Global Change Biology (2015) 21, 1634–1651, doi: 10.1111/gcb.12757

Polygonal tundra geomorphological change in response to warming alters future CO2 and CH4 flux on the Barrow Peninsula MARK J. LARA1, A. DAVID MCGUIRE2, EUGENIE S. EUSKIRCHEN1, CRAIG E. TWEEDIE3, KENNETH M. HINKEL4, ALEXEI N. SKURIKHIN5, VLADIMIR E. ROMANOVSKY6,7, G U I D O G R O S S E 8 , W . R O B E R T B O L T O N 9 and H E L E N E G E N E T 1 1 Institute of Arctic Biology, University of Alaska, Fairbanks, AK 99775, USA, 2U.S. Geological Survey, Alaska Cooperative Fish and Wildlife Unit, University of Alaska, Fairbanks, AK 99775, USA, 3Department of Biological Sciences, University of Texas at El Paso, El Paso, TX 79968, USA, 4Department of Geography, University of Cincinnati, Cincinnati, OH 45221, USA, 5Intelligence and Space Research Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA, 6Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, USA, 7Tyumen State Oil and Gas University, Tyumen, Russia, 8Periglacial Research Unit, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, Germany, 9International Arctic Research Center, University of Alaska, Fairbanks, AK 99775, USA

Abstract The landscape of the Barrow Peninsula in northern Alaska is thought to have formed over centuries to millennia, and is now dominated by ice-wedge polygonal tundra that spans drained thaw-lake basins and interstitial tundra. In nearby tundra regions, studies have identified a rapid increase in thermokarst formation (i.e., pits) over recent decades in response to climate warming, facilitating changes in polygonal tundra geomorphology. We assessed the future impact of 100 years of tundra geomorphic change on peak growing season carbon exchange in response to: (i) landscape succession associated with the thaw-lake cycle; and (ii) low, moderate, and extreme scenarios of thermokarst pit formation (10%, 30%, and 50%) reported for Alaskan arctic tundra sites. We developed a 30 9 30 m resolution tundra geomorphology map (overall accuracy:75%; Kappa:0.69) for our ~1800 km² study area composed of ten classes; drained slope, high center polygon, flat-center polygon, low center polygon, coalescent low center polygon, polygon trough, meadow, ponds, rivers, and lakes, to determine their spatial distribution across the Barrow Peninsula. Land-atmosphere CO2 and CH4 flux data were collected for the summers of 2006–2010 at eighty-two sites near Barrow, across the mapped classes. The developed geomorphic map was used for the regional assessment of carbon flux. Results indicate (i) at present during peak growing season on the Barrow Peninsula, CO2 uptake occurs at -902.3 106gC-CO2 day 1 (uncertainty using 95% CI is between 438.3 and 1366 106gC-CO2 day 1) and CH4 flux at 28.9 106gC-CH4 day 1(uncertainty using 95% CI is between 12.9 and 44.9 106gC-CH4 day 1), (ii) one century of future landscape change associated with the thaw-lake cycle only slightly alter CO2 and CH4 exchange, while (iii) moderate increases in thermokarst pits would strengthen both CO2 uptake ( 166.9 106gC-CO2 day 1) and CH4 flux (2.8 106gCCH4 day 1) with geomorphic change from low to high center polygons, cumulatively resulting in an estimated negative feedback to warming during peak growing season. Keywords: arctic, carbon balance, classification, climate warming, negative feedback, polygonal tundra, thaw-lake cycle, thermokarst Received 4 September 2014 and accepted 9 September 2014

Introduction High latitude permafrost landscapes have accumulated vast amounts of carbon over thousands of years (Smith et al., 2004; Schuur et al., 2008; Strauss et al., 2013), and now contain ~50% of the world’s soil organic carbon pool (Tarnocai et al., 2009). Projections indicate these regions will experience greater increases in temperature, precipitation, and growing season length than Correspondence: Mark J. Lara, tel. 915 240 4277, fax 907 474 7872, e-mail: [email protected]

1634

elsewhere on the globe (Kattsov et al., 2005; Stocker et al., 2013), which suggests that the arctic soil carbon pools may be impacted. There are considerable uncertainties about the present and future state of the atmospheric CO2 sink strength of arctic tundra at the pan-arctic scale (Mcguire et al., 2012), and the associative role climate change may have on land cover change (Pearson et al., 2013). Therefore, it is important to conduct more tangible and comprehensive landscape-level studies in data-rich subregions of the Arctic tundra biome to evaluate how warming is likely to impact carbon exchange trends and dynamics in response to © 2014 John Wiley & Sons Ltd

C H A N G E I N T U N D R A G E O M O R P H O L O G Y & C A R B O N F L U X 1635 future climate and land cover change scenarios, and how such responses may enhance or mitigate greenhouse warming. This study is focused on the Alaskan Arctic coastal plain tundra spanning the Barrow Peninsula, which is a mosaic of ice-wedge polygons, meadows, ponds, lakes, and rivers that are irregularly distributed across Drained Thaw Lake Basins (DTLBs) and interstitial tundra. Sub-surface ice-wedge growing and thawing and related heaving and subsidence of the ground surface in arctic coastal plain tundra influence geomorphology and fine-scale surface microtopography, to affect local hydrology (Liljedahl et al., 2011, 2012), snow pack depth and density (Kershaw, 2008), plant community composition (Villarreal et al., 2012), soil organic carbon storage (Bockheim et al., 2001), and associated landatmosphere CO2 and CH4 fluxes (Rhew et al., 2007; Olivas et al., 2011). Although the relative distribution of lakes, DTLBs, and interstitial tundra on the Barrow Peninsula is well-established (Hinkel et al., 2003; Frohn et al., 2005), the spatial distribution of smaller geomorphic types (i.e., high, flat, low center polygons) which control fine-scale processes, is generally unknown. Although recent progress has been made (Muster et al., 2012, 2013; Skurikhin et al., 2013), difficulty in discriminating between relatively homogeneous moisture and vegetation cover in tundra geomorphic types is challenging. Identifying the spatial distribution of geomorphic types in arctic coastal plain tundra where landscape heterogeneity is high, is essential to reduce uncertainty in model applications and estimates of annual/seasonal carbon exchange. Arctic coastal plain tundra can evolve geomorphologically over long- and short-time scales. The thawlake cycle acts over long-time scales, typically forming over centuries to millennia (i.e., over one to several thaw-lake cycles; Billings & Peterson, 1980). Although many revisions have been made after its initial formulation (Jorgenson & Shur, 2007), this process successively transforms one land form to another, beginning with lake formation and ending with lake drainage (Billings & Peterson, 1980). Specific to the Barrow Peninsula, approximately 28% of this region (i.e., interstitial tundra) has been unaffected by thaw-lake processes during the last 15–20 ka (Hinkel et al., 2003; Eisner et al., 2005). However, the vast majority of surficial features are younger than 5 500–8 300 years and have likely not completed a single thaw-lake cycle (Brown 1965, Hinkel et al., 2003), suggesting geomorphic change is ongoing. Alternate hypotheses have been presented to explain potentially noncyclic geomorphic changes in other arctic coastal tundra sites where ice content and subsurface properties differ (Jorgenson & Shur, 2007; Jones et al., 2012). Abrupt or relatively rapid (i.e., short-time scale) © 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

geomorphic changes can occur in these landscapes with altered drainage patterns as a result of (i) thermal erosion along coasts, river and stream margins, and adjacent to slumps and gullies (Berhe et al., 2007; Fortier et al., 2007), (ii) landscape drying (Smith et al., 2005; Lin et al., 2012), and (iii) thermokarst (i.e., ground subsidence) formation which produces pits, troughs, and small lakes (Mackay & Burn, 2002; Cory et al., 2013; Raynolds et al., 2014). Recent observations suggest ice-wedge degradation may be increasing as mean annual ground temperatures have increased 2–5 °C over past decades (Romanovsky et al., 2003; Shiklomanov et al., 2010), causing a substantial redistribution of surface water, that transitions geomorphology from low to high center polygons. For example, low to high center geomorphic transitions have been observed in response to (i) abrupt increases in thermokarst pits and trough depths, from 1982–2001 near the Colville River, AK (Jorgenson et al., 2006), from 1990–2010 near Prudhoe Bay, AK (Raynolds et al., 2014), and from 1978–2005 near Kobuk Valley National Park, AK (Necsoiu et al., 2013), associated with recent warming, and (ii) accelerated thermal erosion by gullies or streams in DTLBs near Barrow, AK (Britton, 1957), along the Meade river in AK (Billings et al., 1978), and in Bylot Island, CA (Fortier et al., 2007). Thermokarst formation is likely to occur rapidly (e.g., over 10– 20 years) and quickly stabilize with the accumulation of vegetation and organic matter (Jorgenson et al., 2006; Gamon et al., 2012), while altering landscape geomorphic structure. However, the timing, direction, and magnitude of climate feedback to warming associated with long and short-term landscape change processes in arctic tundra regions is highly uncertain (Luo et al., 2011; Anthony et al., 2012; Mcguire et al., 2012). To date, a variety of plot and landscape-level CO2 and CH4 exchange studies near Barrow have focused on characterizing peak growing season (i.e., mid July to early August) carbon exchange using various land cover or geomorphic units across the Barrow peninsula, such as soil moisture (Rhew et al., 2007; Von Fischer et al., 2010), plant functional type (Brown et al., 1980), plant community composition (Lara et al., 2012), microtopographic position (Olivas et al., 2011; Zona et al., 2011), and DTLB age and/or interstitial tundra (Zona et al., 2010; Zulueta et al., 2011; Sturtevant & Oechel, 2013). These studies find differences in CO2 and/or CH4 exchange over small or large spatial scales associated with varying soil moisture and microtopographic variation. However, each study specifically and intensively focused on a particular land cover unit or spatial scale and no study has integrated spatial patterns in carbon flux from plot to landscape scales in a manner that is appropriate to validate previous estimates and

1636 M . J . L A R A et al. reduce uncertainty of landscape-level estimates (Luo et al., 2011). This study evaluates the future impact of 100 years (e.g., 2002–2102) of geomorphic change scenarios on peak growing season carbon exchange including: (i) successional geomorphological changes associated with the thaw-lake cycle and (ii) thermokarst pit formation spanning incremental levels (10%, 30%, and 50%) of those reported near the Colville River (Jorgenson et al., 2006) and Prudhoe Bay (Raynolds et al., 2014). We hypothesized that: (i) successional landscape-level changes caused by the thaw-lake cycle will offset carbon uptake/loss as new polygons develop, while others are degraded, (ii) increased thermokarst pit formation will decrease carbon uptake with geomorphic transitions from wetter low center polygons to drier highcentered polygons, and increase CH4 flux with greater inundated soils caused by pit formation, resulting in a significant reduction of the overall sink strength of the tundra due to the extensive rise in CH4 flux. We develop a 30 9 30 m tundra geomorphology map of the Barrow Peninsula and spatially upscale plot-level fluxes by polygon type in combination with estimates of geomorphic thaw-lake succession and scenarios of thermokarst degradation, to estimate 100-year change in peak growing season fluxes. We assess change in peak growing season CO2 and CH4 fluxes solely associated with geomorphic landscape change associated with the thaw-lake cycle and scenarios of thermokarst formation on the Barrow Peninsula. This study is a contribution to the Next-Generation Ecosystem Experiments (NGEE-Arctic) and draws from data collected by the International Polar Year-Back to The Future (IPYBTF) project.

(Hinkel et al., 2003; Frohn et al., 2005). The landscape generally has a low relief with elevations ranging from 0–22 m.a.s.l. (Brown et al., 1980). Higher elevated regions are accented by erosional remnants that emerged following marine regressions (Brighamgrette & Hopkins, 1995; Eisner et al., 2005). Seasonal freeze-thaw cycles have influenced ice-wedge geomorphological structure, creating a distinct array of geomorphic types and associated plant communities (Webber, 1978; Villarreal et al., 2012).

Polygonal tundra geomorphic types. We define ten geomorphologically and hydrologically distinct geomorphic types (Fig. 1), used in our landscape classification which are similar to that previously identified by Brown et al.(1980): (i) drained slope, (ii) high center polygons, (iii) flat-center polygons, (iv) low center polygons, (v) coalescent low center polygons, (vi) meadow, (vii) polygon trough, (viii) pond, (ix) lake, and (x) river (Table 1). These classes are described below, with respect to geomorphic features (Billings & Peterson, 1980; Brown et al., 1980; Hinkel et al., 2003), plant communities (Villarreal et al., 2012), and functional properties (Lara et al., 2012). 1. Drained Slopes (DS) are characterized by very high relief found on the margins of DTLBs, lakes, and rivers. These geomorphic types have dry soils, low productivity, and high albedo. Two plant communities dominate DS dry

Materials and methods

Study site The ~1800 km² Barrow Peninsula lies at the northernmost tip of the Arctic Coastal Plain on the North Slope of Alaska. The region, is underlain by continuous permafrost >400 m thick (Sellmann & Brown, 1973), with a maximum thaw depth ranging from 30 to 90 cm (Nelson et al., 1998; Hinkel & Nelson, 2003). Mean annual air temperature, precipitation, and snowfall are 11.2 °C, 115 mm, and 958 mm, respectively (climate normal 1981–2010; Alaska Climate Research Center:www.akclimate.org). The region has warmed by approximately ~3 °C since 1950, with the majority of warming occurring since the mid-1980s (Lachenbruch & Marshall, 1986; Stafford et al., 2000; Romanovsky et al., 2002). Barrow-area soils are ice-rich, with silty, deltaic, and loess deposits, which are regionally identified as, ‘true thaw lakes’ (Jorgenson & Shur, 2007) and the thaw-lake cycle is the primary catalyst of landscape change within lakes and DTLBs that cover ~72% of land area

Fig. 1 Tundra geomorphic types considered in the geomorphic classification as shown on 2002 Quickbird satellite imagery. Scale bars are in meters. © 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

C H A N G E I N T U N D R A G E O M O R P H O L O G Y & C A R B O N F L U X 1637 Table 1 Characterization of geomorphology and plant communities of polygonal tundra by geomorphic type Geomorphic Unit

Moisture Regime

Relief

Vegetation Community

Drained Slope

Very Dry

Very High

High-Center Flat Center

Dry Dry-Moist

High Intermediate

Low Center

Moist-Wet

Intermediate

Coalescent Low Center Meadow

Wet

Intermediate

Moist-Wet

Low

Trough

Moist-Aquatic

Low

Pond Lakes & River

Aquatic Aquatic

Low Low

(i) Dry Lichen Heath, (ii) Dry Arctagrostis, Luzula, Poa, Carex graminoid tundra (i) Dry Arctagrostis, Luzula, Poa, Carex graminoid tundra (i) Dry Arctagrostis, Luzula, Poa, Carex graminoid tundra, (ii) Moist Carex, Poa, Luzula graminoid tundra, (i) Moist Carex, Poa, Luzula graminoid tundra, (ii) Wet Carex, Sphagnum graminoid tundra, (iii) Seasonally flooded Carex, Dupontia, Eriophorum graminoid tundra (i) Seasonally flooded Carex, Dupontia, Eriophorum graminoid tundra, (ii) Aquatic Arctophila, Carex, Dupontia graminoid tundra (i) Moist Carex, Poa, Luzula graminoid tundra, (ii) Wet Carex, Sphagnum graminoid tundra, (iii) Seasonally flooded Carex, Dupontia, Eriophorum graminoid (i) Dry Arctagrostis, Luzula, Poa, Carex graminoid tundra, (ii) Moist Carex, Poa, Luzula graminoid tundra, (iii) Wet Carex, Sphagnum graminoid tundra, (iv) Seasonally flooded Carex, Dupontia, Eriophorum graminoid (i) Aquatic Arctophila, Carex, Dupontia graminoid tundra N/A

Adapted from Brown et al., 1980; C.E. Tweedie, C.G. Andresen, R.D. Hollister, J.L. May, D.R. Bronson, A. Gaylord and P.J. Webber, in prep; Hubbard et al., 2013; Villarreal et al., 2012.

2.

3.

4.

5.

lichen heath and dry Arctagrostis, Luzula, Poa, Carex graminoid tundra. High-Center (HC) polygons have a relatively high relief and are generally found in well drained interstitial tundra regions or elevated sections of old-ancient DTLBs. Relief results from the thawing of ice-wedge troughs surrounding low- or flat-centered polygons, with the polygon center remaining as a topographic high once thermo-erosion has lowered the troughs. Dry Arctagrostis, Luzula, Poa, Carex graminoid tundra dominates this geomorphic type. Flat-Center (FC) polygons have an intermediate relief and are most common in dry-moderate soil moisture regimes. Plant communities of FC polygons include dry Arctagrostis, Luzula, Poa, Carex graminoid tundra and moist Carex, Poa, Luzula graminoid tundra. Low Center (LC) polygons are similar geomorphologically to HC polygons with the exception of having a submerged moist-aquatic center. Surrounding this moist-aquatic center are dry-moist rims. Four plant communities typically occur within this geomorphic type and are strongly influence by microtopography. On polygon rims: dry Arctagrostis, Luzula, Poa, Carex graminoid tundra is common whereas in polygon centers: moist Carex, Poa, Luzula graminoid tundra, wet Carex, Sphagnum graminoid tundra, and seasonally flooded Carex, Dupontia, Eriophorum graminoid tundra dominate. Coalescent Low Center (CLC) polygons occur through erosional and fragmentation of LC polygon rims, which creates a series of interconnected ponds or troughs.. This geomorphic type is often found within old-ancient DTLBs. The wet-aquatic soil moisture status of this geomorphic type results in seasonally flooded

© 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

6.

7.

8.

9.

Carex, Dupontia, Eriophorum graminoid tundra and aquatic Arctophila, Carex, Dupontia graminoid tundra being common. Meadows (Mdw) are most common in recently drained areas such as Young-Medium age DTLBs, where icewedge networks have a minimal development. This geomorphic type is characterized by moist-wet soils, low relief, and high productivity (Hinkel et al., 2003). Three plant communities dominate Mdw including moist Carex, Poa, Luzula graminoid tundra, wet Carex, Sphagnum graminoid tundra, and seasonally flooded Carex, Dupontia, Eriophorum graminoid tundra. Troughs (Tr) are drainage channels that are found on the perimeter of HC, FC, LC, and in some cases CLC polygons. These channels can have substantially different soil moisture ranging from moist to aquatic dependent on local drainage patterns. Plant communities can include Arctagrostis, Luzula, Poa, Carex graminoid tundra, moist Carex, Poa, Luzula graminoid tundra, wet Carex, Sphagnum graminoid tundra, and seasonally flooded Carex, Dupontia, Eriophorum graminoid tundra. Ponds or thermokarst ponds are generally small and shallow (1 ha) and ponds (0.241 and NDSWI of < 0.428, or slope >0.95) and Mdw (NDVI > 0.6, NDWI < 0.26 > 0.46, and DTLB age ≤300 years) were developed as the Supervised Classification was unable to differentiate DS and Mdw from HC and LC due to similar spectral reflectance properties. Specific thresholds were defined using spectral indices and slope derived from high resolution satellite imagery and DEMs relative to geomorphology ground control points. A cloud- and snow-free Quickbird satellite image mosaic was acquired for 1 August 2002 from Manley et al. (2006), to: (i) identify geomorphic types not resolved in Landsat data (Tr; 10%), were high-centered polygon (HC), flat-centered polygon (FC), low-centered polygon (LC), and Lakes, representing 11%, 17%, 24%, and 24%, respectively, while the other six geomorphic types [i.e., drained slope (DS), coalescent low center (CLC), meadow (Mdw), trough (Tr), Pond, River] cumulatively represented ~25% of the total land cover on the Barrow © 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

C H A N G E I N T U N D R A G E O M O R P H O L O G Y & C A R B O N F L U X 1641 Table 2 Accuracy assessment for the tundra geomorphology map Reference data Geomorphic Type Classification Urban Drained Slope High-Center Flat Center Low Center Coalescent Low Center Meadow Pond Lake River Total Producer accuracy (%) Overall accuracy (%) Kappa Weighted Kappa

Urban

DS

HC

FC

LC

15 2 4 3

1 43 15 7

1 11 28 17

2 5 92

1

1

5

CLC

Mdw

Pond

Lake

River

3

1 3 1.0

24 0.63

67 0.64

59 0.47

4 17 1

1 2 20

1 8

1 104 0.88

23 0.74

23 0.87

9 0.89

1 1 1 1 41 45 0.91

3 3 1.0

Total

3 17 58 52 125 20 30 9 43 3 360

User accuracy (%)

1.00 0.88 0.74 0.54 0.74 0.85 0.67 0.89 0.95 1.00

0.75 0.694 0.836

Trough (High res. classification) producer and user accuracy was 96 and 73%, n = 142; Kappa: 95% CI = 0.64–0.75, strength of agreement considered ‘good’, weighted Kappa: strength of agreement considered ‘very good’.

Fig. 3 Graphic representation of Drained Thaw Lake Basin age (Hinkel et al., 2003), interstitial tundra, and elevation ranges, used in spatial analysis.

Peninsula (Fig. 2, Table 2). We found Tr to be disproportionately represented among tundra geomorphic types. The dominant geomorphic types where Tr networks may be found are HC, FC, and LC representing ~93% of the total trough area on the Barrow Peninsula (Fig. 2). The distribution of geomorphic types was also determined within DTLBs, interstitial tundra, and within increasing elevation ranges. We found DTLBs, interstitial, and Lakes to represent 44%, 34%, and 24% (total not equal to 100 because lakes present in DTLBs), respectively, of the total land area on the Barrow Peninsula (Fig. 3). Relative DTLB ages of Young, Medium, Old, and Ancient, represented 3%, 8%, 27%, and 6%, respectively, of the total peninsula land area (Table 3). © 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

The primary difference in geomorphic types in Young through Ancient DTLBs was found in LC, CLC, and Mdw, where LC and CLC increased 15% and 11%, from Young to Ancient, respectively, and Mdw decreased 24%. Additionally, FC increased 4% from Young to Ancient DTLBs, with no other changes ≥2% identified. Elevation categories, 0–2, 2–8, 8–13, and >13 m, represented 6%, 73%, 19%, and 2%, of the total elevational range of the Barrow Peninsula, respectively. Generally, the extent of wetter geomorphic types decreased with increasing elevation, as geomorphic types Lakes, LC, CLC, and Mdw decreased by 32%, 8%, 3%, and 3%, respectively, between elevation categories, 0–2 and >13 m; in contrast, dry geomorphic types DS, HC, FC, and Tr increased 3%, 14%, 28%, and 4% (Fig. 4).

1642 M . J . L A R A et al. Table 3 Summary statistics of geomorphic types by terrain features across the Barrow Peninsula. See methods for the definition of geomorphic type Terrain Feature

Area (km²)

Elevation 0–2 m 115 2–8 m 1299 8–13 m 344 >13 m 34 Drained Thaw Lake Basin Young 59 Medium 146 Old 476 Ancient 101 All DTLB 782 Interstitial 609 All tundra 1792

Area (%)

DS

HC

FC

CLC

LC

Tr

Mdw

Pond

Lake

River

6% 73% 19% 2%

11% 7% 10% 14%

8% 10% 17% 23%

9% 14% 26% 36%

7% 8% 7% 4%

22% 26% 28% 14%

2% 3% 5% 6%

3% 3% 0.8% 0.2%

1.3% 1.0% 0.2% 0.1%

36% 28% 7% 4%

0.8% 0.6% 0.1% 0.1%

3% 8% 27% 6% 44% 34% 100%

3% 2% 4% 4% 4% 17% 8%

12% 8% 7% 10% 8% 23% 11%

15% 13% 15% 19% 16% 30% 17%

4% 13% 16% 15% 15% 3% 8%

28% 37% 44% 43% 43% 19% 24%

4% 5% 5% 2% 5% 6% 5%

24% 12% 1.7%* 0.1%* 4% 1.4% 2%

1.0% 0.8% 0.2% 0.2% 0.4% 0.5% 0.9%

5% 5% 1% 0% 2% 1% 24%

0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.6%

*Estimated from manual delineation of quickbird imagery.

Fig. 4 Differences in Geomorphic types (top row), NEE (middle row), and CH4 flux (bottom row) among Drained Thaw Lake Basin ages (left column) and Elevation ranges (right column). Variation in NEE between elevation ranges and DTLB ages correspond to proportional area differences in geomorphology. Carbon fluxes are not calculated for Lakes and rivers. Note logarithmic scale for CH4 fluxes.

Map accuracy assessment The accuracy assessment (Table 2) determined an overall map accuracy of 75%, with a Kappa and Weighted

Kappa of 0.694 and 0.836 respectively, suggesting the strength of agreement between an independent validation dataset and classification to be ‘good’ and ‘very good’ (Fleiss et al., 1969; Congalton, 1988). Given the © 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

© 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

2.9  1.1

0.8  0.6 164.1

525.5  170

222.6  60

6.5

770.7  212

252.4  76

8 10 0.1 0.05 82

    

     35 31 0.3 0.5 245.1

16.3  5.1

6.2  4.4 7 7 0.1 0.04 30

2.6  0.8

1.6  0.4

37 37 0.3 0.5 29.8

3.8  1.0

1.9  0.5

201, 11 1.2  0.4

136, 8

Area (km², landscape %) NEE (gCCO2 m 2 day ¹) GEE (gCCO2 m 2 day ¹) ER (gCCO2 m 2 day ¹) CH4 flux (mgCCO2 m 2 day ¹) TD (cm) WTD (cm) LAI (Index) NDVI (Index) NEE (106gCCO2 day ¹) GEE (106gCCO2 day ¹) ER (106gCCO2 day ¹) CH4 flux (106gCCH4 day ¹) C-CO2-eq (106gCCO2 day ¹)

HC

0.2  0.2

DS

Measurement

     5 3 0.2 0.08 283

233.4

2.3  2.0

713.8  192

1012.7  348

27 6 0.4 0.5 298.9

7.9  6.7

2.4  0.6

3.4  1.2

1.0  0.9

296, 17

FC

     1 1 0.2 0.05 159

226.7

5.6  1.4

1085.5  349

1015.9  334

27 3 0.4 0.5 69.6

12.9  3.3

2.5  0.8

2.3  0.8

0.2  0.4

434, 24

LC

     2 1 0.1 0.02 49

187.1

9.6  3.0

129.9  49

209.9  81

36 0 0.2 0.6 80.1

70.0  22.0

1.0  0.4

1.5  0.6

0.6  0.4

137, 8

CLC

3 2 0.1 0.07 101

85.2

3.9  2.1

304.2  60

498.1  124

    

43.2  23.5

3.4  0.7

5.6  1.4

2.2  1.1

90, 5

34 2 0.6 0.7 193.9

Tr

    

3 3 0.1 0.03 17

14.9

1.9  0.5

138.2  49

207.1  41

36 4 0.4 0.7 68.9

46.1  12.9

3.3  1.2

5.0  1.0

1.7  0.4

42, 2

Mdw

    

4 2 0.1 0.08 18

2.8

1.8  0.8

27.4  13

81.9  28

40 4 0.3 0.4 54.4

114.5  48.1

1.7  0.8

5.1  1.7

3.4  1.1

16, 1

Pond

902.3  464

93.2  428

28.9  16

3147.2  427

4048.7  667

– – – –









1342, 75

Total

Table 4 Summary of plot-level (top) and upscaled landscape-level (bottom) peak growing season means for ecosystem functional properties. Uncertainty estimates (i.e., right of means) represent 95% confidence intervals

C H A N G E I N T U N D R A G E O M O R P H O L O G Y & C A R B O N F L U X 1643

1644 M . J . L A R A et al. Table 5 Landscape-level peak growing season carbon fluxes estimated over 100 years of landscape change associated with the thaw-lake cycle (TLC) and incremental levels of Thermokarst (TK) pit formation. Uncertainty estimates (i.e., right of means) represent 95% confidence intervals Unit gC m

Geomorphic change 2

day ¹

106gC day ¹

2002_Present 2102_TLC 2102_TLC+TK10% 2102_TLC+TK30% 2102_TLC+TK50% 2002_Present 2102_TLC 2102_TLC+TK10% 2102_TLC+TK30% 2102_TLC+TK50%

NEE 0.67 0.67 0.71 0.79 0.88 902.3 900.1 955.9 1069.2 1182.5

GEE          

0.34 0.34 0.34 0.35 0.36 464 464 469 476 476

resolution (i.e., 30 9 30 m) and spectral characteristics of the Landsat-7 imagery, mixed pixels and misclassifications of spectrally similar classes were expected. However, most geomorphic classes were well represented by this geomorphic classification. The lowest accuracy was found in FC polygons, as this geomorphic type was difficult to determine even with the use of both high resolution imagery and DEMs. Additionally, the high resolution Quickbird classification, determined Tr user and producer accuracy to be 73% and 96% (Table 2).

Carbon fluxes by geomorphic type We identified considerable differences in plot-level CO2 and CH4 fluxes among geomorphic types (Table 4). The lowest NEE (e.g., greatest uptake, where negative values indicate carbon uptake from the atmosphere and positive values indicate carbon loss to the atmosphere) was found in aquatic-wet geomorphic types, Pond ( 3.4 gC-CO2 m 2 day 1), Tr ( 2.2 gC-CO2 m 2 day 1), and Mdw ( 1.7 gC-CO2 m 2 day 1), which also had the largest uptake in GEE. Drier geomorphic types, DS, HC, FC, varied less in NEE ( 0.2 to 1.2 gCCO2 m 2 day 1), GEE ( 1.9 to 3.8 gC-CO2 m 2 1 2 day ), and ER (1.6 to 2.6 gC-CO2 m day 1). Further, both CO2 and CH4 fluxes in DS were among the lowest of all geomorphic types (Table 4). Low center NEE was 0.2 gC-CO2 m 2 day 1, as ER was greater than GEE, making LC the only geomorphic type to have a positive NEE (i.e., carbon loss) during the peak growing season. Not surprisingly, CH4 fluxes were highest in aquaticwet anaerobic geomorphic types, Ponds (114.5 mgCCH4 m 2 day 1), CLC (70.0 mgC-CH4 m 2 day 1), Mdw (46.1 mgC-CH4 m 2 day 1), and Tr (43.2 mgCCH4 m 2 day 1). The remaining CH4 fluxes for moistdry geomorphic types were 13 m, representing 0.6, 0.6, 0.7, and 0.8 gC-CO2 m 2 day 1, respectively. While, GEE and ER were estimated at 2.8, 2.9, 3.0, 3.2 gC-CO2 m 2 day 1, and 2.2, 2.3, 2.3, and 2.4 gC-CO2 m 2 day 1 respectively, CO2 uptake increased and respiratory losses remained fairly consistent with increasing elevation. Conversely, CH4 fluxes decreased with increasing © 2014 John Wiley & Sons Ltd, Global Change Biology, 21, 1634–1651

C H A N G E I N T U N D R A G E O M O R P H O L O G Y & C A R B O N F L U X 1645 elevation, as 0–2, 2–8, 8–13, and >13 m, representing 22.5, 21.6, 17.4, and 14.7 mgC-CH4 m 2 day 1, respectively. At the scale of the Barrow Peninsula, we estimate total NEE and CH4 flux to presently account for 902.3 106gC-CO2 day 1 (uncertainty between 438.3 and 1366 106gC-CO2 day 1) and 28.9 106gC-CH4 day 1 (uncertainty between 12.9 and 44.9 106gC-CH4 day 1; Table 5). Across this landscape, the geomorphic types highest in GEE and ER were LC, FC, HC, and Tr, which accounted for 25%, 25%, 19%, and 12%, respectively, of the total daily GEE and 35%, 23%, 17%, and 10%, respectively, of the total daily ER. While GEE and ER were largely neutral in DS, CLC, Mdw, and Pond, representing 6%, 5%, 5%, and 2%, respectively in GEE and 7, 4, 4, and

Polygonal tundra geomorphological change in response to warming alters future CO2 and CH4 flux on the Barrow Peninsula.

The landscape of the Barrow Peninsula in northern Alaska is thought to have formed over centuries to millennia, and is now dominated by ice-wedge poly...
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