil
tem-
peratures
and
high
winds
in
continental
climates,
and
(3)
summer
drought
in
Mediterranean
climates.
Our
review
suggests
that
tree
establishment
occurs
in
any
of
these
climates
when
these
extreme
conditions
are
alleviated.
Pre-
dictions
of
tree
establishment
on
a
fine
scale
are
difficult,
however,
because
establishment
patterns
are
influenced
by
local
variations
such
as
microtopography
and
disturbance
frequency.
Predictions
of
future
climate
are
based
on
numerical
mod-
els
limited
by
incomplete
understanding
of
atmospheric
and
oceanic
processes,
interactions
and
feedbacks.
Furthermore,
current
computer
capacities
limit
spatial
resolution
and
re-
strict
the
coupling
of
climate
system
components
whose
timescales
differ
by
fourteen
orders
of
magnitude
(Schle-
singer
and
Mitchell,
1987;
Cubash
and
Cess,
1990).
Models
predict
temperature
changes
most
accurately
and
are
less
able
to
predict
changes
in
precipitation
and
soil
moisture,
which
are
products
of
complex
interactions
within the
hydrologic
cycle.
Models
also
have
difficulty
in
resolving
subcontinental
variation.
Despite
weaknesses,
climate
predictions
generated
by
general
circulation
models
(GCMs)
provide
the
best
available
description
of future
climate.
We
have
summarized
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97
Table
3
Levels
of
climate
variables
characteristic
of
three
climate
zones,
with
the
extremes
for
each
climate
indicated
by
bold
type.
These
extremes
are
also
the
limiting
factors
for
tree
establishment
for
each
zone.
Table
4
Range
of
changes
in
climate
variables
predicted
by
three
general
circulation
models
at
equilibrium
with
doubled
atmospheric
C02.
Values
indicate
change
from
present.
Models
were
developed
by
the
Canadian
Climate
Centre,
the
Geophysical
Fluid
Dynamics
Laboratory
and
the
United
Kingdom
Meteorological
Office
(Mitchell et
al.,
1990).
Note:
~
Represents
the
available
water
in
the
upper
layers
of
soil
having
field
capacity
of 15
cm.
the
range
of
changes
predicted
for
several
climate
variables
for
the
three
climate
zones
based
on
predictions
of
response
to
doubled
C02
from
three of
the
highest-resolution
models
(Table
4 -
Mitchell et
al.,
1990).
The
most
consistent
conclusions
of the
models
are:
(1)
winter
soil
moisture
will
increase
in
all
three
climates;
(2)
winter
and
summer
temperatures
will
increase
in
all
three
climates,
but
especially
in
the
continental
climate;
(3)
summer
soil
moisture
will
decline
in
maritime
and
continental
cli-
mates ;
and
(4)
distribution
of
precipitation
in
the
maritime
climate
will
be
skewed
even
more
heavily
towards
winter.
Predictions
for
winter
and
summer
precipitation
and
summer
soil
moisture
in
the
Mediterranean
climate
vary,
although
two
of
the
three
models
predict
an
increase
in
each
case.
Climate
model
predictions
also
provide
a
basis
for
specula-
tion
about
future
distribution
of
vegetation,
particularly
trees.
The
predicted
changes
for
all
three
climates
should
alleviate
the
identified
constraint
to
tree
establishment
(Table
3).
Winter
precipitation
is
predicted
to
increase
in
the
maritime
climate,
but
warmer
temperatures
are
expected
to
cause
more
to
fall
as
rain
at
higher
elevations,
resulting
in
less
snowpack.
Summer
drought
is
predicted
to
lessen
in
Mediterranean
climates.
Models
predict
increased
winter
temperatures
and
precipitation
in
continental
areas,
although
temperatures
will
still
be low
enough
to
produce
snow.
This
may
provide
greater
wind
and
cold
protection
than
current
conditions.
However,
changes
that
favour
sub-alpine
establishment
may
be
negated
by
other
changes
such
as
summer
drought,
which
is
predicted
to
intensify
in
maritime
and
continental
zones,
possibly
preventing
tree
establishment.
There
is,
of
course,
a
high
degree
of
uncertainty
about
how
changes
in
different
combinations
of
climatic
variables
might
affect
future
pat-
terns
of
sub-alpine
tree
establishment.
Moreover,
changes
in
other
factors
influenced
by
climate,
such
as
fire
frequency,
may
also
affect
tree
establishment
(Franklin et
al.,
1991).
In
summary,
we
expect
an
increase
in
tree
establishment
(i.e.,
higher
tree-lines,
increased
meadow
invasion)
in
all
three
climate
zones.
Tree-lines
may
be
similar
to
those
of
the
early
Holocene,
when
the
climate
was
similar
to
the
predicted
future
climate.
A
similar
prediction
has
been
made
from
a
landscape
model
for
a
single
site
in
the
continental
zone
(Romme
and
Turner,
1991).
Moreover,
we
may
be
observing
the
beginning
of
a
trend
in
response
to
the
warmer
climates
of
this
century
(Brink,
1959;
Butler,
1986;
Franklin et
al.,
1971;
LaMarche,
1973;
Kearney,
1982;
Rochefort
and
Peterson,
1991;
Woodward et
al.,
1991).
This
very
general
conclusion
must
be
qualified
because
(1)
small-scale
variability,
disturb-
ance,
availability
of
suitable
substrate,
and
other
variables
not
directly
related
to
climate
make
it
difficult
to
specify
precise
situations
in
which
establishment
will
occur;
(2)
loss
of
potential
invasion
sites
due
to
the
imposition
of
new
con-
straints
by
changing
climate
has
not
been
described;
and
(3)
the
predictions
of
GCMs
are
uncertain.
Therefore,
we
predict
that
vegetation
zones
will
not
simply
migrate
up
in
elevation
and
latitude
(Dale
and
Franklin,
1989;
Peters,
1990),
but
that
sub-alpine
species
may
be
distributed
differently
in
the
future
’1!’
than
in
the
past
(Brubaker, 1988;
Davis, 1989).
Detailed
study
of
site
conditions
and
tree
regeneration
requirements
will
help
refine
predictions
of
the
effects
of
climate
on
forest
establishment.
Long-term
monitoring
at
several
sites
will
determine
whether
recently
observed
patterns
of
sub-alpine
tree
establishment
will
continue.
Acknowledgements
We
are
grateful
to
James
Benedict,
Greg
Ettl,
Tom
Hinckley,
Darci
Horner,
John
Innes,
Brian
Luckman,
Kathleen
Mar-
uoka,
David
W.
Peterson,
June
Rugh,
Ed
Schreiner,
David
Silsbee,
Doug
Sprugel
and
an
anonymous
reviewer
for
helpful
comments
on
the
manuscript.
Beth
Rochefort
assisted
with
graphics.
This
study
was
supported
by
the
US
National
Park
Service
Global
Change
Program
and
the
USDA
Forest
Service
Pacific
Northwest
Research
Station.
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98
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