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8
PERMAFROST - Seventh International Conference (Proceedings),
Yellowknife (Canada), Collection Nordicana No 55, 1998
?
THERMOKARST VEGETATION IN LOWLAND BIRCH FORESTS ON THE
TANANA FLATS, INTERIOR ALASKA, U.S.A.
Charles H. Racine1, M. Torre Jorgenson2, James C. Walters3
1. U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755 U.S.A.
e-mail: [email protected],mil
2. Alaska Biological Research Inc. , P.O. Box 80410, Fairbanks, Alaska 99708 U.S.A.
e-mail: [email protected]
3. Department of Earth Science, University of Northern Iowa, Cedar Falls, Iowa 50614 U.S.A.
e-mail: [email protected]
Abstract
The thawing of ice-rich permafrost beneath birch forests in the Tanana Flats area of Interior Alaska has produced thermokarst features colonized by a range of species and wetland vegetation types. As the forest drowns
along its border with fens, an open-water moat is colonized by minerotrophic species and a floating mat develops. At the same time, thawing in the birch forest interior produces water-filled thaw pits and collapse scar
bogs in which ombrotrophic vegetation develops through several stages to Sphagnum bogs. As the thawing
front moves into the birch forest from the fen, these latter features are incorporated into the floating mat, accelerating the expansion of fens.
Introduction
In boreal forests with discontinuous permafrost,
"drowning" of forests can occur as the surface subsides
due to thawing of ice-rich soils. Such thawing can be
initiated by climate warming, forest fire, thermal heat
transfer from adjacent water bodies, tree fall and
changes in groundwater flow and other factors.
Recently, Racine and Walters (1994) and Jorgenson et al.
(1996) identified an area of widespread and rapid permafrost degradation and forest drowning in the Tanana
Flats area of interior Alaska. Here the most intense
thawing occurs in lowland birch forests (Figure 1)
rather than in black spruce woodland where most studies of taiga thermokarst development have occurred
(Drury, 1956; Luken and Billings, 1983; Zoltai, 1993).
Possible reasons for more rapid thawing in birch forests
than in black spruce woodlands are described elsewhere in these proceedings (Walters et al. 1998). The
objective of this study is to describe the thermokarst
features and associated vegetation in birch forests on
the Tanana Flats.
Study area
The Tanana Flats occupies a 3600 km2 portion of the
Tanana lowland basin bordered on the north by the
Figure 1. Low level oblique aerial photo across the Tanana Flats showing a
forested island bordered on both sides by floating mat fen. The forest is predominately Betula papyrifera with darker patches of Picea mariana forest.
Vegetation, soils, permafrost and water were sampled along a 600 m transect
positioned across this island and adjacent fen in the direction of viewing.
Visible thermokarst features include the fen and moat along the edge of the
birch forest and a collapse scar bog visible in the middle of the island. A small
embayment area along the foreground edge is visible and represents a collapse scar bog being incorporated into the fen.
Tanana River, Fairbanks and the Yukon-Tanana uplands
and on the south by the Alaska Range. The Tanana Flats
is therefore situated on the toe slope of a large alluvial
fan complex built out from the north side of the Alaska
Range. Both surface and subsurface water move across
the Tanana Flats on a very low gradient from southeast
to northwest, flowing from Alaska Range glaciers to the
Charles H. Racine, et al.
927
Figure 2. Cross-sectional profile along an actual transect in the Tanana Flats from floating mat fen and moat (left) through a birch forest with thaw pits and
collapse scar bogs showing relationships to topography, soils and permafrost.
Tanana River. The subsurface groundwater portion
appears to discharge to the surface in springs in the
northwest corner and probably accounts for the extensive development there of fens (Racine and Walters,
1994). Here the taiga vegetation is a complex mosaic of
these fen meadows, paper birch, mixed birch-spruce
and black spruce forests, alder or ericaceous shrubscrub and scattered bogs (Figure 1). This study concerns
thermokarst development in the birch forests which
border fens.
The climate of Fairbanks is continental and subarctic,
with a mean annual temperature of -3.3ûC with a pronounced warming trend from 1976 to the present
(Osterkamp,1994). The climate is subarid, with little or
no water surplus; yearly precipitation averages 28.4 cm,
with a minimum in late winter and maximum in late
summer (Viereck et al. 1993).
Methods
TRANSECTS
Vegetation, topography, soil, permafrost and water
sampling were conducted in detail along two 600-m
long transects over four km apart, located in birch
forests and adjacent floating mat fens (Figure 1). In
addition, less detailed information was obtained at a
number of remote sites on floating mat fens and in
928
birch forests and associated thermokarst features. On
transects, relative elevations of the ground and water
surface were measured at 2 m intervals with an autole
vel. Permafrost presence and active layer thickness
were determined in late August at the same intervals
with a 4 m-long tile probe. Soil pits were dug and peat
cores were obtained to describe profiles.
VEGETATION SAMPLING
Birch forest vegetation was sampled at four sites
along the two transects and in eight other birch forest
stands (n=12) at widely scattered sites in the northwest
Tanana Flats. A 20 x 20m quadrat was established in
each birch stand and the height and diameter of all
trees and shrubs with stem diameters over 5 cm were
measured. Four to six of the largest-diameter birch trees
were cored at 15 cm above the ground surface to determine their age. Percent cover estimates of all species in
both the overstorey and understorey were also recorded. The vegetation of 42 thermokarst sites associated
with the sampled birch forest stands was also sampled
by estimating percent cover of all species in 5 x 5 m
quadrats. A number of additional fen sites were also
sampled. Surface water pH and conductivity were measured at most of the sample sites with a YSI 3500 water
quality meter. Vegetation data analysis was carried out
using summary statistics and multivariate methods
including cluster analysis and ordinations using
The 7th International Permafrost Conference
face features related to thermokarst development. The
birch forest here is raised 1 to 2 m above the fen water
table. At the forest-fen border there is a 0.5 to 1.5-m
deep open water moat with dead or dying birch trees
(Figure 3a) extending out into the fen for a distance of
10 to 50 m. Within the forest, there are both small openings or thaw pits, 2 to 15-m-wide (mean = 13 m,
SD = ±9, n = 15) (Figure 3b) and larger 75 to 100-mdiameter collapse scar bogs (Figure 3c). Up to one half
of the transect distance through the birch forest may be
occupied by thermokarst pits and bogs.
Permafrost is sporadic in the moat area, and absent
beneath the fen, collapse scar bogs and the larger thaw
pits in Figure 2. In the birch forest, the active layer profile in late August is deep (1 m) and uneven in relation
to developing thaw pits. The organic soil horizon in the
sampled birch forests was relatively thick (0.5-1 m).
Peat horizons in the collapse scar bogs are on the order
of 1 m. In the fen, the floating mat is 0.5 to 0.7 m thick
consisting of small rootlets, rhizomes and some peat,
overlying a 0.2 to 0.4 m thick sapric organic layer over
the silt. Part of this lower organic layer may represent
the decomposed birch forest organic horizon shown in
Figure 2 as continuous beneath the thaw pits, bogs and
fen. Silts underlie the organics and range in depth from
3 to 5 m where gravels and sands are present. These
sands and gravels probably serve as a conduit for the
movement of groundwater.
VEGETATION
BIRCH FORESTS
Figure 3. Three thermokarst features in lowland birch forests on the Tanana
Flats include: a) open water moat along birch forest-fen edge with standing
dead birch trees, b) a thaw pit in the birch forest floor with open water and
floating Lemna minor and c) a collapse scar bog.
Detrended Correspondence Analysis (DCA) (Hill and
Gauch, 1980) in PC-Ord, a commercial analysis
package.
The 10 to 15-m-tall canopy of all 12 sampled stands
was clearly dominated by Betula papyrifera with occasional understorey trees of Salix bebbiana, Picea glauca or
P. mariana. Tree cover was 60 to 80% and diameters
(dbh) were mostly in the 10-25 cm range with stand
basal area from 15 to 37 m 2 ha 1 (mean = 21,
SD = (7.4, n = 9) and densities from 1000 to 3000 trees
ha 1 . (mean = 1450, SD = (695, n=9). The age of the
largest trees was 50 to 60 years in all stands. There was
little or no evidence of tree regeneration (either spruce
or birch) in any of the sampled stands. Understorey
vegetation varied from a continuous 1-2 m-tall shrub
layer of Rosa acicularis or a grass layer of Calamagrostis
canadensis to almost no understorey vegetation or with
only scattered shrubs of Rosa acicularis, Ribes triste,Salix
bebbiana, Ledum palustre and Rubus idaeus. Other ground
cover species included Vaccinium vitis-ideae, Moehringia
latifolia, Pyrola acerifolia and Epilobium angustifolium.
Moss and lichen cover was absent or very sparse (<5%).
Results
THAW PITS
TRANSECT PROFILE
The profile from a fen into and through a birch forest
(Figure 2) shows the distribution of surface and subsur-
Thaw pits occurred at variable frequency, diameter
and vegetation composition (Table 1) on the shaded
birch forest floor in all sampled birch stands (Figure
Charles H. Racine, et al.
929
Table 1. Summary table of major plant species that compose
the vegetation of four thermokarst features in birch forests
on the Tanana Flats, interior Alaska. H = high frequency
(over 50% of sampled stands); L = low frequency (10 to 50%
of sampled stands). I, II and III under thaw pits refers to
vegetation type developmental stage in Figure 4a.
Figure 4. Ordinations of vegetation samples using detrended correspondent
analysis (DCA) from (A) 21 thaw pit samples and (B) 34 fen samples showing outlines of different vegetation groups or types.
3b). Within a single birch stand, the cover in different
pits may range from open water with floating aquatics
to continuous vegetation. Ordination of the vegetation
in 21 sampled pits (Figure 4a) shows three stand clusters which may represent three stages of vegetation
development following thaw subsidence.
Group I samples represent an early stage in pit formation with deeper (20-50 cm) standing water and standing dead or dying birch trees (Figure 3b). The dominant
species is the floating aquatic, Lemna minor (Table 1).
Pits in group II contain less water, with only a few shallow pools, exposed wet organic soils, woody birch twig
debris and small clumps of vegetation. The annual
Bidens cernua is frequently dominant (>50% cover) by
late August and there are clumps of Carex canescens and
Carex aquatilis, Calamagrostis canadensis and forbs
including Ranunculus gmelini and Cicuta virosa. Group
III stands have a continuous vegetation mat of variable
composition but characterized mainly by the presence
of Spagnum squarrosum and Potentilla palustris.
930
The 7th International Permafrost Conference
Table 2. Surface water chemistry for thermokarst features and associated vegetation types
Associated species include Calla palustris and the mosses, Sphagnum riparium and Calliergon sp.
Permafrost is generally present in group I, occasional
in II and absent in III (Figure 2). The lowered water
levels in group II stands may be related to thawing of
the permafrost and drainage of pore space beneath
these pits. The surface water in Group I had the highest
pH and conductivities of the three groups while those
in Group III had the lowest values (Table 2).
COLLAPSE SCAR BOGS
Collapse scar bogs occur as large openings (30 to
150 m in diameter; mean = 75m) in the birch forest
(Figures 1, 2 and 3c). A moat is common around the
periphery of these bogs but was not sampled. The vegetation composition of the 13 sampled bogs was relatively uniform and simple with Sphagnum riparium clearly
dominant with no or few additional species of moss.
Scattered sedges (Carex limosa, C. aquatilis and
Eriophorum scheuchzeri) covered about 5 to 10% and low
shrubs of Oxycoccus microcarpus, Ledum groenlandicum,
Chamaedaphne calyculata and Spirea beauverdiana account
for an additional 5 to 25% cover (Table 1). In some bogs
there are also scattered and stunted trees of Larix laricina, Picea mariana and Betula papyrifera. The pH and conductivities were lowest of any thermokarst feature
described here (Table 2).
MOATS
Along the birch forest/fen border of rapid thawing
and collapse (Figures 2, 3a), there is a broad or narrow
tension zone of open water, remnant birch forest, stan-
ding dead and dying birch trees and developing vegetation. Water depths are variable but may be up to 1 m
deep near the birch forest bank; the water is frequently
seen to be flowing in these moats unlike water in the
other features. A diverse assemblage of aquatic species
can occur here including Utricularia vulgaris, Sparganium
sp., Lemna minor and Riccia fluitans. The emergent
Hippuris vulgaris and Glyceria maxima were found only
in moats and not in the other thermokarst features
(Table 1). On the moat edge where there is wet organic
soil, the vegetation is similar to that of thaw pits with
Bidens cernua, Epilobium palustre, Calamagrostis sp. and
occasional sedges. Floating mat development begins in
these moats with loose mats of Calla palustris. As the
floating mat develops, Menyanthes trifoliata eventually
invades and completely replaces these Calla palustris
mats. Water pH in these moats averaged 6.3 (SD = ±0.5;
n
=
7)
and
conductivity
averaged
-1
300 µs cm (SD = ±105, n = 7).
FLOATING MAT FENS
Floating mat fens consist of a floating rhizome, root
and peat mat composed of various species of forbs,
graminoids and in some places low shrubs and moss
(Table 1). The vegetation composition of these floating
mats varies locally across a single fen and regionally
over their wide extent. The most basic and extensive
floating mat community consists of almost pure stands
of tall (0.5 m) buckbean (Menyanthes trifoliata) forming a
group of stands (I) clearly visible on the left in Figure
4B. From this basic buckbean type, increasing amounts
of Potentilla palustris, sedges and low shrubs (Salix candida) form a second group (II in Figure 4B) followed by
a third distinct group (III) with Equisetum fluviatile do-
Charles H. Racine, et al.
931
minant. A fourth type of floating mat fen is represented
by a group of two outlying stands dominated by sedge
meadow (Carex aquatilis or C. lasiocarpa) meadow with
little or no buckbean. Minerotrophic moss species such
as Calliergon sp. are fairly common in group II and III
stands. Other species that are usually present but not
abundant at most floating mat sites include Cicuta
virosa, Typha latifolia and Rumex arctica (Table 1). Water
chemistry of surface water in fens is quite variable but
is generally circumneutral with a pH over 6.5 and high
conductivities of about 275 µs cm-1 (Table 2).
Discussion
The thawing of permafrost underlying birch forests in
the Tanana Flats has produced several distinct
thermokarst features and associated wetland vegetation
types. Two different pathways of wetland vegetation
succession are represented: (1) a minerotrophic
sequence involving forest drowning adjacent to
groundwater fens and development of a highly productive forb-dominated floating vegetation mat; (2) an
ombrotrophic sequence within the birch forest interior
involving the development of small water-filled thaw
pits with dead birch trees and possibly progressing
through several stages to simple Sphagnum bogs.
Convergence of these two pathways occurs when the
progressive thawing front (subsidence of the birch forest along its boundary with floating mat fens) moves
into the birch forest and incorporates the thaw pits and
collapse scar bogs within the birch forest into the floating mat fen. We observed several sites on aerial photos
and in the field where this is occurring (Figure 1). This
process accelerates the expansion of the fens and may
account for variation in the floating mat vegetation.
Under this scenario, the final birch forest thermokarst
stage is the fen. We have seen little evidence for reestablishment of forest or permafrost on these floating mat
fens although this is a common stage in peatland development (Zoltai, 1993).
in birch forests. This observation and one radiocarbon
date of 490 ±70 BP (Beta-97563) at the bottom of the collaspse scar bog in Figure 2 suggest that the bogs here
have formed within the past 500 years (cf. Zoltai, 1993).
The thawing of ice-rich permafrost beneath lowland
birch forests in the Tanana Flats represents a major
ecosystem change from terrestrial forest to wetland
with associated changes in biological productivity, biomass, gas exchange, nutrient cycling, vegetation patterns and biodiversity. With respect to the effects of
thermokarst on biodiversity, over 60 species of
hydrophytic plants were sampled in the four types of
thermokarst features associated with these birch forests
(fens, moats, pits and bogs). This represents at least 20%
of the flora of the Tanana Flats (Racine et al., 1997). In
addition, the majority (76%) of the species associated
with the thermokarst complex (Table 1) occur in only
one (44%) or two (32%) of the four features.
In interior Alaska, birch forests are usually associated
with upland areas on well-drained and permafrost-free
south-facing slopes. Here they usually represent a 30 to
60 year-old fire-succession with white spruce replacing
the birch after 100 years (Van Cleve et al., 1996). It is
therefore difficult to understand the origin of the lowland birch forests described here and why they are
associated with ice-rich permafrost areas of rapid thermal degradation. It is clear however, that they represent
one of the most sensitive ecosystem in interior Alaska
to the pronounced climate warming which is occurring
there (Osterkamp, 1994),
Acknowledgments
This work was funded by US Army Alaska Integrated
Training Area Management program under the direction of William Gossweiler, Gary Larsen, and Pam
Bruce. Field assistance was provided by Peggy
Robinson (CRREL), Marilyn Racine, and Robert Lichvar
(CRREL).
Most studies of thermokarst development in boreal
forests take place in black spruce woodlands rather
than birch forests (Drury, 1956; Thie, 1974; Luken and
Billings, 1983; Zoltai, 1993; Vitt et al. 1994; Halsey et al.,
1995; Laberge and Payette, 1995). These studies all list
Sphagnum riparium and Carex limosa and sedge lawns as
the major features of early vegetation development
with Sphagnum fuscum, S. angustifolium, low ericaceous
shrubs and black spruce appearing later in succession.
In the Tanana Flats birch forests, only the Sphagnum
riparium-Carex limosa vegetation of collapse scar bogs
resemble this black spruce thermokarst vegetation type.
Bogs in the latter stages of thermokarst succession (with
S. fuscum, S. angustifolium and abundant shrubs) are rare
932
The 7th International Permafrost Conference
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