Recent stabilization of active sand dunes on the Canadian prairies and relation to recent climate variations

Geomorphology 68 (2005) 131 – 147 www.elsevier.com/locate/geomorph

Recent stabilization of active sand dunes on the Canadian prairies and relation to recent climate variations Chris H. Hugenholtza,*, Stephen A. Wolfeb b

a Department of Geography, University of Calgary, Calgary, Alberta, TN, Canada T2N 1N4 Geological Survey of Canada, Terrain Sciences Division, Ottawa, Ontario, Canada K1A 0E8

Received 13 November 2003; received in revised form 1 March 2004; accepted 1 April 2004 Available online 28 December 2004

Abstract The historical activity of sand dunes on the southern Canadian prairies was investigated in order to determine the relation with recent climate variations. Changes in dune activity were determined from historical aerial photographs at seven sites with active parabolic dunes in Saskatchewan and Manitoba. The methodology developed combines digital image processing and geographical information system (GIS) analysis to measure changes in dune activity from the aerial photographs, as well as correlation analysis to determine the relation between dune activity and climate variations. Overall, dune activity has been decreasing since the early to mid 1900s as evidenced by greater vegetation colonization than sand dune migration or reactivation. The decrease in dune activity is significantly correlated with decadal-scale variations in aridity and is coincident with a decrease in annual wind speed. Evidence suggests that this response is superimposed on a longer trend towards stabilization, possibly in place since the late 1700s. Taken together, the effect of this complex response is that dune recovery on the southern Canadian prairies is prolonged well after an initial disturbance and therefore gives the impression of anomalous levels of activity for extended periods thereafter. For this reason, dune mobility indices predicated on short-term climate variations (years to decades) do not fully describe the present level of dune activity on the southern Canadian prairies. D 2004 Elsevier B.V. All rights reserved. Keywords: Continental dunes; Wind transport; Climate; Canada

1. Introduction Sand dunes and aeolian sand sheets are major features of the surficial geology of Canada’s prairie * Corresponding author. Tel.: +1 403 220 5584; fax: +1 403 282 6561. E-mail addresses: [email protected] (C.H. Hugenholtz)8 [email protected] (S.A. Wolfe). 0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.04.009

provinces. Stratigraphic and geochronological evidence accumulated in the last 30 years has demonstrated that many dune fields on the Canadian prairies are not merely relict features from deglaciation, but mobilized recurrently throughout the Holocene and late Holocene in response to climate change (David, 1971; Wolfe et al., 2000, 2001, 2002). Although most dunes and sand sheets are presently stabilized by vegetation under the modern climate regime, there are

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several dune fields in the southern prairie provinces with active dunes. David (1993, 1998) suggested that, under the present climate, the long-term trend of these sand dunes is toward stabilization, an observation supported by Wolfe et al. (1995, 2000). At present, however, the factors that have caused diminished dune activity on the Canadian prairies, and the rate and regional synchronicity of dune stabilization, remain poorly understood. Active sand dunes are sensitive to atmospheric conditions that limit sediment transport capacity, as well as surface conditions that limit sediment supply and availability. In turn, transport capacity is proportional to the sand-moving power of wind, and sediment supply and availability are inversely proportional to vegetation cover (Lancaster, 1988). From these relations, several wind erosion or dune mobility indices have been developed for various parts of the world (Chepil et al., 1962; Talbot, 1984; Ash and Wasson, 1983; Wasson, 1983; Lancaster, 1988). The most widely used index is Lancaster’s (1988) dune mobility index (M): M ¼ W =P : PE

ð1Þ

where W is the percent of time wind above the threshold velocity for sand transport, and P:PE is the ratio of precipitation to potential evapotranspiration, which serves as a proxy for vegetation cover. Lancaster’s index has proven successful in a wide variety of environments. On the Great Plains, the index has been used to estimate the potential impacts of future climate change scenarios (Muhs and Maat, 1993; Wolfe, 1997), as well as to hindcast the climatic controls on recent historical dune activity (Muhs and Holliday, 1995). However, most occurrences of active sand dunes on the northern Great Plains of Canada (Canadian prairies) are not well explained by Lancaster’s index (Muhs and Wolfe, 1999). In most instances, these dunes exhibit higher levels of activity than is predicted by Lancaster’s index. Although small in areal extent, there are no other dune fields on the central Great Plains of the United States that show the same level of activity (Muhs and Wolfe, 1999). Wolfe (1997) speculated that some of the active dunes may represent the last areas to stabilize following major droughts in the late 1700s (cf. Wolfe et al., 2001), suggesting that the dunes have experienced a protracted period of stabilization.

If sand dunes on the Canadian prairies are still recovering from a disturbance in the late 1700s, as has been speculated, then there are implications with respect to the utility of dune activity as a proxy indicator of short-term (annual to decadal) environmental change. With this in mind, a study was undertaken to investigate whether dune activity has responded to recent climate variations. The approach used in this study consisted of measuring the changes in sand dune activity from aerial photographs for the past 75 years and comparing the changes with historical climate data. By comparing numerous sites on the Canadian prairies, this study also provides an indication of the rates and regional synchronicity of historical dune activity, as well as the recent geomorphic evolution of the aeolian landscape.

2. Setting 2.1. Regional geology, climate, and Holocene activity In Canada, the most active continental dune occurrences are located within the southern prairies in the region referred to as the Palliser Triangle (Fig. 1). Outside of this region, only the Lake Athabasca dune system in northern Saskatchewan and Alberta, and the Brandon Sand Hills of Manitoba have significant areas of active dunes under the present climate. Because most of the southern Canadian prairies were glaciated during the Late Wisconsinan, the surficial sediments are dominantly glacigenic. Consequently, the sand supply to most dune fields in the Canadian prairies is derived from sandy glaciolacustrine, glaciofluvial, and deltaic sediments (David, 1977), which are relict from deglaciation. This is an important characteristic because it implies that most of Canada’s continental dune systems effectively are closed systems with finite sediment supplies; therefore, variations in climatic factors that control sediment availability and/or transport capacity are the main controls on dune activity (Muhs and Wolfe, 1999). The climate of the southern prairie provinces is continental with long cold winters and short but warm summers. The Rocky Mountains to the west form an effective barrier to the moderating influence of warm moist air from the Pacific Ocean. Average annual precipitation is generally less than 450 mm (Wolfe,

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Fig. 1. Map of sand dune areas in the southern Canadian prairie provinces. Sand transport diagrams derived from the PSD reports, published between 1983 and 1985 by the Canadian Climate Program, Atmospheric Environment Service of Environment Canada. RDDs (Fryberger, 1979) given by arrows. The RDD is defined as the net directional trend of sand drift. DP (Fryberger, 1979) is a measure of the energy of surface winds in terms of sand movement (measured in vector units). RDP (Fryberger, 1979) expresses, in vector units, the net sand transport potential when winds from various directions interact.

1997). Summers are normally hot and dry with local convective precipitation. The latter produces great variability in precipitation both spatially and temporally, and frequently there is an annual soil moisture deficit (Longley, 1972). Because of its dependence on large-scale circulation patterns for precipitation, particularly in winter, the region is highly susceptible to drought. Recent drought in the 1980s is attributed to persistent anticyclones over continental North America, which effectively blocked cyclonic activity over the Great Plains region (Weber, 1996). According to the United Nations Environmental Programme (1992), the prairies may be classified as subhumid to dry subhumid, but certain regions have become temporarily semiarid during severe droughts of the last century. Winds on the prairies are in the high-energy category, and are even higher than those for desert basins where some of the world’s largest sand seas occur (Muhs and Wolfe, 1999). Using average monthly wind data from Environment Canada (1993) for the period 1961–1990, Wolfe (1997) showed that wind is above the threshold velocity for dry sand (W; Lancaster, 1988) between

25% and 45% of the time. Resultant drift directions (RDDs; using the method of Fryberger, 1979) show that winds in the southern prairies are generally from the west, southwest, and northwest (Fig. 1). RDD is defined as the net directional trend of sand drift. The bulk of dune development on the Canadian prairies probably began shortly after deglaciation when there was limited vegetation development, an abundance of sandy glacigenic source sediments, and strong adiabatic winds. Studies conducted at a number of dune fields indicate that many dunes became active repeatedly during the Holocene, probably in response to climate forcings (David, 1971; Wolfe et al., 1995, 2000, 2001, 2002). For example, in the Great Sand Hills (GSH) of southwest Saskatchewan (Fig. 1), Wolfe et al. (2001) obtained evidence of widespread dune activity as recently as the last 200 years in response to reduced precipitation in the late 1700s. In southwest Manitoba, David (1971) and Wolfe et al. (2000) found evidence of recurrent intervals of dune activity in the past 5000 years (late Holocene) bracketed by at least six periods of dune stabilization.

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As with many examples from the Great Plains of the United States (e.g., Holliday, 1995; Forman et al., 1995, 2001; Muhs et al., 1997a,b; Stokes and Swinehart, 1997), the studies conducted in the Canadian prairies demonstrate the importance of arid intervals in the mobilization of the aeolian landscape. 2.2. Study sites In order to obtain a record of recent dune activity, an analysis of historical aerial photographs was undertaken. Black and white aerial photographs provide one of the few data sources for investigating dune activity over the last 50–70 years and offer reasonable temporal resolution (5–10 years between photographs) and accuracy (F1 m). Based on a review of available records, a total of seven sites were chosen from Saskatchewan (n=5) and Manitoba (n=2) to quantify recent historical sand dune activity (Fig. 1). Selection criteria depended on the availability and temporal record of aerial photographs, as well as the similarity of the illumination conditions that affect shadowing and reflectance of objects in the photographs. For these reasons, two other sites with active sand dunes could not be included in this study. The active dune occurrences examined in this study

fall into one of three broad regional zones (geographic distinction only): southwest Manitoba, southcentral Saskatchewan, and southwest Saskatchewan (Fig. 1). The predominant dune morphology at all sites is parabolic association. The term dparabolic dune associationT refers to a grouping of parabolic dunes and related features commonly found within the Palliser Triangle (cf. Wolfe and David, 1997; David, 1998). Two submorphologies were examined: (1) discrete parabolic dunes, and (2) compound parabolic dunes. The Brandon Sand Hills of southwestern Manitoba (49850VN, 99815VW) are one of the easternmost dune occurrences on the Canadian prairies and lie substantially outside the Palliser Triangle (Fig. 2). Although largely inactive, there are two large compound parabolic dune complexes in the south-central portion of the Brandon Sand Hills that are presently active (known locally as the Bald Head Hills). The Brandon South dune complex exhibits two broad lobes, with the southern lobe oriented slightly more to the southeast. The Brandon North dune has only one broad lobe. The Elbow Sand Hills of south-central Saskatchewan (51802VN, 106822VW) are located in the upper reaches of the Qu’Appelle Valley (Fig. 3). The Elbow Sand Hills are comprised of two dune fields situated

Fig. 2. Location map of two study sites in southwestern Manitoba. Aeolian deposits are in black. Aerial photographs are from the Canadian National Air Photo Library. Photograph numbers are: Brandon North (A27312-222) and Brandon South (A27604-14). Photograph scales are equivalent.

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Fig. 3. Location map of study sites in southern Saskatchewan. Aeolian deposits are in black. Aerial photographs obtained from the Canadian National Air Photo Library (NAPL). Photograph numbers are: GSH NW (A27726-189), GSH WC (A27312-184), Elbow (N/A), Tunstall (A27728-33), and Seward (A27728-70).

along the east and west sides of Lake Deifenbaker. Dune morphologies found in the area include blowout dunes, circular and parabolic dunes, and a few ridgesided dunes (David, 1977). The largest active dune occurs in the eastern dune field of the Elbow Sand Hills. This dune, known locally as Big Dune, is a compound parabolic dune oriented towards the east– southeast.

The southwestern region of Saskatchewan has the highest number of active sand dunes in the prairie provinces. Four sand hills from this region with active dunes were examined in this study (Fig. 3). The easternmost site is the Seward Sand Hills, located west of Swift Current (50815VN, 108814VW). The Seward Sand Hills form an elongate body roughly 30 km long, oriented towards the northeast. Sand source

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is from the underlying glaciofluvial and glaciolacustrine deposits (David et al., 1999). Numerous discrete parabolic dunes occur throughout the Seward Sand Hills, with the bulk of active dunes concentrated in the central portion of the dune field. Approximately 60 km to the northwest of the Seward Sand Hills is the GSH area (50852VN, 109803VW), which forms the largest contiguous dune occurrence of the Canadian prairies (David, 1977). Two prominent areas of active dunes occur in the northwest (GSH NW) and westcentral (GSH WC) portions of the GSH (Fig. 3). GSH NW contains numerous discrete parabolic dunes with some conjoined forms. The active dunes at GSH WC are also predominantly parabolic, but the main active area consists of one large conjoined parabolic dune. Southwest of the GSH lies the Tunstall Sand Hills (50809VN, 109843VW). The Tunstall Sand Hills is an elongate dune tongue oriented towards the east (David, 1977), and is almost 29 km in length and generally 2.5 km wide (Fig. 3). Several active dunes occur in the Tunstall Sand Hills, but the largest active dune is a compound parabolic dune located at the leading edge of the dune tongue.

3. Methodology 3.1. Digital aerial photograph analysis A review of available aerial photographs for the study sites was undertaken in order to select photographs with comparable properties. Many sites had lengthy photograph coverage extending into the early 1940s, and some had coverage in the early 1930s and late 1920s. However, most of the photographs could not be used in the digital analysis due to significant differences in the illumination conditions or poor film quality. Ideally, the selected photographs for a given site should have the same sun illumination, have principle point location relative to the area of interest, and have been taken at a similar time of year and at a similar time of day (cf. Shoshany, 2002). Because this was not entirely the case in the present study, some level of compromise with respect to these conditions was required. Although the acquisition times, sun elevation, and azimuth differed for some photographs, those ultimately chosen for analysis had negligible shadowing effects.

An overview of the methodology used in the processing and analysis of the aerial photographs is presented in Fig. 4. Following scanning, the images were subjected to two types of rectifications to produce planimetrically true images. The first procedure involved resampling to correct for camera orientation. During this procedure, the fiducial marks were measured and a photograph coordinate system was established for each image. The second procedure involved resampling to correct for exterior orientation. The approach developed for this procedure was based on the selection of a master photograph for each site. The master photograph was rectified to a cubic polynomial surface using a total of 8–13 ground control points (GCPs) collected from Canada’s National Ground Control Database, 1:50,000 National Topographic Survey (NTS) maps, and larger-scale maps, where available. The remaining photographs were rectified relative to the master photograph using 8–13 common points. The resultant root mean square error (RMSE) values were all below the spatial resolution of the images, indicating minimal positional error. The rectified images were subsetted to isolate the area of interest at each site. It should be mentioned that high-resolution DEMs were not available, and, as such, the effect of relief distortion was not removed. However, because the photographs did not have markedly different principle point locations with respect to the areas of interest, differences in relief distortion between images were minimal. An example of a sequence of rectified aerial photographs is shown in Fig. 5 for the Brandon North dune complex. In order to normalize the range of grey levels between images, relative radiometric calibration was performed in PCI Geomatica v8.2 by matching the histograms from each image to the master image for a particular site. The calibrated images were then classified using a supervised classification scheme with a maximum likelihood classifier. Two classes were defined: (1) bare sand and (2) vegetation. The classification was facilitated by the fact that the bare sand surface on the dunes and surrounding vegetation had significantly different grey level values. A sample of no less than 200 training pixels for each class was obtained in each photograph. The classified images were rescaled to 1 bit (1=sand and 0=vegetation). Change detection was carried out in ERDAS v8.5. Two user-specified thresholds were defined wherein

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Fig. 4. Overview of digital image processing and GIS methodologies: (A) preprocessing; (B) image analysis I; (C) image analysis II; and (D) GIS analysis.

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Fig. 5. Sequence of rectified aerial photographs showing changes occurring at the Brandon North dune.

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pixel values that increased or decreased beyond the thresholds were shown on an output image as new sand (increase) or new vegetation (decrease). In order to summarize changes at each site, three 1-bit images were combined using image stacking to create an 8-bit multitemporal color composite image with 23 different possible outcomes: AAA, AAV, AVV, AVA, VVV, VVA, VAA, or VAV (where A denotes active sand and V denotes vegetation covered). Each pixel in the multitemporal image was classified into one of the foregoing classes. As an example, a pixel corresponding to the class AVA indicates that the pixel was classified as active sand in the first image, vegetation covered in the second image, and active sand again in the third image. Although some sites had four different images, stacking all four images to create 24 possible outcomes was not pursued for the sake of consistency. Before computing various metrics for the active sand patches, unwanted anthropogenic and natural features included in the classification results (e.g., drained lakes, sloughs, and oil wells) were removed. This was accomplished by converting the 1-bit images for each site and the 8-bit multitemporal composite images into vector polygons using the geographical information system (GIS) vector module in ERDAS v8.5. The output vector polygons were subjected to three operations: (1) removal of unwanted polygons, (2) vector cleaning to fix broken polygons, and (3) building of vector topology. Patch metrics were computed from the polygon attribute tables. The metrics include: the number of patches, patch size (mean and standard deviation), and total patch area. 3.2. Climate data and analysis Climate data were obtained from stations near the study sites. In all cases, the data were obtained from stations located at airports (denoted by dAT hereafter). Separated by geographic region, the climate stations are Brandon A (southwest Manitoba), Saskatoon A (south-central Saskatchewan), Swift Current A (southwest Saskatchewan), and Medicine Hat A (southeast Alberta) (Fig. 1). Brandon A was compared with dune activity at the Brandon dune occurrences; Saskatoon A was compared with dune activity at Elbow; Swift Current A was compared with dune activity at Seward; and Medicine Hat A was compared with dune activity at GSH NW, GSH

WC, and Tunstall. The maximum distance separating a study site from the nearest climate station was approximately 120 km, and the minimum distance was 25 km. Because of these distances, it is recognized that there may be significant differences in the climate values between the climate stations and the study sites, and, for this reason, the main focus of this study is on the regional climate trends. Data for selected climate parameters were obtained from Environment Canada’s (1994) Canadian Monthly Climate Data (CMCD), containing climate data for stations in Canada extending back to the late 1800s and up to 1993. The climate parameters obtained from CMCD included annual and monthly records of wind speed, temperature, and precipitation. The annual and monthly wind speeds were computed from average hourly wind speeds. Raw hourly data were not used and therefore Lancaster’s (1988) W and Fryberger’s (1979) drift potential (DP) were not computed. Annual potential evapotranspiration was computed using the modified Thornthwaite method (Thornthwaite and Mather, 1957) from mean monthly temperature data. Linear regression was used to determine relations between the climate parameters and time. Principle station data (PSD) reports from Environment Canada showed that all four stations had instrument moves, as well as adjustments to anemometer heights over their records. It is assumed that instrument moves (without height changes) did not affect wind speed or precipitation readings by changing the local surface roughness. This was corroborated by examining historical aerial photographs, which showed no significant changes to the landscape (e.g., buildings, trees, etc.) in the immediate vicinity of the stations. From 1970 onward, anemometer heights at all four stations were maintained at 10 m. An accepted approach is to standardize wind speed to a common anemometer height (typically 10 m) (cf. Touma, 1977; Robeson and Shein, 1997; Klink, 1999). However, because the exact timing of the anemometer height changes could not be determined from the PSD reports or other sources, corrections to the wind speed data for the preceding years (b1970) were not performed. Therefore, monthly wind speed data from the selected stations were examined only for the period between 1970 and 1993, corresponding to when anemometer heights were kept at 10 m.

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4. Results

Table 1 Stabilization rates between photograph intervals

4.1. Dune activity

Site

Years

Rate (ha/year)

Brandon South

1970–1980 1980–1990 1928–1956 1956–1970 1970–1988 1944–1970 1970–1996 1970–1979 1979–1991 1946–1970 1970–1979 1979–1991 1946–1970 1970–1988 1945–1969 1969–1979 1979–1991

7.6 17.7 6.2 3.3 1.8 1.9 0.4 1.2 3.8 7.2 7.6 10.5a 1.4 1.3 0.1 3.6 0.6

Historical dune activity (Fig. 6) reflects the balance between two major geomorphic changes to the landscape: (1) vegetation encroachment, which reduces the area of active sand; and (2) dune migration and reactivation of stabilized surfaces, which increase the active area. Although the temporal record of aerial photographs varied considerably between sites, all showed a general trend towards decreasing dune activity for the period of record (Fig. 6). The one notable exception to this trend was GSH NW, where elevated dune activity between 1979 and 1991 increased the area of active sand by 51 ha. It is possible that the increase in dune activity at GSH NW was the result of the mid to late 1980s drought, which produced semiarid conditions within the core of the Palliser Triangle (Wolfe, 1997). However, it is reasonable to expect that other dunes affected by this drought, such as Tunstall, Seward, and GSH WC, would also show signs of increased activity, but this is not observed in the aerial photographs. The isolated increase in activity at GSH NW may be the result of the combined effects of mid to late 1980s drought and grazing stress from livestock. This speculation is based on the occurrence of several watering holes in the vicinity of the area where the increase in activity occurred. As such, this may only be a response to localized factors. Alternatively, there may be a lag in the response time from the other sites, but this cannot be determined because the aerial photograph record does not extend beyond 1991 for all but one site.

Fig. 6. Historical changes in dune activity at study sites.

Brandon North

Elbow Seward GSH NW

GSH WC Tunstall

a

Rate of dune activation.

The greatest decrease in the area of active sand occurred at both the Brandon North and South dune complexes where 102 ha of active sand at each site was stabilized by vegetation (Fig. 6). Although the temporal record of photographs used in this study for the South dune at Brandon was only 20 years, a 1947 photograph not suitable for digital processing showed that the area of active sand was at least 50% greater in 1947 than in 1970. Therefore, this would increase the active area at Brandon South to approximately 350 ha in 1947 and yield a cumulative reduction of approximately 60% over 43 years. Comparatively, despite its smaller size, the Brandon North dune showed a more significant transformation from a fully active dune in 1928, reduced to only 10 ha of active sand by 1988. In this regard, the area of active sand at Brandon North has decreased by 91% in 60 years (1928–1988). Overall, the temporal measurements at the Brandon dune occurrences are in line with those previously reported by Wolfe et al. (2000). Examination of the rates of dune stabilization shows that the tempo of dune stabilization was not synchronous between sites (Table 1). For example, there was a declining trend in the rate of stabilization at Brandon North, Elbow, and GSH WC, whereas increasing trends occurred at Brandon South and Seward. At Brandon North, where the most significant transformation occurred, the exponential decrease in the rate of stabilization demonstrates

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the ease with which the initial vegetation colonization took place in the moist interdune areas between 1928 and 1956, and became progressively slower in later periods in colonizing the exposed dune crests where wind stress is higher and soil moisture is lower. The data presented in Table 1 also show that, for certain intervals in the photograph records at Tunstall and Elbow, values were equal to or below 0.6 ha/year. This is an important characteristic because a value of zero represents a threshold that separates the states of dune stabilization versus dune activation. Therefore, for the three intervals in which values are close to zero in Table 1, the stabilization at Tunstall and Elbow was virtually balanced by sand migration or small areas that were reactivated. From a visual analysis, the low stabilization rate at Elbow between 1970 and 1996 is, in part, a result of an overall increase in patch complexity (rougher edge), despite stabilization of the stoss-side from vegetation colonization. It should also be noted that the compound parabolic dune at Tunstall migrated at an average rate of approximately 10 m/year, which is considerably higher than the rates at any of the other sites. A possible explanation for this rapid migration is that, despite its large areal extent, the volume of sand transported at Tunstall is relatively small. The characteristics of active sand patches were not consistent between sites (Table 2). Patch size steadily increased or decreased at some sites (e.g., Brandon South and Tunstall, respectively), while at others, the number of patches fluctuated during the period of record (e.g., Brandon North and Elbow); therefore, the number of patches is not reflective of the general trend towards stabilization. Mean patch size generally decreased at both Brandon dune complexes over the photograph record, producing an overall increase in the fragmentation of these compound dunes. Conversely, mean patch size showed an increasing trend at GSH NW, which is attributable to coalescence of adjacent parabolic dunes and stabilization of smaller patches of active sand. The patch size standard deviation showed declining trends at Elbow, Brandon North, and Brandon South, which indicates that stabilization produces more uniformly sized patches of active sand at sites with compound parabolic dunes. Unlike the patch characteristics, the results from the change detection analysis reveal a coherent pattern

Table 2 Characteristics of active sand patches Site

Year Number Total area Mean patch Patch of patches (ha) size (ha) size S.D.

Brandon South 1970 1980 1990 Brandon North 1928 1956 1970 1988 Elbow 1944 1970 1996 Seward 1970 1979 1991 GSH NW 1946 1970 1979 1991 GSH WC 1946 1970 1988 Tunstall 1945 1969 1979 1991

210 375 387 72 84 78 53 19 8 36 99 49 54 340 154 107 130 52 40 70 12 10 6 5

234 204 132 112.7 41.9 23.4 10.4 62.0 42.3 38.5 81.3 77.0 58.7 207.2 137.5 109.8 160.7 133.4 120.3 111.0 50.0 49.2 34.8 31.6

1.1 0.5 0.3 1.6 0.5 0.3 0.2 3.3 5.3 1.1 0.8 1.6 1.1 0.6 0.9 1.0 1.2 2.6 3.0 1.6 4.2 4.9 2.7 6.3

8.0 5.2 2.8 12.4 1.4 1.0 0.4 13.8 13.5 5.6 1.7 2.4 1.9 2.0 1.8 1.9 3.5 8.1 6.4 7.0 12.9 14.5 7.7 12.2

at all sites in the manner in which dune stabilization took place by vegetation encroachment (Fig. 7). The general pattern is largely dependent on dune morphology. For sites with discrete parabolic dunes (e.g., Fig. 7c), vegetation encroachment occurred almost exclusively along the upwind margin relative to the prevailing winds (usually from the west) on the stoss-side and deflation depression. For sites with compound parabolic dunes (e.g., Fig. 7a), vegetation encroachment occurred both along the upwind margin and along interdune areas. The net result of this pattern is that the local supply of sand available for dune migration is reduced at a rate inversely proportional to the rate of vegetation encroachment. Although some vegetation does take hold on the slipface (e.g., Psoralea lanceolata, Rumex venosus), even a modest rate of slipface advance (e.g., 0.5 m/ year) can be sufficient to bury the vegetation and give the impression that no appreciable colonization occurs on the slipface between the long intervals separating the aerial photographs. Over the long term, the most active surfaces, which include the dune head, crest,

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Fig. 7. Results of change detection at selected sites. A denotes active sand and V denotes vegetation covered. The sequence of photographs used to create (a) is shown in Fig. 5.

and slipface, are kept clear of vegetation due to the processes of erosion and deposition, and due to the fact that these surfaces are well-drained, preventing favorable growing conditions for vegetation. 4.2. Climate trends and relations with dune activity Summaries of temperature, precipitation, potential evapotranspiration, and wind speed from the four climate stations are presented in Table 3. For the period of record, Saskatoon A had the lowest mean annual air temperature and Brandon A had the highest precipitation. Mean annual wind speeds were lowest in the easternmost and westernmost stations (Brandon A and Medicine Hat A, respectively), while extreme wind speeds were highest at the two westernmost stations (Swift Current A and Medicine Hat A). Fig. 8 shows that considerable variation in wind speed occurs throughout the year.

Generally, wind speed is at a minimum during the months of July and August. The highest wind speeds occur in winter (December and January) for Swift Current A, whereas the highest wind speeds occur in spring (April and May) for the other three stations. Overall, winds are higher in winter and spring because the equator-to-pole temperature and pressure gradients are greatest (Klink, 1999), indicating that aeolian activity is likely to be most pronounced in these seasons, and generally at a minimum during July and August. Trends in average annual wind speed and annual P:PE are presented in Fig. 9. The longest record is the P:PE at Medicine Hat A, which spans more than a century (1888–1993). Over the entire P:PE record at Medicine Hat A, there is no trend towards increasing or decreasing values. The P:PE time series from Saskatoon A exhibits a weak decreasing trend over the entire record and also has a lengthy

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Table 3 Summary of mean annual climate parameters for climate stations Station

Mean annual temperature (8C)

Mean annual precipitation (mm)

Mean annual PE (mm)

Mean annual wind speed (m/s)

Mean extreme wind speed (m/s)

Brandon A Saskatoon A Swift Current A Medicine Hat A

1.9 1.7 3.6 5.5

458 361 377 341

537 542 539 593

4.27 4.44 5.60 3.97

18.72 18.02 19.85 19.12

All values for wind speeds were derived for the period between 1970 and 1993. Values for P:PE were derived from different periods: Brandon A (1958–1993), Saskatoon A (1900–1993), Swift Current A (1939–1993), and Medicine Hat A (1888–1993).

period between 1910 and 1950 in which values were frequently above the average. At Brandon A, there is a weak increasing trend throughout the record. It should be noted that P:PE values are closer to unity at Brandon A than at the other stations, indicating that moisture deficits are generally less pronounced at this station. Although none of the linear regressions for the P:PE time series was significant at the 90% confidence level, the linear regressions for the average annual wind speeds were significant at the 97.5% confidence level or better (Brandon A: 97.5%, Saskatoon A: 99.7%, Swift Current A: 98.3%, and Medicine Hat A: 99.9%). Overall, the trend at each station was one of decreasing average annual wind speed from 1970 to 1993. The greatest decreases occurred at Swift Current A and Medicine Hat A. In addition, extreme wind speed decreased at all four stations, although none was significant at the 90% confidence level.

Fig. 8. Monthly variability in wind speed based on hourly wind data from 1970 to 1993 for Brandon A, Saskatoon A, Swift Current A, and Medicine Hat A.

In order to examine the response of dune activity to the climate variables, process response models were developed based on correlation analysis (cf. Lancaster, 1997). Average values for P:PE and wind speed were calculated for the time periods between aerial photograph intervals from the nearest climate station and were compared with the decrease in dune activity at nearby sites (see Section 3.2) for the same periods. In Fig. 10, the average values of wind speed and P:PE are plotted against the decrease in dune activity during the same interval. In this case, dune activity is expressed by the percent decrease in the area of active sand. It should be noted that due to the limited duration of the climate records at some stations, certain photograph periods could not be included (mainly for wind). Results in Fig. 10 show that the correlation for average annual wind speed is relatively weak compared to the correlation for average P:PE, which is significant at the 95% confidence level. R 2 values in Fig. 10 indicate that the average annual wind speed explains only 4% of the variability in dune stabilization and P:PE explains 28%. As such, there is a large amount of variability that is not explained by these variables. Fig. 10b confirms that periods of cooler temperatures and/or greater precipitation tend to increase the rate of dune stabilization, whereas more arid periods tend to decrease the rate. Given that Brandon A generally had cooler temperatures and higher precipitation than the other climate stations examined, the relation in Fig. 10b provides some indication as to why the Brandon North and South dune complexes had the highest rates of stabilization. However, the changes in dune activity at Brandon North also demonstrate that, at some point, the stabilization rate is likely to diminish irrespective of the influence of P:PE due simply to difficulty in

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Fig. 9. Historical trends, depicted as departures from the mean, for average annual wind speed (based on hourly wind data) and P:PE derived using modified Thornthwaite method (Thornthwaite and Mather, 1957). Circled numbers for P:PE time series at Medicine Hat A referred to in text.

stabilizing the more wind- and moisture-stressed locations such as the dune crest. Collectively, these results are in line with previous studies on the North

American Great Plains, which have suggested that vegetation cover, as expressed by P:PE, is more of a limiting factor on dune activity than wind speed

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Fig. 10. Results of correlation analysis between the percent decrease in active area between aerial photographs versus values of (a) wind speed (n=11) and (b) P:PE (n=14) for the same time intervals.

(Muhs and Maat, 1993; Muhs and Holliday, 1995; Wolfe, 1997).

5. Discussion 5.1. Recent historic dune activity Digital image processing and GIS analysis of historical aerial photographs confirm that recent historic dune activity on the southern Canadian prairies has been decreasing since the early to mid 1900s. This trend has occurred despite pronounced droughts in the 1930s and 1980s, which contrasts with the central and southern Great Plains of the United States where many dunes reactivated in response to drought in the 1930s (Muhs, 1998). These observations lend support to the argument that dune activity on the Canadian prairies has not responded to recent climate variations and may have been carried over from a period of greater activity, possibly during the late 1700s (cf. Wolfe, 1997; Wolfe et al., 2001).

In this study, however, several lines of evidence demonstrate that active dunes on the Canadian prairies have responded to recent climate variations. Specifically, the decrease in dune activity has been shown to be significantly correlated with decadalscale changes in P:PE ( p=0.05) and coincident with a decrease in annual wind speed from 1970 to 1993. Based on these relations, it is tempting to reject the argument that dune activity has been carried over from the late 1700s. Yet it is important to recognize that the R 2 values in Fig. 10 indicate that wind speed and P:PE explain only a small part of the variability in terms of dune stabilization. Therefore, rather than being associated only to one or the other, these results suggest that dune activity has responded to recent climate variations, but this response is superimposed on a longer-term trend towards stabilization in place over the last 200 years. In this way, the trend of decreasing dune activity observed in the historical aerial photographs is a complex response, which reflects short-term (decadal) climate variations, particularly aridity, as well as long-term recovery from a disturbance in the late 1700s. This interpretation provides some clarification as to why Lancaster’s dune mobility index (M) does not work well for the southern Canadian prairies (cf. Muhs and Wolfe, 1999). The suggestion that dunes are still recovering from a disturbance in the late 1700s brings to question why it has taken so long for them to recover. For example, in areas on the central Great Plains of the United States, where the climate is warmer and drier than on the Canadian prairies, near-complete revegetation of dunes that were active during the 1930s drought has occurred within one to several decades (Muhs and Maat, 1993; Muhs et al., 1997a; Muhs, 1998). A possible explanation for this difference is that the shorter growing season on the Canadian prairies could lead to slower revegetation of dunes after major disturbances or arid intervals (cf. Muhs and Wolfe, 1999). In this regard, the natural recovery of sand dunes on the Canadian prairies is much slower than on the central and southern Great Plains of the United States. Furthermore, as evidenced by the response of dune activity to historical climate variations presented here, the natural recovery process of dunes may be exacerbated by responses to minor climate variations superimposed on the recovery phase. Ultimately, these

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elements may combine to prolong dune recovery on the Canadian prairies. A major assumption inherent in dune mobility indices is that the level of dune activity is in equilibrium with climate. However, evidence presented in this study suggests that, in some instances, dune activity exhibits a complex response whereby the level of activity at any given time reflects a combination of recent climate variations and past disturbances. In these situations, calculation of mobility indices can give the impression of anomalous levels of activity. Therefore, if mobility indices are adapted or developed for the Canadian prairies, they will have to include parameters to deal with complex responses. 5.2. Climate controls Our analysis of historical climate data from selected stations across the southern prairie provinces indicates that recent dune activity is more strongly related to decadal-scale changes in aridity than it is to changes in average annual wind speed. Correlation between wind speed and dune activity yielded a relatively weak relation. In part, this may reflect the fact that the correlations were made with changes in the area of active sand rather than volumetric changes, which are probably more characteristic of the effects of wind on the landscape. Although volumetric changes can be assessed for aerial photographs using photogrammetric techniques (cf. Gaylord and Stetler, 1994; Brown and Arbogast, 1999), these were not presented here due to processing difficulties and significant measurement errors. Nevertheless, an important area for future study is to examine the association between volumetric changes in dune activity and wind characteristics. A significant issue raised in this paper concerns historical trends in wind speed, as well as the impact of future climate change on wind speed. The decrease in average annual wind speeds from 1970 to 1993 at all four stations examined in this study suggests a common underlying control. It is possible that the declining trends represent simultaneous deterioration in anemometer performance. However, annual wind speed time series from several other stations outside the southern prairies did not show the same trend, indicating that simultaneous instrument degradation is unlikely. An alternative explanation is that, during the

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same period, there was a concomitant trend in synoptic-scale phenomena, which are the dominant controls on surface winds in the North American Midwest (Robeson and Shein, 1997; Klink, 2002). Evidently, several recent studies have determined that the frequency of Northern Hemisphere mid-latitude cyclones and anticyclones has decreased in the last half of the 1900s (Serreze et al., 1997; Key and Chan, 1999; McCabe et al., 2001), which may have influenced the annual surface wind speed. In addition, a decrease in annual surface wind speed has also been observed in Minnesota during the same interval examined here (Klink, 2002). Further work is required to correlate surface wind speed with synoptic phenomena in order to begin to understand the potential impacts of future climate change on aeolian processes.

6. Conclusions The main findings from this study indicate that sand dune activity has been decreasing across the southern Canadian prairies since the early to mid 1900s. This trend is a response to recent climate variations, particularly decadal-scale changes in aridity ( P:PE), which are superimposed on a longer trend towards stabilization, possibly established since the late 1700s (cf. Wolfe et al., 2001). The main effect of this complex response, in association with the short growing season, is that dune stabilization may be prolonged well after an initial disturbance and thereby give the impression of anomalous levels of activity for an extended period thereafter. For this reason, dune mobility indices for the southern Canadian prairies do not well describe the present level of dune activity. Although there was a relatively weak correlation between changes in the area of active sand (a surrogate for dune activity) and decadal-scale variations in wind speed (R 2=0.04), the general decrease in dune activity for the period between 1970 and 1993 was coincident with a decrease in annual wind speed at nearby climate stations. The decreasing trends in annual wind speed at the four stations examined were significant at pb0.03. Rather than suggesting that wind speed is not important in dune activity, variations in wind speed may be more closely associated with volumetric changes of dune activity.

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