JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, G04002, doi:10.1029/2005JG000150, 2006 Click Here for Full Article

Shrinking ponds in subarctic based on 1950–2002 remotely sensed images Brian Riordan,1 David Verbyla,1 and A. David McGuire1,2 Received 5 December 2005; revised 2 June 2006; accepted 20 June 2006; published 10 October 2006.

[1] Over the past 50 years, Alaska has experienced a warming climate with longer growing seasons, increased potential evapotranspiration, and permafrost warming. Research from the Seward Peninsula and Kenai Peninsula has demonstrated a substantial landscape-level trend in the reduction of surface water and number of closed-basin ponds. We investigated whether this drying trend occurred at nine other regions throughout Alaska. One study region was from the Arctic Coastal Plain where deep permafrost occurs continuously across the landscape. The other eight study regions were from the boreal forest regions where discontinuous permafrost occurs. Mean annual precipitation across the study regions ranged from 100 to over 700 mm yr1. We used remotely sensed imagery from the 1950s to 2002 to inventory over 10,000 closed-basin ponds from at least three periods from this time span. We found a reduction in the area and number of shallow, closed-basin ponds for all boreal regions. In contrast, the Arctic Coastal Plain region had negligible change in the area of closed-basin ponds. Since the 1950s, surface water area of closed-basin ponds included in this analysis decreased by 31 to 4 percent, and the total number of closed-basin ponds surveyed within each study region decreased from 54 to 5 percent. There was a significant increasing trend in annual mean temperature and potential evapotranspiration since the 1950s for all study regions. There was no significant trend in annual precipitation during the same period. The regional trend of shrinking ponds may be due to increased drainage as permafrost warms, or increased evapotranspiration during a warmer and extended growing season. Citation: Riordan, B., D. Verbyla, and A. D. McGuire (2006), Shrinking ponds in subarctic Alaska based on 1950–2002 remotely sensed images, J. Geophys. Res., 111, G04002, doi:10.1029/2005JG000150.

1. Introduction increased growing season and primary production [Zhou et al., 2001; Stow et al., 2003]. [2] Recent environmental changes associated with cli- [3] In subarctic interior Alaska, the growing season mate warming at high latitudes have been summarized climate regime switched from a predominantly cool and [Serreze et al., 2000; Overland et al., 2002, 2004; Hinzman moist to hot and dry after a Pacific-wide regime shift in et al., 2005] and include abiotic changes such as reduction 1977 [Mantua et al., 1997; Hare et al., 1999; Hare and in annual snow cover [Brown and Goodison, 1996; Brown Mantua, 2000; Dickson, 2000]. On the basis of tree-ring and Braaten, 1998], reduction in sea ice extent [Bjørgo et reconstructed climate and recorded summer temperature al., 1997; Cavalieri et al., 1997], warming and thawing of observations, some of the warmest summers over the past permafrost [Osterkamp and Romanovsky, 1999; Camill, 200 years in interior Alaska have occurred since the 1970s, 2005], increased thermokarsting [Jorgenson et al., 2001], while the coolest growing seasons occurred in the early to warming of lake water [Hobbie et al., 2003], and earlier mid 1900s [Barber et al., 2004]. On the basis of tree-ring spring river and lake ice break-up [Keyser et al., 2000; analysis, drought stress associated with a warming climate Magnuson et al., 2000]. Biotic changes at high latitudes in Alaska may be widespread at nontreeline sites [Barber et have also occurred including expansion of boreal forest into al., 2000; Lloyd and Fastie, 2002; Wilmking et al., 2004]. tundra [Suarez et al., 1999], increased shrubbiness in tundra [4] Climate warming may cause a significant change in [Chapin et al., 1995; Sturm et al., 2001], expansion of regional hydrology in subarctic regions. For example, shrubs into drying lake beds [Klein et al., 2005], and Roulet et al. [1992] estimated a drop in fen water table level of 190 to 240 mm under a climate warming scenario. While precipitation in interior Alaska has not changed 1Bonanza Creek Long Term Ecological Research Program, University substantially in recent decades, warming temperatures have of Alaska, Fairbanks, Alaska, USA. 2 led to increases in potential evapotranspiration (PET) for Now at U.S. Geological Survey, University of Alaska, Fairbanks, Alaska tundra regions [Oechel et al., 2000]. In this paper, we Alaska, USA. use remotely sensed imagery from the past 50 years to in- Copyright 2006 by the American Geophysical Union. vestigate whether there was a landscape-level change in 0148-0227/06/2005JG000150$09.00

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Figure 1. Alaska study regions and long-term first order weather stations used in the study. Study regions: 1, Arctic Coastal Plain; 2, Steven Village Area; 3, Yukon Flats National Wildlife Refuge; 4, Minto Flats State Game Refuge; 5, Denali National Park; 6, Talkeetna Area; 7, Innoko National Wildlife Refuge; 8, Tetlin National Wildlife Refuge; 9, Copper River Basin, Wrangell St. Elias National Park. Weather Stations: Ba, Barrow; Be, Bettles; Fa, Fairbanks; Gu, Gulkana; Mc, McGrath; No, Northway; Ta, Talkeetna. closed basin ponds associated with the recent climate warm- and shorter growing season and underlain by continuous ing in Alaska. permafrost of hundreds of meters. We also selected one region (Talkeetna) from the Cook Inlet Basin, south of the 2. Methods Alaska Range. Talkeetna, which lies in a more maritime climate, had approximately twice the mean annual precip- 2.1. Study Areas itation relative to interior Alaska study regions and was the [5] Our main study area is the subarctic boreal region of 2 only study region with a mean annual temperature above interior Alaska. This region spans over 5 million km , 0°C (Table 1). The Copper River Basin is south of the bounded on the north by the Brooks Range and on the Alaska range, but has a continental climate similar to south by the Alaska Range. These mountain ranges act as interior Alaska because it is in a rain shadow of the barriers to marine air masses and consequently Interior surrounding Chugach, Wrangell and Alaska mountain Alaska is semi-arid with an annual precipitation of 500 to ranges. All study regions were alluvial or lacustrine plains <200 mm [Fleming et al., 2000; Hammond and Yarie, with a maximum slope gradient of less than 5 percent. 1996]. Interior Alaska has experienced an increase in growing season length [Keyser et al., 2000] and a warmer, 2.2. Remote Sensing Methods drier climate [Barber et al., 2000, 2004]. It is also a region [7] To estimate surface water changes, we used remotely of discontinuous permafrost that is warming and degrading sensed imagery from the 1950s to 2002. For each study [Osterkamp and Romanovsky, 1999; Jorgenson et al., 2001] region, we analyzed scanned panchromatic aerial photo- which may lead to drier soils in some boreal sites [Camill, graphs from the 1950s, scanned color infrared aerial photo- 2005]. graphs from 1978–1982, and digital images from the [6] Our selection of study regions in interior Alaska was Landsat Enhanced Thematic Mapper Plus sensor (ETM+) constrained by the availability of cloud-free aerial photog- from 1999–2002. Landsat Thematic Mapper TM images raphy and satellite imagery. Primarily on the basis of from 1991–1995 provided an extra observation period for imagery availability, we selected low-lying tectonic basins some study regions (Table 1). Smoke from wildfires and (Figure 1) with imagery from at least three time periods clouds were a major limitation in acquiring useful satellite ranging from the 1950s to 2002. To contrast our study imagery from the summer months. Because of the high regions from interior Alaska, we also selected a study region latitude, cloud shadows cover a greater area than clouds; for from the Arctic Coastal Plain, an area with a much colder

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Table 1. Study Regions Climatology and Imagery Datesa Mean Annual Temperature Mean Precipitation Study Region Area Number of Water Study Region 1971–2000, °C 1971–2000, mm Weather Station (1000 ha) Bodies Delineated Imagery Analyzed Arctic Coastal Plain Region Arctic Coastal Plain 11.8 102 Prudhoe Bay 26 1426 1954 aerial photography 1978 aerial photography 3 Aug 1999 Landsat ETM+

Interior Alaska Regions

Tetlin National Wildlife Refuge 4.9 238 Northway 95 2152 1954 aerial photography: 22 August ALASKA BOREAL ACROSS PONDS SHRINKING AL.: ET RIORDAN 1978 aerial photography: 1 July 1981 aerial photography: 1 August 1999 Landsat ETM+: 4 August Yukon Flats National Wildlife Refuge 3.9 193 Circle 202 1370 1952 aerial photography: 14 June,24 June, 5 July, 18 July, 21 August, 13 September 1979 orthoquad maps 2000 Landsat ETM+: 16 August Stevens Village 3.9 193 Circle 107 1218 1951 aerial photography: 27 June, 11 July 1978 aerial photography: 1 July 1991 Landsat TM: 29 June 2000 Landsat ETM+: 16 August 2002 Landsat ETM+: 6 August 3of11 Minto Flats State Game Refuge 2.9 262 Fairbanks Airport 81 1390 1951 aerial photography: 27 June, 11 July 1954 aerial photography: 7 August 1980 aerial photography: 1 July 1991 Landsat TM: 29 June 2000 Landsat ETM+: 16 August 2002 Landsat ETM+: 6 August Copper River Basin 2.7 283 30 101 1955 aerial photography: 5 July 1957 aerial photography: 29 July 1978 aerial photography: 28 August 1995 Landsat TM: 8 August 2001 Landsat ETM+: 7 August Denali National Park 2.8 380 McKinley Park 82 876 1951 aerial photography: 28 August 1952 aerial photography: 20 July, 5 August 1954 aerial photography: 7 August 1979 aerial photography: 1 August 1981 aerial photography: 23 August 2000 Landsat ETM+: 16 August Innoko Flats National Wildlife Refuge 2.8 445 McGrath Airport 63 1205 1952 aerial photography: 14 June, 24 June 1980 aerial photography: 1 July 2001 Landsat ETM+: 18 August

Cook Inlet Basin Region Talkeetna 1.1 716 177 1052 1951 aerial photography: 3 July 1953 aerial photography: 3 July, 11 July 1954 aerial photography: 17 June

1980 aerial photography: 1 August G04002 2000 Landsat ETM+: 16 August aTemperature and precipitation source: Alaska Climate Research Center (http://climate.gi.alaska.edu/Climate/Normals/). G04002 RIORDAN ET AL.: SHRINKING PONDS ACROSS BOREAL ALASKA G04002

Table 2. Area Estimates of Closed Pond Surface Water by Study Regiona Aerial Landsat TM/ETM+ Percent Change Photography Imagery (1950s – Most Study Region 1950s 1978–1982 1991–1995 1999–2001 2002 Recent Estimate) Innoko Flats National Wildlife Refuge 2126 1532 ... 1483 ... 31 Copper River Basin. Wrangell 130 95 85 93 ... 28 St. Elias National Park Minto Flats State Game Refuge 4066 3763 3952 3319 3060 25 Yukon Flats National Wildlife Refuge 7773 8097 ... 6359 ... 18 Stevens Village 5132 5106 4768 4746 4398 14 Talkeetna 1530 1479 ... 1455 ... 5 Denali National Park 1758 1964 ... 1681 ... 4 Tetlin National Wildlife Refuge 4811 4566 ... 4597 ... 4 Arctic Coastal Plain 3162 3127 ... 3208 ... 1 Totals 30,488 29,729 26,941 11.6 aUnits are hectares. example an image with 25 percent cloud cover would be surface water was estimated between each time period and over 50 percent unusable owing to cloud and associated the 1950s period. cloud shadows covering the Earth’s surface. 2.3. Meteorological Data [8] The 1950s panchromatic aerial photographs were paper prints at an original scale of 1:40,000 obtained from [12] For the closest long-term weather station to each the U. S. Geological Survey (USGS), Anchorage, Alaska. study region, we obtained Summary of the Day data set The 1978–1982 aerial photographs were 1:60,000 in color (daily maximum temperature, daily minimum temperature, infrared transparencies from the Alaska High Altitude and daily total water equivalent precipitation, including Aerial Photography (AHAP) Program and available from snow) from the Alaska Climate Research Center in Fairbanks, the GeoData Center, University of Alaska Fairbanks. All Alaska. Unfortunately, Alaska has few stations with weather photographs were scanned at 600 dpi to create digital records spanning more than 50 years and therefore some images. Panchromatic digital ortho photographs were avail- stations used in the study are located hundreds of kilometers able for the Yukon Flats study area as a product from the from the center of a study region (Figure 1). AHAP program and were obtained from the Yukon Flats [13] For each weather station, we computed mean monthly National Wildlife Refuge headquarters, Fairbanks, Alaska. temperature as Landsat TM and ETM+ digital images were ordered from "#"#! the EROS Data Center, Sioux Falls, South Dakota. All Xn Xn images were from the period of mid-June to mid-August. Max=n þ Min=n =2; ð1Þ i¼1 i¼1 [9] All digital images were georectified to the UTM map projection, NAD27. We used the statewide coverage of where n is number of days in each month and Max, Min are 1:63,360 topographic maps as the source for control points. daily maximum/minimum temperatures. We used monthly Each digital image was rectified using a second-order mean temperature to estimate annual potential evapotran- polynomial model based on at least 25 control points. Each spiration (PET) using the Thornthwaite’s method rectification model had a maximum Root Mean Squared [Thornthwaite, 1948]. We then estimated water balance at Error of less than 1 satellite image pixel (30 m). After each weather station as precipitation – PET for each year. completing the rectification process, we had a time series of spatially aligned images for each study region. [10] We used closed ponds (lacking inlets or outlets) 3. Results because they are sensitive to changes in climate in semiarid [14] All study regions in subarctic Alaska had a reduction regions such as interior Alaska [Barber and Finney, 2000]. in the area of closed ponds, ranging from 31 to 4 percent Initially we attempted automated classification of closed (Table 2). The Arctic Coastal Plain region had negligible ponds. We abandoned automated spectral approaches com- change. The number of closed ponds decreased over the monly used in digital image processing due to problems 50-year period for all study regions (Table 3). As surface from (1) sunglint near the edge of aerial photographs water decreased, smaller multiple ponds were sometimes creating bright ponds that were confused with other bright created (Figure 2), and therefore the loss of closed basin targets such as meadows, and (2) cloud shadows creating ponds may be a conservative estimate in some areas. Most dark patches that were spectrally similar to water. of the decrease in number of ponds occurred after a [11] We visually delineated all closed water bodies from climatic regime shift [Mantua et al.,1997;Hare and each image that were above a minimum area of 0.2 ha. The Mantua, 2000] in the 1970s (Table 3). shoreline of each closed pond was manually traced as a [15] Except for Barrow, we found a significant (p < 0.05) polygon area using a Geographic Information System (GIS). linear increase in mean annual temperature and annual total An inventory of closed water bodies from each time period PET from all first-order weather stations (Table 4 and was then analyzed using the GIS. For each study region, the Figure 3). There was no significant trend for total annual change in the number of closed water bodies and the area of precipitation at these stations. Water balance, defined as

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Table 3. Total Number of Shallow, Closed-Basin Ponds Inventoried by Study Region

Aerial Landsat TM/ETM+ I Percent Change Photography magery (1950s – Most Study Region 1950s 1978–1982 1991–1995 1999–2001 2002 Recent Estimate) Innoko Flats National Wildlife Refuge 1205 1053 ... 839 ... 30 Copper River Basin 101 53 37 46 ... 54 Minto Flats State Game Refuge 1390 1203 820 934 891 36 Yukon Flats National Wildlife Refuge 1370 1267 ... 1228 ... 10 Stevens Village 1141 1218 850 896 933 18 Talkeetna 1052 1026 ... 876 ... 17 Denali National Park 876 964 ... 834 ... 5 Tetlin National Wildlife Refuge 2152 2293 ... 1725 ... 20 Arctic Coastal Plain 1426 1448 ... 1265 ... 11 Totals 10,713 10,525 8643 19 annual total precipitation – annual total PET (Table 4) had a There was a reduction in closed pond area for 6 of the consistently negative trend for all stations except Bettles. 8 subarctic study regions from 1950s to 1978–1982, based The Bettles station is located in the foothills of the Brooks exclusively on aerial photography (Table 2). Range, 260 km away from the Yukon Flats study region [17] If the first period in the time series was an unusually (Figure 1) and may not have been representative of that wet year, then the trend might be due to a sampling artifact study region. within the range of natural variability of precipitation. We normalized precipitation of every year corresponding to our 4. Discussion study image years as

[16] There are several nonclimatic potential reasons for the long-term trend in pond shrinkage. One source of ðÞPrecip Mean Precip =Std: Dev:; ð2Þ potential error in our estimates is the larger pixel size (30 m) in our latest period using satellite imagery relative where Precip is September through August total precipita- to the 1950s and 1978–1982 aerial photography (3 m pixel tion for image year, Mean Precip is Mean September size). However, we believe there was a tendency to over- through August total precipitation from 1950–2004, and estimate pond areas using satellite imagery since water Std. Dev. is Standard Deviation of September through strongly absorbs midinfrared radiation and subpixel water August total precipitation. For most of the periods absorption typically leads to an overestimate of ‘‘water pixels.’’ corresponding to our remotely sensed images, the normal-

Figure 2. Shrinkage of open water within the Yukon Flats National Wildlife Refuge. Notice that it is likely to increase the number of smaller ponds as the area of surface water steadily decreases from larger ponds. The series are typical examples of the water loss patterns occurred during the 50-year study period.

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Table 4. Linear Temporal Trends in Mean Annual Temperature, Annual Precipitation, Annual Potential Evapotranspiration (PET), and Water Balance Based on the Closest Long-Term, First-Order Weather Station to Each Study Regiona Mean Annual Total Annual Total Annual Annual Water Temperature, °C Precipitation, mm PET, mm Balance Study Regions Weather Station Slope R2 P Slope R2 P Slope R2 P Slope R2 P Arctic Coastal Plain Barrow Airport 0.001 0.044 0.116 0.482 0.050 0.089 0.761 0.098 0.016 1.243 0.150 0.002 (1941–2002) Yukon Flats 0.046 0.259 0.0001 1.189 0.035 0.190 0.607 0.131 0.009 0.582 0.007 0.559 (1951–2002) Minto Flats/Stevens Village/ Fairbanks Airport 0.037 0.209 0.0005 0.707 0.018 0.340 0.716 0.170 0.002 0.707 0.176 0.340 Denali National Park (1946–2002) Tetlin National Wildlife Northway Airport 0.034 0.209 0.0008 0.491 0.0178 0.350 0.379 .068 0.064 0.111 0.0007 .8511 Refuge (1949–2002) Copper River Basin Gulkana Airport 0.016 0.063 0.055 0.150 0.002 0.741 0.392 0.010 0.015 0.243 0.004 0.621 (1941–2002) Innoko Flats National McGrath Airport 0.026 0.136 0.003 0.251 0.002 0.743 0.466 0.150 0.002 0.717 0.015 0.354 Wildlife Refuge (1939–2002) Talkeetna Talkeetna Airport 0.024 0.116 0.007 0.354 0.001 0.771 0.349 0.070 0.039 0.703 0.005 0.572 (1940–2002) aWater balance is annual precipitation – PET. ized precipitation was within 1 standard deviation of the creased in interior Alaska by 5.5 mm year1 since the 1960 1950–2004 mean (Figure 4). [Hinzman et al., 2005]. On the basis of tree-ring analysis, [18] We believe that increased evapotranspiration due to drought-stress of white spruce trees in interior Alaska has warmer and longer growing seasons is one factor associated occurred during the warming period since 1950 [Barber et with the long-term trend of shrinking pond surface areas. al., 2000; Lloyd and Fastie, 2002]. There has also been a On the basis of analysis of 1988–2002 satellite microwave substantial reduction in the area and number of water bodies data [Smith et al., 2004], the growing season (spring thaw to on the Kenai Peninsula, Alaska, due to a decrease in water autumn freeze) increased about 5 days decade1, and within balance since 1950 [Klein et al., 2005]. In contrast, we Alaska was highest in our study region of interior Alaska. found no reduction in the area and number of water bodies While our analyses did not identify a significant increase in in coastal Arctic Alaska, although summer water deficits annual water deficit since 1950, other studies have identi- have increased in that region during recent decades [Oechel fied that summer (June–August) water deficits have in- et al., 2000] [see also Hinzman et al., 2005]. Coastal Arctic

Figure 3. Ten-year running means of potential evapotranspiration (PET) for long-term, first-order weather stations. The linear trend since 1970 was positive and significant (P < 0.01) for all stations analyzed.

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Figure 4. Growing season precipitation (May through August) scaled as normal standard deviate by first-order weather stations closest to each study area.

Alaska is characterized by a thaw-lake cycle that involves 1950s while neighboring ponds decreased substantially in drainage leading to several thousand years of vegetation surface area (Figures 5a and 5b). This pattern may be related succession followed by lake development that ultimately to the distribution of discontinuous permafrost and subsurface results in drainage [Hopkins, 1949; Billings and Peterson, drainage related to warming permafrost [Yoshikawa and 1980; Hinkel et al., 2003; Bockheim et al., 2004]. Our Hinzman, 2003]. analysis suggests that the thaw-lake cycle in our study [20] Wildfire in the Alaska boreal forest can accelerate region of Arctic Alaska has been stable over the last eventual warming of permafrost by removing much of the 50 years. insulating duff layer and increasing soil thermal conductiv- [19] Shrinking pond surface areas may become a common ity and ground heat flux [Yoshikawa et al., 2003; Chambers feature in discontinuous permafrost regions as a consequence and Chapin, 2002]. In a warming climate, the loss of a of warming climate and thawing permafrost [Yoshikawa and frozen soil aquiclude may promote long-term soil drying Hinzman, 2003]. Another mechanism for pond drainage is [Swanson, 1996], especially with the increase in transpira- the expansion of a layer of unfrozen soil (talik) allowing tion as prefire black spruce is replaced by early succession drainage. Smith et al. [2005] used satellite imagery from the broadleaf species [Betts et al., 1999]. Thus wildfire legacies 1970s and 1997–2004 to document the decline in lake may be another factor responsible for the observed pattern abundance and surface area for thousands of lakes in Siberia. of shrinking ponds. On the basis of known fires recorded by Their analysis showed an increase of total lake area and the Alaska Fire Service, some of the study regions had over numbers in a continuous permafrost region, and a contrasting 25 percent of their areas covered by known wildfires since decrease in regions of discontinuous, sporadic, and isolated the 1950s (Table 5). permafrost. Yoshikawa and Hinzman [2003] used remotely [21] Since wildfire, permafrost thawing, and evapotrans- sensed imagery to document a decrease in surface area of piration may have increased in our study regions since the thermokarst ponds in western Alaska. They used ground 1950s, it is difficult to determine the dominant driving penetrating radar to detect talik that would allow subsurface mechanism behind the observed pattern of shrinking ponds. drainage below the shrinking ponds. Smith et al. [2005] All three mechanisms have been documented in boreal observed permanently drained lakes commonly alongside Alaska. For example, in a study of 11 wildfire burns in undrained neighboring lakes, suggesting a spatially patchy interior Alaska, Yoshikawa et al. [2003] found that all burns process related to permafrost distribution rather than a direct older than 10 years had lower soil moisture content relative climatic mechanism such as increased evaporation. We also to adjacent unburned control areas. The long-term effect of found a heterogeneous pattern of shrinking ponds within wildfire on decreased soil moisture may be due to both some study regions. For example, in the Yukon Flats study increased transpiration and thawing of permafrost. In the region, there was a slight increase in closed pond surface area permafrost-dominated Seward Peninsula, Yoshikawa and at a higher elevation area, while lower elevations had a Hinzman [2003] found a reduction in thermokarst ponds decrease of over 18 percent. Even within the extent of a over ice wedge terrain exceeding 80 percent based on 1950 single aerial photograph, there was a heterogeneous pattern of to 1981 aerial photography. They concluded the major shrinking ponds, with some ponds remaining stable since the mechanism for shrinking ponds was formation of taliks

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Figure 5. Heterogeneous pattern of shrinking ponds. For example, ponds A, B, and C exhibit little change in surface water area, while ponds D, E, and F shrink substantially since the 1950s.

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Figure 5. (continued)

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Table 5. Percent of Study Regions Burned by 1950–2000 Fires [23] Acknowledgments. This project was funded through support from the NASA Land Cover/Land Use Change Program (NAF-11142) Mapped by the Alaska Fire Service and the Bonanza Creek LTER (Long-Term Ecological Research) program Study Region Area Burned, % (funded jointly by NSF grant DEB-0423442 and USDA Forest Service, Pacific Northwest Research Station grant PNW01-JV11261952-231). Innoko Flats National Wildlife Refuge 20.0 Copper River Basin. Wrangell 0.2 References St. Elias National Park Minto Flats State Game Refuge 32.4 Barber, V., and B. P. Finney (2000), Late Quaternary paleoclimatic recon- Yukon Flats National Wildlife Refuge 18.3 structions for interior Alaska based on paleolake-level data and hydro- Stevens Village 37.5 logic models, J. Paleolimnol., 24, 29–41. Talkeetna 0.0 Barber, V., G. P. Juday, and B. Finney (2000), Reduced growth of Alaskan white spruce in the twentieth century from temperature-induced drought Denali National Park 13.5 stress, Nature, 405, 668–673. Tetlin National Wildlife Refuge 10.9 Barber, V., G. P. Juday, B. P. Finney, and M. Wilmking (2004), Reconstruc- Arctic Coastal Plain 0.0 tion of summer temperatures in interior Alaska from tree-ring proxies: Totals Evidence for changing climate regimes, Clim. Change, 63, 91–120. Betts, A. K., M. Goulden, and S. Wofsy (1999), Controls on evaporation in a boreal spruce forest, J. Clim., 12, 1601–1618. Billings, W. D., and K. M. Peterson (1980), Vegetational change and ice- allowing internal drainage through the permafrost. In the wedge polygons through the thaw-lake cycle in Arctic Alaska, Arct. Alp. permafrost-free Kenai Peninsula of Alaska, Klein et al. Res., 1, 413–432. Bjørgo, E., O. M. Johannessen, and M. W. Miles (1997), Analysis of [2005] found a reduction in pond area exceeding 70 percent merged SSMR-SSMI time series of Arctic and Antarctic sea ice para- for two subregions based on 1950 to 1996 aerial photogra- meters 1978–1995, Geophys. Res. Lett., 24, 413–416. phy. The authors concluded the major mechanism for Bockheim, J. G., K. M. Hinkel, W. R. Eisner, and X. Y. Dai (2004), Carbon pools and accumulation rates in an age-series of soils in drained thaw- shrinking ponds was decreased water balance in a warming lake basins in Arctic Alaska, Soil Sci. Soc. Am. J., 68, 697–704. and drying climate. Brown, R. D., and R. O. Braaten (1998), Spatial and temporal variability [22] The reduction in the surface area of closed-basin of Canadian monthly snow depths, 1946–1995, Atmos. Ocean, 36, 37–54. lakes that we have detected in this study may be the initial Brown, R. D., and G. E. 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