THERMOKARST VEGETATION IN LOWLAND BIRCH FORESTS ON THE TANANA FLATS, INTERIOR ALASKA, U.S.A.

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1 THERMOKARST VEGETATION IN LOWLAND BIRCH FORESTS ON THE TANANA FLATS, INTERIOR ALASKA, U.S.A. Charles H. Racine 1, M. Torre Jorgenson 2, James C. Walters 3 1. U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire U.S.A. cracine@crrel.usace.army,mil 2. Alaska Biological Research Inc., P.O. Box 80410, Fairbanks, Alaska U.S.A. tjorgenson@abrinc.com 3. Department of Earth Science, University of Northern Iowa, Cedar Falls, Iowa U.S.A. walters@uni.edu Introduction 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. 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 km 2 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

2 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 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 928 The 7th International Permafrost Conference

3 TRANSECT PROFILE The profile from a fen into and through a birch forest (Figure 2) shows the distribution of surface and subsurface 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. 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. Results VEGETATION BIRCH FORESTS 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 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%). THAW PITS 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

4 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

5 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, standing 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 300 µs cm -1 (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

6 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

7 References Drury, W.H., Jr. (1956). Bog flats and physiographic processes in the Upper Kuskokwim River region, Alaska. Contributions of the Gray Herbarium, 178 pp. Halsey, L.A., Vitt, D.H and Zoltai, S.C.(1995). Disequilibrium response of permafrost in boreal continental western Canada to climate change. Climatic Change, 30, Hill, M.O. and Gauch, H.G.(1980). Detrended correspondence analysis: an improved ordination technique. Vegetatio, 42, Jorgenson, M.T., Roth, J.E., Raynolds, M.K., Smith, M.D., Lentz, W. and Zusi-Cobb, A.L. (1996). An Ecological Land Classification for Fort Wainwright, Alaska. Draft Report to U.S.A. CRREL. ABR Inc., Fairbanks, Ak. Laberge, M.J. and Payette, S. (1995). Long-term monitoring of permafrost change in a palsa peatland in northern Quebec, Canada. Arctic and Alpine Research, 27, Luken, J.O. and Billings, W.D. (1983). Changes in bryophyte production associated with a thermokarst erosion cycle in a subarctic bog. Lindbergia, 9, Osterkamp, T.E. (1994). Evidence for warming and thawing of discontinuous permafrost in Alaska. Eos, Transactions American Geophysical Union, 75, 86. Racine, C.H. and Walters, J.C. (1994). Groundwater discharge wetlands in the Tanana Flats, Interior Alaska, USA. Arctic and Alpine Research, 26, Racine, C., Lichvar, R., Murray, B., Tande, G., Lipkin, R. and Duffy, M. (1997). A floristic inventory and spatial database for Fort Wainwright, Interior Alaska. CRREL Special Report Thie, J. (1974). Distribution and thawing of permafrost in the southern part of the discontinuous permafrost zone in Manitoba. Arctic, 27, VanCleve, K., Viereck, L.A. and Dyrness, C.T. (1996). State factor control of soils and forest succession along the Tanana River in interior Alaska, U.S.A. Arctic and Alpine Research, 28, Viereck, L.A., Van Cleve, K,. Adams, P.C. and Schlentner R.E. (1993). Climate of the Tanana River floodplain near Fairbanks, Alaska. Canadian Journal of Forest Research, 23, Vitt, D.H., Halsey, L.A. and Zoltai, S.C. (1994). The bog landforms of continental western Canada in relation in climate and permafrost patterns. Arctic and Alpine Research, 26, Walters, J. C., Racine, C.H., and Jorgenson, M.T. (1998). Characteristics of permafrost in the Tanana Flats, Interior Alaska. In Proceedings 7 th International Conference on Permafrost (This volume). Zoltai, S. C. (1993). Cyclic development of permafrost in the peatlands of northwestern Alberta Canada. Arctic and Alpine Research, 25, Charles H. Racine, et al. 933