Juliane Wolter
1,2, Hugues Lantuit
1,2, Ulrike Herzschuh
1,21Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, Germany
2Institute for Earth and Environmental Sciences, University of Potsdam, Germany
Thermokarst lake history and stable tundra vegetation since the 18th century in a Low
Arctic setting
Yukon Territory, Canada
Past Gateways
Third International Conference and Workshop
May 18
th– 22
nd, 2015 Potsdam, Germany
Results Discussion
1. Recent climatic warming and related vegetation change
The regional climatic warming that took place during the last century (Burn and Zhang 2009, Figure 6) is not well represented in the pollen record. The local to subregional vegetation largely remained stable. We attribute the slight increase in Alnus pollen since about AD1910 to either an approaching Alnus shrubline or an increase in Alnus within its current distribution range south and east of the study area.
2. Lake level changes
Changes in organic carbon content and carbon to nitrogen ratio are in accordance with changes in pollen from semiaquatic vegetation (Fig. 5). We attribute this to changes in lake marginal vegetation productivity and fluctuations in the ratio of aquatic to terrestrial vegetation debris. We suggest that either partial draining and
refilling (Fig. 7b) or to geomorphological change caused by thawing permafrost (Fig. 7c) led to a lower and more variable lake level.
Study area
The Yukon Coastal Plain stretches over 200 km from the Yukon-Alaskan Border to the Mackenzie Delta along the Beaufort Sea coast (Fig. 1).It is part of a Low Arctic transition zone between low-shrub tundra and dwarf-shrub tundra, where the response of vegetation to warming is predicted to be fastest (Lantz et al. 2010).
Wetlands and lakes cover about 25-50 % of the plain (Hagenstein et al. 1999), the typical vegetation consisting of sedges, mosses and dwarf shrubs or low shrubs (Fig. 1d, Walker et al. 2005).
Research Questions
1. How did the regional vegetation react to recent climatic warming?
2. How did the lake basin develop during the last centuries?
Figure 1 . Location of study area. (a) The studied lake is situated on the Yukon Coastal Plain within the reconstructed limit of Quaternary glaciation (white line).
Map based on Landsat imagery. (b) Sedges, mosses and dwarf shrubs characterize the flat treeless landscape (photograph of studied lake: J. Wolter). (c) The short core was retrieved from a rubber dinghy using a gravity corer.
(d) Vegetation zones of the wider study region (modified after Walker et al. 2005).
25 25 25 25 25 25 75 75 .25 .75 .75 .25 .25 .25 .25 .25 .25 .25 .75 .25 .25 .25 .25 .25 .25 .75 .25 .25 .25 .75 .25 .25 1730
1750 1770 1790 1810 1830 1850 1870 1890 1910 1930 1950 1970 1990 2010
0 20 40
Cyperaceae
0 20
Ranunc ulus
0
Equisetum
0 20 40 60 80 100
sand(
%) silt(%)
clay(
%)
0 5 10
TOC
0 1
N
5 15
CN
-30 -29 -28 -27
d13C 1 3 5
CONISS
zone 2
zone 1
0 10 20 30 40 50
Core depth (cm) 1700
1800 1900 2000
Calendar years AD
Figure 4. Age depth model based on 210Pb/137Cs dating. The filled dots represent 210Pb/137Cs dates, the open dots are extrapolated from the mean of dated samples. Dots with error bars represent AMS 14C dates which are within a similar range but show a much higher uncertainty and were not used in the age model.
Figure 5. Stratigraphic diagram of semiaquatic and sedimentary parameters.
2012
1730 - 1910
Change in water level
(partial draining and refilling) Change in basin depth (thermokarst)
1730 1750 1770 1790 1810 1830 1850 1870 1890 1910 1930 1950 1970 1990 2010
Picea
PinusTsuga Populus
20 40
Alnus
20 40
Betula
20
Ericales Salix
20
Poaceae
Artemisia Asteraceae t
otal
Asteraceae S aus
surea type
Asteraceae S enec
io type
Brassicaceae Caryoph
yllaceae Faba
ceae Lam
iaceae Polygonum
bistorta t ype
Potent illa Rum
ex acetosa type
20 40
Cyperaceae
20
Ranunc ulus ac
ris type
Equisetum Potam
oget on
Polypodi aceae s
pore monol et
Asco myce
tes spores total
Botryo coccu
s
20
trilete spores tot al
Sphagnum
20000
pollen c onc
entration (
grains/cm³)
0.5
alloch/autoch ratio
2 3 4
trees hrub/her
b ratio
trees and shrubs herbs aquatics and semiaquatics NPP
Regional vegetation
Lake related vegetation
Methods
We analyzed a short sediment core from a thermokarst lake (Fig. 1) for pollen, 210Pb/137Cs, AMS 14C, grain size distribution, stable carbon isotopes, and carbon and nitrogen contents.
Figure 2. Pollen percentage diagram. The pollen sum is based on terrestrial pollen excluding Cyperaceae and Ranunculus acris type, which represent semiaquatic vegetation in the core.
Figure 3. Principal component analysis biplot.
The sample scores from the upper part of the core (Zone 2) differ from samplescores from the lower part (Zone 1).
Figure 6. Climatic warming in the Western Canadian Arctic during the last 100 years (Burn and Zhang 2009).
References
Frost GV & Epstein HE (2014) Tall shrub and tree expansion in Siberian tundra ecotones since the 1960s, Global Change Biology, 20, 1264-1277.
Hagenstein R, Sims M, Mann G, Ricketts TH (1999) Arctic Coastal Tundra. In T. H. Ricketts, (ed.): Terrestrial Ecoregions of North America: A Conservation Assessment. 398pp. Washington, DC, USA: Island Press.
Lantz T, Gergel S, Kokelj S (2010) Spatial Heterogeneity in the Shrub Tundra Ecotone in the Mackenzie Delta Region, Northwest Territories: Implications for Arctic Environmental Change. Ecosystems, 13, 194-204.
Tape KD, Sturm M, Racine C (2006) The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology, 12, 686-702.
Walker DA, Raynolds MK, Daniëls FJ, Einarsson E, Elvebakk A, Gould WA, Katenin AE, Kholod SS, Markon CJ, Melnikov ES (2005) The circumpolar Arctic vegetation map. Journal of Vegetation Science, 16, 267-282.
Figure 7. Conceptional sketch of lake development. (a) Present lake basin and water level. Changes in amount of lake marginal vegetation are brought about by either (b) Draining and refilling of lake water or © thermokarst- induced changes in lake basin depth.
mean sedimentation rate: 0.17 cm/yr
Key findings
1. Stable regional vegetation during the last 300 years, slight increase of extraregional Alnus over the last century 2. Higher amount of lake marginal vegetation pre 1910 → lake level changes.
Zone 1 Zone 2
b) c)
a)
