Fire and the Forest History of the North Cascade Range
Author(s): Les C. Cwynar
Reviewed work(s):
Source: Ecology, Vol. 68, No. 4 (Aug., 1987), pp. 791-802
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1938350 .
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Ecology, 68(4), 1987, pp. 791-802
© 1987 by the Ecological Society of America
FIRE AND THE FOREST HISTORY OF THE
NORTH CASCADE RANGE1
LES C. CWYNAR2
Departmentof Botany, Universityof Toronto,Toronto,OntarioM5S IA1, Canada
Abstract. Postglacial vegetation changes are often ascribed to the direct effects of
climate change. I studied pollen, plant macrofossils, and sediment charcoal in order to
determine the potential role of changes in the disturbance (fire) regime in the postglacial
development of local vegetation at Kirk Lake in the foothills of the North Cascade Range
in northwestern Washington. Five pollen assemblage zones are recognized: a Pinus-Populus
zone > 12 000 BP, a Picea-Alnus sinuata zone from > 12 000 to 11 030 BP, an Alnus
rubra-Pteridium zone from 11 030 to 6830 BP, a Cupressaceae zone beginning at 6830
BP, and a late Holocene Pinus-Alnus rubra zone from 2400 to 900 BP.
The first forests (> 12 000 BP) were an open mixture of conifers and deciduous trees,
chiefly Tsuga mertensiana, Abies, Pinus contorta, and Populus on a landscape subject to
erosion. Just before 12 000 BP, the pioneer species Picea sitchensis, Alnus rubra, and A.
sinuata became important constituents of the forest. Although pollen accumulation rates
were high, the abundance of Alnus sinuata indicates an open-canopy forest. Beginning
11 200 BP, climatic warming initiated major changes in forest composititon and the fire
regime. Tsuga heterophylla migrated into the region, rapidly expanded, then declined
shortly thereafter, while Pseudotsuga menziesii, Alnus rubra, and Pteridium expanded, and
Pinus contorta, Picea sitchensis, Populus, and Alnus sinuata declined. The abundance of
Pseudotsuga, Alnus rubra, and Pteridium between 11 030 and 6830 BP corresponds with
increased influxes of charcoal into the sediment; this zone is interpreted as a closed forest
with a relatively high fire frequency and composed of a mosaic of postfire successional
communities in which fire-adapted Pseudotsuga and Alnus rubra predominated over firesensitive Tsuga heterophylla. Pinus monticola became locally important m8000 BP. Between 6800 and 6400 BP Thuja plicata arrived, Tsuga heterophylla expanded, and Alnus
rubra, Pseudotsuga, and Pteridium declined. These changes are accompanied by a reduced
fire frequency, inferred from lower charcoal accumulation rates, and they indicate a shift
to wet-temperate climate similar to today's. The late Holocene fossil record shows the
development of the adjacent peatland, which Pinus contorta eventually invaded.
Key words: CascadeRange; charcoal;fire;forest history;paleoecology;palynology;plant macrofossils; Quaternarystudies.
INTRODUCTION
Recent studies of vegetation emphasize the importance of disturbance (White 1979, Mooney and Godron 1983, Pickett and White 1985). The composition
of forests is partly determined by the disturbance regime, which includes average rates, distribution in
space, and severity of disturbances (Loucks 1970,
Mooney and Godron 1983, Shugart 1984, Pickett and
White 1985, Runkle 1985). In forests, disturbances
occur on various scales, ranging from small gaps created by the death of one or a few trees, leading to gapphase regeneration, to large gaps created by a variety
of external factors, such as fire or wind, leading to
secondary succession (Runkle 1985).
Delcourt et al. (1983) developed an hierarchical
model of how vegetation patterns arise from the in-
teraction of physical and biological processes on three
time scales: micro (1-5000 yr, 1-106 m2), macro (5 x
103-106 yr, 106-1012 m2), and mega (>106
yr, >10'2
m2). They concluded that "the Quaternary paleoecological record demonstrates the importance of climatic
change as the dominant influence upon vegetational
processes and patterns at the macro-scale of spatialtemporal resolution" and that "even at the micro-scale,
prevailing disturbance regimes that affect plant succession must be viewed within the context of their prevailing macroclimate." When macroclimate changes,
vegetation may therefore be expected to respond not
only to the direct effects of change in climate, but also
to the indirect effects of change in the disturbance regime.
Although fire is an important agent of disturbance
in the modern evergreen coniferous forests of the PaManuscriptreceived 8 April 1986; revised 15 September cific Northwest (PNW) and some forest types depend
1986; accepted 19 September1986.
2
Presentaddress:Departmentof Biology,SirWilfredGren- on it for their regeneration (Franklin and Dyrness 1973,
Agee 1981, Hemstrom and Franklin 1982), the postfell College, Memorial University of Newfoundland, Corer
Brook,NewfoundlandA2H 6P9, Canada.
glacial development of PNW forests is generally inter-
792
Ecology,Vol. 68, No. 4
LES C. CWYNAR
preted as resulting from the direct effects of climate
change. The general sequence of postglacial vegetation
change at lowland sites in the northern Puget Trough
(Hansen 1938, 1947, 1948, Hansen and Easterbrook
1974, Baker 1983, Heusser 1983, Barnosky 1984) begins with open forests of various admixtures of temperate lowland (Pinus contorta, Populus) and montane
taxa (Abies lasiocarpa, Tsuga mertensiana). At - 11 200
BP temperate lowland species began to replace montane species so that by 10 000 BP open forests dominated by Pseudotsuga and Alnus rubra had developed
at most sites. These forests persisted until -6000 BP
when modern closed forests of temperate conifers
(Pseudotsuga, Thuja plicata, Tsuga heterophylla) developed. The climates inferred from this sequence are
cool-maritime conditions initially, gradually warming
to a warmer and drier period relative to modern climate between 10 000 and 6000 BP, and then the development of the modern humid climate after 6000
BP.
The role of fire in either maintaining plant communities or in initiating vegetation change has increasingly attracted the attention of Quaternary paleoecologists, especially since charcoal is abundant in lake
sediments and easily quantified (Waddington 1969,
Swain 1973, Cwynar 1978, Green 1981, Grimm 1983,
Lamb 1985, Winkler 1985a, b, Dunwiddie 1986). Some
researchers have recognized the potential significance
of fire in the postglacial development of PNW forests
(Hansen 1947, Heusser 1973a, Mathewes 1973, Leopold et al. 1982), but only Sugita and Tsukada (1982)
and Dunwiddie (1986) have provided detailed studies
of charcoal from lake sediments, and Mathewes (1985)
has stressed the need for direct evidence of past fire
regimes from sedimentary charcoal profiles for a full
understanding of postglacial vegetation change in the
PNW. My primary objective in this paper is to reconstruct the history of local vegetation around Kirk Lake,
Washington in relation to disturbance by fire. I specifically address the following questions: (1) Are postglacial changes in vegetation associated with changing
fire regimes? (2) What was the nature of the vegetation
between 10 000 and 6000 BP and what was the relative
importance of change in climate vs. the disturbance
regime in initiating the development and maintenance
of this vegetation, which has generally been interpreted
as an open Pseudotsuga woodland possibly maintained
by fire and resulting from the direct effects of a much
drier climate relative to modern? I argue that the vegetation was a closed forest, but more importantly, that
a change in the fire regime and not the direct effects of
climate change initiated and maintained this forest.
SITE DESCRIPTION
Kirk Lake is small (0.6 ha), relatively deep (7.6 m),
and is on the western flank of the North Cascade Range
at 190 m altitude (Fig. 1; 121°37' W, 48°14' N; Section
23 N/P, Township 32 N, Range 9 E). It lies just within
the maximum extent of the Cordilleran Ice Sheet (Waitt
and Thorson 1983) of the Fraser Glaciation (circa
25 000-10 000 BP). The North Cascades near Glacier
Peak were deglaciated before 12 250 BP (Beget 1984).
The vicinity of Kirk Lake was deglaciated before
I 1 500 BP, and the lake probably formed as a kettle
in outwash or kame terrace (J. E. Beget, personal
communication). Glaciers in the region re-advanced in
the early Holocene
8400-8300 BP (Beget 1984). No
bedrock is exposed within the drainage basin of Kirk.
Surrounding mountains are composed of crystalline
rocks, principally schists (McKee 1972).
The lake lies within the Tsuga heterophylla Zone
(Franklin and Dyrness 1973), which regionally is dominated by Pseudotsuga menziesii (Douglas-fir), Tsuga
heterophylla (western hemlock), and Thuja plicata
(western red cedar). Abies amabilis (Pacific silver fir),
A. grandis (grand fir), Picea sitchensis (Sitka spruce),
Pinus contorta (lodgepole pine), and P. monticola
(western white pine), are minor constituents. The most
common deciduous tree is Alnus rubra (red alder). Near
the lake, Thuja plicata and Tsuga heterophylla are the
most common trees together with some Pseudotsuga
menziesii. A peatland on the east side supports a stand
of Pinus contorta with scattered Picea sitchensis and a
shrub understory of Ledum groenlandicum (Labrador
tea) and some Alnus sinuata (Sitka alder). A small stand
of Tvpha latifolia (cattail) grows at the north end of
the lake.
METHODS
I collected a 12.44-m core from peat on the west
shore of the lake with a 5 cm diameter modified Livingstone piston sampler (Wright 1967) and help. I removed samples of 0.5 cm3 with a calibrated brass sampler (Birks 1976) for pollen analysis (at 20-cm intervals
above 9 m, and 5- or 10-cm intervals below 9 m) and
to determine percent mass-loss on ignition (LOI) at
550°C. Tablets of Eucalyptus pollen or Lycopodium
spores were added to pollen samples for determination
of pollen concentration and pollen accumulation rates
(PARs). I prepared samples for pollen analysis using
standard methods (Faegri and Iversen 1975) and, in
addition, sieved basal inorganic sediments to remove
fine particles (Cwynar et al. 1979). I tallied a minimum
sum of 300 upland pollen types (trees, shrubs, terrestrial herbs, pteridophyte spores, and unknowns) for
each sample and used it to calculate pollen percentages.
The Haploxylon/Diploxylon determination of pine
pollen was based on a minimum of 25 grains per sample. I zoned the pollen diagrams using constrained cluster analyses (Gordon and Birks 1972). Charcoal was
analyzed by the method of Waddington (1969). Clark
(1984) has shown that charcoal counts derived from
pollen preparations are sensitive to the preparation
techniques used; I treated all samples identically during
sample preparation.
For plant macrofossil analysis, I removed 5 cm long
FORESTAND FIRE HISTORYOF NORTH CASCADES
August 1987
core segments, measured their volumes (generally 6080 cm3) by displacement in a graduated cylinder, wetsieved each through screens of 1000-, 425-, and 106Almmesh (clayey or silty samples were disaggregated
in a 5% sodium pyrophosphate solution), and then
picked, identified, and enumerated all seeds, fruits, conifer needles, and other recognizable plant parts from
the residues. Identifications were based on modern reference material. I embedded in wax several conifer
needles from basal sediments and sectioned them for
critical identifications. I included only samples with a
minimum of 20 conifer needles in the preparation of
the percentage diagram of conifer macrofossils (Dunwiddie 1987), with the exception of two samples from
the early postglacial period; I amalgamated some samples in order to raise the needle sum to 20.
793
r
STRATIGRAPHY
Numerous coarse roots and wood fragments prevented the collection of the uppermost 1 m of sediment. None of the core reacted with 10% HC1, indicating the absence of carbonates. The sediment
stratigraphy was as follows (Munsell Color descriptions
[Munsell Color 1975] are given in parentheses):
1200-1244 Banded sandy clay with some silt. Coarse
sand at the very bottom (N 3/0). Inorganic.
Depth
below
surface
(cm)
100-360
360-715
FIG. 1. Location of Kirk Lake and the sites frequently
mentioned in the text.
Description
Peat, well-decomposed at the bottom and
coarser at the top (7.5 YR 3/4).
Coarse gyttja grading to decomposed peat
Below the ash couplet LOI was <6% (see Fig. 3),
whereas above the ash LOI rose rapidly to 40-75%
throughout most of the core, increasing to a maximum
of 94% in the upper peat.
(5 YR 4/3).
715-1151 Fine detritus gyttja grading upward to
coarse gyttja. (Grading in color from 7.5
YR 2/1 to 5 YR 2/3.) A volcanic ash occurred between 775-771 (2.5 YR 8/1).
1151-1184 Predominantly silt (N 4/0) with complex
banding of thin sand and clay layers and
some brownish, slightly organic layers. A
couplet of volcanic ash layers occurred at
1151-1153 cm; each layer was -2 mm
thick and formed a sharp contact with adjacent sediment. Slightly organic, brownish silt lay between the ash layers.
CHRONOLOGY
Seven radiocarbon dates (Table 1) were obtained
from the lower half of the core where most of the pollen
stratigraphic events occur. The geographic distribution
of Holocene tephra layers (Sarna-Wojcicki et al. 1983),
the pollen stratigraphy, and radiocarbon dates indicate
that the tephra at 771-775 cm is from Mount Mazama.
Its age of ;6800 BP (S. Porter, personal communication) is used as another point on the age-depth curve.
Deposition times were calculated by linear interpolation (Fig. 2). The two oldest dates (14 610 and 13 250
BP) may not be reliable because the sediment dated
has a low organic content (<6%), which may result in
spuriously old dates (Olsson 1979). If these dates are
TABLE
1. Radiocarbondates from Kirk Lake, Washington. correct, the sedimentation rate would have been very
slow, which seems unlikely because of the many small
Sediment depth
Radiocarbon age
Laboratory
layers of sand, which probably sedimented rapidly. An
number
(BP + 1 SD)
interval (cm)
hiatus below the ash couplet is an alternative to slow
+
40
8600
865.0-870.0
QL-1614
continuous sedimentation. In either case, reliable deQL-1615
960.0-965.0
9580 + 60
position times cannot be based on the two oldest dates.
10510 + 60
QL-1616
1060.0-1065.0
Therefore, I extrapolated deposition times downward
QL-1617
1105.0-1110.0
11 180 + 60
11500 + 160
QL-1618
1125.0-1130.0
only to the top of the ash couplet at 11.51 m and
13 250 + 90
1153.5-1158.0
QL-1649
upward to the change in sediment type from fine to
14610 + 220
1165.0-1173.0
QL-1619
coarse detritus gyttja.
LES C. CWYNAR
794
Ecology,Vol. 68, No. 4
associated PARs (Fig. 4), charcoal accumulation rates
(CHARs; Fig. 4), and plant macrofossils (Figs. 5 and
gyttja
14
I Siltgyttja
6) are summarized in Table 2.
I
_
Although it is clear from the superposition of the
,
CQ 12pollen zone boundaries onto the macrofossil concen6.00
tration diagram (Fig. 5) that there is excellent agree14.89
ment between the pollen and macrofossil data, there
co
_
I
O
10,-.
9.30
i
are, nevertheless, several important local changes in
the
vegetation that are not as evident in the pollen
2
LxJ
1
/10.3
I
profiles as in the macrofossil record. In the mid-HoI
19
8
locene there is a shift from Diploxylon to Haploxylon
pine pollen, although the overall abundance of pine
A
remains low. The macrofossil record, however, shows
10
11
12
6
9
8
7
that P. monticola became important in the local vegeDEPTH(m)
tation when this shift occurred. Similarly, when Picea
FIG. 2. Age-depth curve for Kirk Lake. · Mazamatephra pollen appears consistently in the late Holocene at val(6800 BP), · radiocarbon dates used in the calculation of ues < 2% (a subtle change that occurs at other sites and
depositiontimes in yearsper centimetre, dates that are not is generally disregarded in vegetation reconstructions),
consideredreliable.
leaves of Picea sitchensis recur indicating its local reappearance.
RESULTS AND INTERPRE'TATION
The percentage macrofossil diagram for conifers (Fig.
Five local pollen assemblage zo nes are informally
6) indicates several interesting trends. Pseudotsuga was
recognized on the basis of constrairled cluster analysis
clearly the dominant conifer in the early Holocene, but
(Fig. 3). The chief features of the,se zones and their
Tsuga heterophylla was important locally despite its
Xp
14
-
t
Coarse
--detritus
ti
Fine detritus g
KIRK LAKE, Washington
Trees
>s
\C
(0 b\
.c\
\Cs?
5.5.y
Co
*
*$o
Hrr,
k4-rkkr-s
-
E
0
a
and Shrubs
S
O
I
e
Q(5
k I
I
S
,r' /^'_.
6
" a?4
iiii,HVr~mkr-n-,-r
hhlF,-, hH
' '
.\
to
/
a
S~o'
cj~osb~b·'~
·~ive~'o
HH
H
0 H
HHHc
c [hllh
' '
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4-1-
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Pollen
05.sseemblage zones
:
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.
.
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.
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.
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.
*:
.
.
.
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I
.
.
.
PinusAlnus rubro
.
3-
LI
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5-
Cupressaceae
ii
1-
.E
.0
6-
7-
I
ash
j~~I
14c
8-
I
.1
- 8600
I
9-
: dates
(yr BP)
:
.a)
Glacier
{Peak
couplet
Silt
a y
clay
i!
10 -
U.
1 -
-2 -
llrrrTrfll
0
IC
I
' ". .'
w
Ir-f
60
i i'
m rI
r
.8
10
"T
r
r rpp-p-r-m
r I I I,, r-,
T
M
ppP
: -9580
Alnus rubraPteridium
, .:-10
!
510
. _ 11 180
: :
Picea*
'**
0m1 500 KL-2
* : | ' i *i i *
:
-11
Alnus sinuafa
!' : .
t:** ' : : :: : . :
. . :
. : : -13 250 KL-1Pinus-Populus
-rrr14610
-
iI 1 1
0
.
KL-3
I
I1' : :
Pl rrPpplpprprnp
50
FIG.3. Pollen percentagediagramfor Kirk Lake.
FORESTAND FIRE HISTORYOF NORTH CASCADES
August 1987
the lake muds of Kirk Lake. Second, Pseudotsuga and
other conifers have been selectively logged so that sediments show a striking decrease of conifer pollen and
an increase of Alnus pollen, attributable to A. rubra,
which colonizes disturbed environments (Davis 1973,
Heusser 1978a, c, Barnosky 1981, Sugita and Tsukada
1982, Dunwiddie 1987). Pollen spectra immediately
below the settlement horizon better describe the pollen
rain of the natural forests of the region (Davis 1973,
Heusser 1978d), but they are too few in number to
form an adequate basis for comparison.
Kirk Lake is small, so most of the pollen it receives
is derived from local or extra-local sources, i.e., from
plants growing within several hundred metres of the
lake (Jacobson and Bradshaw 1981). Plant macrofossils
are also derived from local source areas, but areas
smaller than those for pollen (Watts 1978, Birks 1980).
The reconstructions possible from the record at Kirk
are therefore of the local vegetation, but because of the
lack of modern pollen and macrofossil samples for
comparison with the fossil assemblages, the reconstructions are necessarily qualitative and based on the abundances of various taxa in the pollen and macrofossil
records. When interpreting the pollen record, I have
low pollen percentages. Furthermore, Pseudotsuga declined in local abundance in the mid-Holocene as Tsuga heterophylla and Thuja plicata increased. The percentage diagram highlights these trends, but in general
there is little difference between the concentration (Fig.
5) and percentage (Fig. 6) macrofossil diagrams. When
interpreting these diagrams, however, we should bear
in mind that nothing is known about the potential for
differential preservation of macrofossils and that deciduous species were important (Populus and Alnus
rubra) early in the postglacial period.
Basis for interpretation
Paleoecologists now often use quantitative methods
to compare fossil pollen assemblages with pollen assemblages produced by modern vegetation so that
vegetation reconstructions may be refined (MacDonald
and Ritchie 1986). This approach was not possible in
this study for two reasons. First, pollen spectra from
different types of deposits are not directly comparable
(e.g., Ritchie 1974, Birks 1977). Most samples of modern pollen spectra from the PNW are derived from
moss polsters (Heusser 1973b, 1978a, c, d) and therefore cannot be compared with the fossil spectra from
Herbs
P
os
,. /
s
·/
795
Aquatics
and
Plants
Marsh
/
·
len
I
I
Lih
I`
i
I ~ 'i'~ HH
VT-d-iHHHF4-4,sV hFH
~l? H
I;:.,!.1hh~-1hhh
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-· Ihh
I
'i-- n.
H h, -·, ih rr. ... hhh'hhhhhhll
.
· I
:r
E
...
r
i-
.-1
j~~~~~~~~~~
I
.~:·::::
I
r-mlrlm r rPP
0 20 10
rrlrnrPrrrrrrrnrpnrTlmll
FIG. 3.
-
__
i
0 500
1
m nn""rr
P
010
Continued.
i"
0
500 0.1
103 grains/cm3
0.3
0.5
LES C. CWYNAR
796
Ecology, Vol. 68, No. 4
I
U0
.l . I I
II
5
FIG. 4.
.I
.l
10
7l
11 I
15
TI
l
5
I
60
I
2.5
5.0
7.5
2
c-2
0 pm ·cm
-myr
10
yr
Pollen and charcoal accumulation rate diagram.
Trees and Shrubs
/I
is~
9'$
~Q
O~
I
f~
9 ,1~
<
\9QNC~
\ (
I 1
.4
~
\.
I
I
,.
I
i
I
I
r.6
I
I .
I·"","
FV
V
6
I
*=
I
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E:
_.
I
:p-
__
*:
,·
" I
iE
SandyTInTIF'
50
'
']' I
510 50100
'-- 10
-I
N
2
I
·
100
i
·
I
100
I
400
10
100
3
MacrofossilConcentration
(macrofossils/100 cm )
FIG. 5. Concentration diagram of plant macrofossils from Kirk Lake. For the trees and shrubs, all macrofossils are leaves
unless otherwise shown. For all other plants, they are fruits or seeds, unless otherwise indicated. The concentration data are
plotted on a log scale.
FORESTAND FIRE HISTORYOF NORTH CASCADES
August 1987
borne in mind that Pseudotsuga and Tsuga heterophylla are generally under-represented and Alnus rubra
over-represented in modern pollen spectra relative to
their abundance in the vegetation (Heusser 1973b,
1978a, c, d). Dunwiddie (1985, 1987) has shown that
in the PNW, conifer needles can be identified to species
and are abundant in the sediments of small lakes and
ponds; their abundance in recent sediments is proportional to the importance of the species in the local
vegetation (within 30 m of the site). Therefore, although I have considered the pollen and macrofossil
records together, I have relied more on the specific
determinations and abundances of macrofossils to reconstruct the local vegetation. The pollen record has
been primarily used to correlate the record at Kirk with
the records at other sites.
Swain (1973) showed that past fires around a lake
could be identified by the presence of distinct charcoal
peaks in the sediments of the lake. Changes in the
amount of charcoal accumulating in lake sediments
(CHARs) may therefore be taken as an indication of
changes in the fire regime. However, it is not possible
to determine whether the fire regime is changing with
respect to the frequency or magnitude of fires.
4
Herbs
/4 --:/--
-
Trees and Shrubs¢
E
·o\\\Oc
·
Emergent Aquatics
,
_
o
&
.
0
a
°,°
aI
Paleoecological reconstructions
Pinus-Populus zone (KL-1).--This zone is present
at most sites in the PNW (Barnosky 1981) and is generally interpreted as representing a pine woodland
dominated by Pinus contorta (confirmed at Kirk Lake
by the predominance of Diploxylon-type pollen and
macrofossils), Populus, and Tsuga mertensiana. The
inorganic nature of the sediment indicates that the
vegetation cover was probably too sparse to stabilize
the fresh soil surfaces. The occurrence ofAlnus sinuata,
which today grows in the Cascade Range on inorganic
soils where forest canopies are open (Oliver et al. 1985),
also indicates that the forests were open.
Picea-Alnus sinuata zone (KL-2).--Similar pinedominated pollen assemblages from western Washington have generally been interpreted as representing
open, woodland vegetation. This interpretation is particularly appropriate for sites farther south in the glaciated areas of southwestern Washington (e.g., Barnosky 1981, Tsukada and Sugita 1982) where PARs
are comparatively low (5000-10 000 grains cm-2 yr- ),
the sediment organic content is low, and open-ground
taxa are common (Thalictrum, Epilobium, Saxifraga
and Submerged-
-Floating
.,*,,,S
0Aquatics,6x
\
","^,4'
797
:
:
.
\_
2E-~~~~~~~~
*....':~~~~~~~~~~~~~~..~
*__
-
3-
z-m
:
Pollen
o
°,
Gb
assemblage
/
Pinus -
:Alnus
...L.~
rubra
-
4-:
_
1=
-
.
,
l
.-
5-
r-
. . .
6-
zonesS
KL-4
Cupressaceae
PI
: -"r
7-:
I;
8-.
14C
dates
(yr B P)
9--
F
;.
10-'
::
11
.
'
·
'-
,
··-
. r-
-8600
i:
cl
.
:
-9580
=:16
i_
·
o
510
'
'o
10
--
I r'
2r2
, '' 1
10
r- , -
F
FIG. 5.
·
·
,
:
F
KL-3
Alnus rubra Pteridium
r n
Continued.
m
n m7
I
- rm
I
I
-10 510 KL-2 Picea _11 180 Alnus sinuata
-11 500
-13 250
"14 610
/
LES C. CWYNAR
798
E
Ecology, Vol. 68, No. 4
110 1
, 0
I ,,
9^~')
0
0
NSR
I-
* 0)
II ,
H-1
I
-
O1
I
I-
I
I
I
I
I
I
I
I
1
KL-5
2-.7
3-
I
4-
5-
KL-4
U
6-
7-
8-
I
9KL-3
10-
I
I--
II-
KL-2
0
60
20'
20
I
I-60
60
%
FIG. 6.
20
I
20
i
100
100
100
Percentage diagram of conifer-leaf macrofossils.
tricuspidata-type). At Kirk Lake the abundance of Alnus sinuata indicates that the forest canopy must also
have been partially open. The higher PARs at Kirk,
however, suggest that forests there were less open than
those in southwestern Washington. Because PARs are
a function not only of pollen productivity by vegetation
but also of pollen transport to the lake by streams (Peck
1973, Bonny 1976, Pennington 1979), other explanations for these high PARs are possible, although they
seem improbable. For example, erosion of polleniferous soil into the lake may have inflated the PARs somewhat. However, it is unlikely that that is solely responsible for the high PARs, because PARs remain
high at the end of the zone where the organic component (LOI) of the sediment reaches 60% of sediment
dry mass, in comparison with 6% at the beginning of
the zone. Another possible explanation is that these
high PARs result from changing patterns of sediment
deposition within the lake (Davis et al. 1984). Multiple
cores would be needed to test this hypothesis rigorously, but it seems unlikely because the core was recovered from the side of the basin, far from the presumed center of deposition where the water is 7.6 m
deep today.
The local vegetation around Kirk Lake during this
period was, therefore, a well-developed but nonetheless
somewhat open coniferous forest of Pinus contorta,
Picea sitchensis, Tsuga mertensiana, and Abies lasiocarpa. The abundance of Picea sitchensis probably reflects its ability to colonize open habitats. At present,
both Picea sitchensis and Pinus contorta colonize dunes
along the PNW coast and invade prairie in southwestern Washington
(Franklin and Dyrness 1973).
Populus and Alnus rubra were important deciduous
trees. A. sinuata was most prominent during this time,
probably growing in canopy gaps. Fire was a minor
influence on this vegetation. CHARs in this zone are
moderate relative to the rest of the record and are
consistent with dominance by P. contorta ssp. contorta,
the coastal type that occurs in the PNW today. It differs
from ssp. latifolia, the inland or Rocky Mountain type,
in having open cones that shed their seeds seasonally
(Critchfield 1980). This is generally regarded as an adaptation to a less severe fire regime than that experienced by ssp. latifolia in the drier interior.
Alnus rubra-Pteridium zone (KL-3). -This zone has
been variously interpreted as representing "structurally
open forest, unlike the forest of today" (Heusser 1978b),
unstable transitional open woodlands (Tsukada et al.
1981), woodland (Barosky
1981, Sugita and Tsukada
1982), or either open woodland or a forest mosaic (Leopold et al. 1982, Barnosky 1984). The interpretation
TABLE 2.
799
FOREST AND FIRE HISTORY OF NORTH CASCADES
August 1987
Summary charactersitics of pollen zones and associated plant macrofossils and charcoal.
Pollen zone
KL-1
Informal name
Pinus-Populus
Depth (cm)
Age (BP)
1180-1157.5
>12000
Majorpalynomorphs
Total PARs (103
grains.cm-2
yr-1)
Majorterrestrial
plant macrofossils
CHARs (106
cm-2.
Amm2
yr-1)
Pinus, Populus,
Alnus sinuatatype, Tsuga
mertensiana
...
Pinus contorta
(rare)
***
KL-2
Picea-Alnus sinuata-type
1157.5-1097.5
12 000-11 030
KL-3
Alnus rubraPteridium
1097.5-772.5
11030-6830
Alnus rubraPinus, Alnus sinutype, Pteridata-type,Picea,
ium
Alnus rubra-type
30-65
15-30
KL-4
Cupressaceae
772.5-275
6830-;2400
Cupressaceae,
Tsuga heterophylla, Alnus
rubra-type
that this zone represents an open forest community is
commonly based on two observations. First, Pteridium
is a heliophytic fern in which sporulation decreases
with increasing shade (Conway 1957, Page 1976). Its
spores are abundant in this zone, implying the presence
of an open community. Secondly, the percentages for
Pseudotsuga pollen in this zone are maximal for the
postglacial period, and they exceed considerably percentages found in surface samples from modern closed
forests in the Puget Lowland and adjacent areas (Heusser 1973b, 1978a, c, d). Since trees generally flower
most profusely when grown in open stands (Faegri and
Iversen 1975), and the highest values for Pseudotsuga
pollen in surface samples come from Pseudotsuga
woodlands in the rainshadow of the Olympic Mountains (Heusser 1978a), a woodland hypothesis was
widely accepted. There are several difficulties with this
hypothesis.
First, the analogy with open, dry rainshadow communities is not compelling because Alnus rubra and
Pteridium, both of which increase in this zone together
with Pseudotsuga, are infrequent in these communities
(Franklin and Dymess 1973, Fonda and Berardi 1976).
This analogy is also linked with the climatic interpretation that increased Pseudotsuga implies greater aridity, but it seems paradoxical that pollen ofAlnus rubra,
a species of moist habitats, increases in abundance
simultaneously with that of Pseudotsuga. Secondly, the
low percentages of Pseudotsuga pollen in surface samples may be an artifact of selective logging of Pseudotsuga coupled with the over-representation of Alnus
rubra pollen.
Several lines of evidence suggest that the forest was
closed. First, at Lake Washington, where the natural
presettlement forest was dominated by closed stands
< 1-8
Cupressaceae,Pinus, Alnus rubratype
15-25
Picea sitchensis,
Pseudotsuga,
Thujaplicata,
Abies (lasiocarPseudotsuga,
Tsuga heteroTsugaheteropa), Pinus contorphylla
ta, Alnus rubra,
phylla, Pinus
and Alnus sinumonticola,Picea sitchensis
ata
2-4
KL-5
Pinus-Alnus rubratype
275-100
;2400-900
Pinus contorta,
Ledum, Thuja
plicata, Tsuga
heterophylla
< 1-2
of Pseudotsuga menziesii, Thuja plicata, and Tsuga
heterophylla (Davis 1973, Scott 1980), pollen spectra
from sediment immediately below the settlement horizon contain 5-60% Pseudotsuga pollen (Davis 1973,
Leopold et al. 1982). Similarly, at Mineral and Hall
Lakes, presettlement Pseudotsuga percentages are - 15
and 9%, respectively (recalculated from Figs. 4 and 5
in Sugita and Tsukada [1982] using the total pollen
and spores of upland plants as the pollen sum, as at
Kirk Lake). Dunwiddie (1987) found 20% Pseudotsuga
pollen in the surface sediment of a pond on Mount
Rainier located within a dense old-growth stand of
Pseudotsuga that formed 80% of the total basal area.
The 10-17% Pseudotsuga pollen at Kirk Lake is therefore consistent with expected percentages for Pseudotsuga in closed forests.
Secondly, at sites where total PARs have been
calculated for this zone, they are high: 20 x 103
grains-cm-2 yr-1 at Kirk Lake, 20-45 x 103 grainsat Davis Lake in the southern Puget
cm-2.yr-1
Lowland (Barnosky 1981), 10 x 103 grains cm-2 yr-1
at Mineral Lake, and 16 x 103 grains cm-2 yr-1 at
Hall Lake (Sugita and Tsukada 1982). There is no
change to a more-minerogenic sediment in this zone,
so these high PARs do not result from increased erosion associated with an open forest. Presettlement PARs
for Pseudotsuga at Mineral and Hall Lakes (1200 and
2500 grains cm-2 yr-1, respectively
[Sugita and Tsu-
kada 1982]), where closed forests dominated by Pseudotsuga, Thuja plicata, and Tsuga heterophylla grew
before settlement, are similar to the mean values for
this zone at Kirk (2050 ± 920 grains cm-2 yr-1). Such
a correspondence indicates that this zone represents a
closed forest.
Thirdly, as previously mentioned, Alnus sinuata does
800
LES C. CWYNAR
not grow in closed forests (Oliver et al. 1985). A. sinuata-type pollen declines as Pseudotsuga increases at
the beginning of this zone, implying that the forest
canopy became closed.
How can the evidence for both closed and open forests be reconciled? Hansen (1948), Heusser (1973a,
1978a), Barosky (1981), Leopold et al. (1982), Mathewes (1973, 1984, 1985), and Tsukada et al. (1981)
argued that fire has been important for maintaining
open communities during this time. The charcoal profile from Kirk Lake clearly shows increased charcoal
accumulation rates in this zone. Similarily, at Hall Lake,
80 km southwest of Kirk Lake, high percentages and
PARs of Pseudotsuga, Alnus, and Pteridium in the early
Holocene are accompanied by postglacial maximum
CHARs (Sugita and Tsukada 1982). Thus, disturbance
by fire was more important during this time.
Both Pseudotsuga and Alnus rubra are well adapted
to survive and regenerate after fire (Fowells 1965), and
they are the most important trees following disturbance
in the Tsuga heterophylla Zone (Munger 1940, Franklin and Dyrness 1973). In contrast, Tsuga heterophylla
is especially sensitive to fire owing to its thin bark and
shallow roots, which are often exposed (Fowells 1965).
Pteridium is also abundant during early successional
stages following logging or fire, particularly during the
1st 5 yr after disturbance (McMinn 1951, Franklin and
Dyrness 1973).
Repeated fires leave an irregular, patchy distribution
of vegetation on the landscape in various stages of
succession. A higher fire frequency would result in a
higher proportion of patches in relatively young successional stages, and thus a greater abundance of Alnus
rubra and Pteridium on the landscape. In addition,
pollen percentages of fire-sensitive Tsuga heterophylla
declined early in this zone and remained at low values
as the percentages of Pseudotsuga pollen increased. In
the absence of disturbance, T. heterophylla invades
stands of Pseudotsuga, replacing it in late seral stages.
Because of the higher fire frequency during zone KL-3
time, however, the development to later successional
stages was preempted more frequently so that Pseudotsuga was less often replaced by T. heterophylla, except probably at the wettest sites in the drainage where
fire frequency remained relatively low.
I interpret this zone as representing a closed forest
dominated by Pseudotsuga, Alnus rubra, and Tsuga
heterophylla as a mosaic of postfire seral communities
of different ages. These forests were similar to modem
closed forests of the region except that they lacked
Thuja, Tsuga heterophylla was less abundant, and the
proportion of landscape patches in early successional
stages was relatively high because of a higher frequency
of disturbance by fire.
Cupressaceae zone (KL-4).-The increased abundances of Thuja plicata and Tsuga heterophylla pollen
and macrofossils indicate the development of forests
similar to those of today in which Pseudotsuga, Thuja
Ecology,Vol. 68, No. 4
plicata, and Tsuga heterophylla trees predominate. The
coincident increase of Thuja and decline of Alnus (both
of which grow in moist habitats) in both the pollen and
macrofossil records suggest that Thuja plicata replaced
Alnus rubra trees on the wettest sites least prone to
fire. Pinus monticola, a tree of well-drained sites, became a minor but consistent local component of the
forest.
The greater particle size of the sediment in the upper
part of this zone and the occurrence of Ledum and
Gaultheria shallon macrofossils indicate the encroachment of the growing peatland toward the coring site.
Both Pinus contorta and Picea sitchensis once again
grew locally. They were probably restricted to the developing peatland, as they are today at Kirk and other
similar sites in western Washington (Franklin and Dyrness 1973).
Pinus-Alnus rubra zone (KL-5).-The initiation of
peak deposition indicates the growth of the peatland
over the coring site. The most extensive peatland in
the basin is on the east side of the lake opposite the
coring site. The increase in Diploxylon pine pollen and
of Pinus contorta macrofossils reflects the continued
colonization of the peat by pine as well as its greater
proximity to the coring site. The increased amplitude
in the abundance of pollen of most taxa, such as Alnus,
probably results from greater local heterogeneity of
fossil deposition on peat surfaces relative to lakes. It
is not clear, for example, whetherAlnus rubra or Pteridium were actually more abundant at this time. The
vegetation changes of this period can be attributed to
local peat growth and expansion, and the forests of
surrounding uplands may not have changed.
DISCUSSION
Pickett (1976) developed an evolutionary model of
succession in which he viewed landscapes as "mosaics
of successional habitats, generated by random periodic
disturbance." In this model, vegetation on the scale of
landform units depends in part on the frequency of
disturbance. Hansen (1947) argued that the postglacial
development of vegetation in the Puget Lowland was
influenced "regionally by fire and locally by soil conditions, and to a limited extent by climate." Although
the ultimate cause of postglacial vegetation change was
climate change, the charcoal record at Kirk indicates
that change in the fire regime was a proximate cause
for some of the postglacial changes in vegetation. Specifically, the increase of Pseudotsuga, Alnus rubra, and
Pteridium coincident with a decline of Tsuga is readily
explained in terms of a shift to a relatively younger
mosaic of postfire successional communities as a consequence of more frequent fires. The trend to warmer
and/or drier climate in the PNW during zone KL-3
time is well established (Heusser 1983, Barosky 1984),
but if greater dryness itself were the proximate cause
of these changes from zone KL-2 to KL-3, then the
increase of A. rubra, which grows in moist habitats,
August 1987
FOREST AND FIRE HISTORY OF NORTH CASCADES
would be paradoxical. Rather, a small change to drier
climate probably triggered a relatively large change in
the disturbance regime by increasing fire frequency.
Similarly, the later increased abundances of Tsuga heterophylla and Thuja plicata trees in the vegetation and
concomitant
decreases of Pseudotsuga and A. rubra
may be attributed to less frequent fires.
The importance of the disturbance regime itself as
a factor influencing forest composition is apparent in
the PNW where forests have recently changed drastically in response to an altered disturbance regime dissociated from climate change. Logging has artificially
elevated the frequency and severity of disturbance. The
result is a mosaic of relatively young successional communities in which A. rubra and Pteridium are abundant. Recent pollen spectra contain abundant A. rubratype pollen and Pteridium spores (Davis 1973, Heusser
1978d), and they are remarkably similar in this respect
to spectra from zone KL-3 (Davis 1973, Heusser 1978d).
ACKNOWLEDGMENTS
I thank W. A. Watts for the opportunity to do this research.
Detailed reviews by C. Barnosky and E. Grimm greatly improved an earlier version of this paper. P. W. Dunwiddie, G.
M. MacDonald, R. W. Mathewes, J. C. Ritchie, S. Sugita,
and W. A. Watts made useful suggestions on the manuscript.
C. Barnosky, P. W. Dunwiddie, E. Grimm, and W. A. Watts
helped to core the site. P. W. Dunwiddie helped to identify
some of the macrofossils. M. Stuiver kindly supplied the radiocarbon dates. A. Christmas provided clerical assistance.
This work was funded by a National Science Foundation grant
to E. B. Leopold and an Operating Grant (U0242) to me from
the Natural Sciences and Engineering Research Council of
Canada.
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