Stephen A. Wolfe

SHEAR STRESS PARTITIONING IN SPARSELY VEGETATED DESERT CANOPIES

Shear velocity ratios were derived for six neighbouring localities representing rangeland, abandoned farmland and a playa, in the semi-arid region of the Sonoran Desert. Near-surface and above-canopy wind speeds, mostly below saltation threshold, were used to evaluate the partitioning of shear velocities and shear stresses between the plant canopy and the surface. The technique utilized represents an alternative to the threshold shear velocity ratio (Rt) approach of Musick and Gillette (Land Degradation and Rehabilitation, 1990, 2, 87–94) and results demonstrate the need for partitioning over a range of wind speeds. Shear velocity ratios were variable over the playa and abandoned farmland, while variance in near-surface wind speeds was highest over shrub-dominated sites. The relationship between lateral cover (Lc) and shear velocity (R) is comparable to previous findings utilizing Rt, although methodological differences result in mean shear velocity ratios less than shear velocity ratios derived for threshold conditions. With respect to the model of Raupach et al. (Journal of Geophysical Research,1993, 98-D2, 3023–3029), this may be attributed to the difference between measured shear stresses and the average shear stress on the exposed surface. At high lateral covers, shear velocity ratios are lower than threshold shear velocity ratios due to increased wake interactions or decreased drag on the shrub elements at higher wind speeds.

Barchan dunes stabilized under recent climate warming on the northern Great Plains

We use light detection and ranging (LIDAR) imagery and optical stimulation luminescence dating to show that stable parabolic dunes on the Canadian prairies originated from active barchan dunes ~200 years ago. Residual dune ridges, marking former lower stoss slope positions of migrating dunes, record the transformation of barchan dunes to parabolic dunes between A.D. 1810 and 1880. Parabolic dunes stabilized by ca. A.D. 1910, with a few larger dunes and blowouts still active today. A dry, cool climate permitted sand transport to out-compete vegetation stabilization and, with lowered water tables, maintain desert-like barchan dunes with bare interdune sand sheets. These findings explain why dune fields of the southern Canadian prairies are currently more active than those of the United States Great Plains and the observation that dunes have stabilized under twentieth century warming. Our results emphasize the importance of viewing dune field responses to short-term disturbances in the context of longer-term system response, particularly when relatively modest climatic changes can cause major shifts in dune activity.

Chronology and geochemistry of late Holocene eolian deposits in the Brandon Sand Hills, Manitoba, Canada

Abstract Accelerator mass spectrometry and conventional radiocarbon age determinations of organic matter from paleosols indicate that the Brandon Sand Hills area of southern Manitoba has been subjected to recurrent intervals of eolian activity in the past 5000 years. Although precise regional correlations are precluded by dating uncertainties, periods of most notable paleosol development occurred around 2300 to 2000, 1400 to 1000, and 600 to 500 cal yr BP with eolian activity occurring before and after each of these periods. Episodes of eolian activity may correspond to periods of regional drought, whereas paleosols mark periods of increased moisture availability and stabilization by vegetation. The geochemistry of the eolian sands, paleosols and source sediments indicates that partial leaching of carbonates occurs from pedogenesis during humid climatic phases, and that this is probably the primary mechanism of carbonate depletion of eolian sands in this area. Recent trends in sand dune activity from historic aerial photography and early explorers’ accounts indicate that the few active dunes that presently exist have stabilized at a rate of 10–20% per decade, despite several severe droughts in the 20th century. This may be attributed to pre-settlement droughts that were more severe than those in historic times although regional dune stabilization may also be related, in part, to the spread of forest cover in the past few hundred years.

Distribution and activity of ice wedges across the forest‐tundra transition, western Arctic Canada

Remote sensing, regional ground temperature and ground ice observations, and numerical simulation were used to investigate the size, distribution, and activity of ice wedges in fine-grained mineral and organic soils across the forest-tundra transition in uplands east of the Mackenzie Delta. In the northernmost dwarf-shrub tundra, ice wedge polygons cover up to 40% of the ground surface, with the wedges commonly exceeding 3 m in width. The largest ice wedges are in peatlands where thermal contraction cracking occurs more frequently than in nearby hummocky terrain with fine-grained soils. There are fewer ice wedges, rarely exceeding 2 m in width, in uplands to the south and none have been found in mineral soils of the tall-shrub tundra, although active ice wedges are found there throughout peatlands. In the spruce forest zone, small, relict ice wedges are restricted to peatlands. At tundra sites, winter temperatures at the top of permafrost are lower in organic than mineral soils because of the shallow permafrost table, occurrence of phase change at 0°C, and the relatively high thermal conductivity of icy peat. Due to these factors and the high coefficient of thermal contraction of frozen saturated peat, ice wedge cracking and growth is more common in peatlands than in mineral soil. However, the high latent heat content of saturated organic active layer soils may inhibit freezeback, particularly where thick snow accumulates, making the permafrost and the ice wedges in spruce forest polygonal peatlands susceptible to degradation following alteration of drainage or climate warming.

The influence of vegetation and soil characteristics on active-layer thickness of permafrost soils in boreal forest.

Abstract Carbon release from thawing permafrost soils could significantly exacerbate global warming as the active‐layer deepens, exposing more carbon to decay. Plant community and soil properties provide a major control on this by influencing the maximum depth of thaw each summer (active‐layer thickness; ALT), but a quantitative understanding of the relative importance of plant and soil characteristics, and their interactions in determine ALTs, is currently lacking. To address this, we undertook an extensive survey of multiple vegetation and edaphic characteristics and ALTs across multiple plots in four field sites within boreal forest in the discontinuous permafrost zone (NWT, Canada). Our sites included mature black spruce, burned black spruce and paper birch, allowing us to determine vegetation and edaphic drivers that emerge as the most important and broadly applicable across these key vegetation and disturbance gradients, as well as providing insight into site‐specific differences. Across sites, the most important vegetation characteristics limiting thaw (shallower ALTs) were tree leaf area index (LAI), moss layer thickness and understory LAI in that order. Thicker soil organic layers also reduced ALTs, though were less influential than moss thickness. Surface moisture (0–6 cm) promoted increased ALTs, whereas deeper soil moisture (11–16 cm) acted to modify the impact of the vegetation, in particular increasing the importance of understory or tree canopy shading in reducing thaw. These direct and indirect effects of moisture indicate that future changes in precipitation and evapotranspiration may have large influences on ALTs. Our work also suggests that forest fires cause greater ALTs by simultaneously decreasing multiple ecosystem characteristics which otherwise protect permafrost. Given that vegetation and edaphic characteristics have such clear and large influences on ALTs, our data provide a key benchmark against which to evaluate process models used to predict future impacts of climate warming on permafrost degradation and subsequent feedback to climate.

Potential impacts of climate change on grazing capacity of native grasslands in the Canadian prairies

Relationships between climate and native grassland production in the Canadian prairies were modelled and used to estimate the potential impacts of climate change on grazing capacity. Field measurements of production were related to climate variables and water balance estimates using regression analysis. Historical time series showed that year-to-year production is most closely correlated with annual actual evapotranspiration, whereas geographic patterns revealed that average production is most closely related to the annual water deficit. Climate and production estimates from the US Great Plains represent potential analogues for the Canadian prairies in the 2050s. Analysis of geographic patterns using Canadian and US data showed that production can be related to actual evapotranspiration (Model 1) or the ratio of actual to potential evapotranspiration (Model 2). The proportion of warm-season (C4) grasses has a significant effect on production in these models. A third independent model (Model 3) using US pr...

Evaluation of RADARSAT-2 DInSAR Seasonal Surface Displacement in Discontinuous Permafrost Terrain, Yellowknife, Northwest Territories, Canada

Abstract. Differential Interferometric Synthetic Aperture Radar (DInSAR) is an increasingly viable method for assessing permafrost terrain stability, but the accuracy and performance within discontinuous permafrost terrain has not been well studied. We used a RADARSAT-2 DInSAR data stack for a 120-day period in the summer of 2010 to map seasonal surface displacement in the discontinuous permafrost terrain of Yellowknife, Northwest Territories. Calculated displacement was compared to surficial geology and municipal land use zones. Displacement results reveal that glaciofluvial, glaciolacustrine, humanly modified, and organic terrain are increasingly unstable, in contrast to predominantly stable bedrock. Within municipal zones, increased proportional displacement is related to higher proportions of glaciolacustrine sediments and organic terrain. Organic terrain, associated with the highest proportion of the moderate downward displacement (−3.0 cm to −6.0 cm), occupies less than 6% of the total area. Widespread glaciolacustrine sediments (30% total area) are associated with most of the downward displacement in municipal zones. Semi-quantitative field and geotechnical validations indicate that most areas of moderate seasonal downward displacement in developed areas also represent areas of long-term subsidence. This work shows that even a short InSAR data stack and a simple stack processing method can yield information that is useful for municipal knowledge and planning. Résumé. L’interférométrie différentielle par radar à synthèse d’ouverture (DInSAR) est une méthode de plus en plus viable pour évaluer la stabilité des terrains en zone de pergélisol, mais la précision et la performance dans les zones de pergélisol discontinu ne sont pas bien étudiées. Nous avons utilisé une pile de données DInSAR de RADARSAT-2 d’une période de 120 jours au cours de l’été 2010 pour cartographier le déplacement saisonnier de la surface du sol dans la zone de pergélisol discontinu de Yellowknife, Territoires du Nord-Ouest. Le déplacement calculé a été comparé à la géologie de surface et les zones municipales d’utilisation des terres. Les résultats de déplacements révèlent qu’en ordre de stabilité, du plus stable au moins stable, on trouve les zones fluvio-glaciaires, glacio-lacustres, humainement modifiées et organiques, tandis que le substrat rocheux est essentiellement stable. Dans les zones municipales, le déplacement proportionnel accru est lié à des proportions plus élevées de sédiments glacio-lacustres et du terrain organique. Le terrain organique, associé à la plus forte proportion du déplacement modéré vers le bas (−3.0 à −6.0 cm), occupe moins de 6% de la superficie totale. Les sédiments glacio-lacustres répandus (30% de la superficie totale) sont associés à la plupart des déplacements vers le bas dans les zones municipales. Les validations semi-quantitatives de terrain et géotechniques indiquent que la plupart des zones de déplacement saisonnier modéré vers le bas dans les régions développées représentent également des zones d’affaissement à long terme. Ce travail montre que même une pile de données InSAR de courte durée et une méthode de traitement de pile simple peuvent donner des informations utiles pour la connaissance et la planification municipale.

Palliser’s Triangle: Reconstructing the ‘central desert’ of the southwestern Canadian prairies during the late 1850s

Between 1857 and 1860 the British North American Expedition, led by Captain John Palliser, explored and surveyed the Canadian Prairies primarily to establish its suitability for agriculture and settlement. Historical and paleoclimate records indicate the Expedition coincided with below normal precipitation, leading to the perception of an arid or semi-arid region that would ‘forever be comparatively useless’ for agriculture. Today, this part of the Canadian Prairies is known as the Palliser Triangle, and is Canada’s productive dryland agricultural region. Here we present historic, geomorphologic, and chronometric evidence to reconstruct the landscape encountered by the Expedition. We contend that Palliser’s perception of the region was strongly influenced by his experience travelling through active sand dunes in the Middle Sand Hills of southeastern Alberta. At present, the dunes are entirely stabilized by vegetation, in contrast to Palliser’s report of ‘miles of burning sand’. Archival aerial photographs and optical ages of near-surface samples are used to reconstruct the landscape encountered by the Expedition in the Middle Sand Hills. Optical ages of presently stabilized sand dunes date primarily to between ad 1850 and 1934, peaking in c. ad 1925, and are indicative of a dune field undergoing reduction in activity, prior to the onset of 20th century droughts. Ages of interdune sand sheets further attest to regional dune activity occurring at least since ad 1750, concurrent with activity in other southern Canadian Prairie dune fields. Collectively, this evidence supports observations by Palliser of severe travelling due to bare sand conditions in 1857–1859. These conditions and Palliser’s inference of their extent influenced his perception of a ‘central desert’, thus delaying construction of the Canadian Pacific Railway along a southern route and postponing the westward colonization of Canada.

Geological and meteorological controls on icing (aufeis) dynamics (1985 to 2014) in subarctic Canada

Icings are widespread yet poorly understood winter hydrological phenomena that develop over the winter by freezing successive overflows of groundwater to the surface. Groundwater hydrology in arctic regions is constrained by geological setting and permafrost extent, and overflows are possibly driven by cold winters, winter warming intervals, high antecedent autumn rainfall, and low early winter snowfall. Consequently, icings are spatially recurrent but not necessarily annually nor to the same extent. We test the significance of identified meteorological forcing variables against a long-term data set of icing dynamics and distribution we developed for the Great Slave region around Yellowknife, Northwest Territories. Climate is regionally consistent, but variable geology and permafrost create hydrological conditions representative of much of the subarctic. We mapped 5500 icings in the study area (21,887 km2) with a semiautomated approach utilizing late spring Landsat archival images (1985 to 2014). Individual icing size, ranging 3 orders of magnitude (1.8 × 10−3 km2 to 4.1 km2), is related to return frequency. Infrequent ice (25% return frequency) accounts for 94% of the total icing area (86 km2). Winter warming intervals (≥5°C; typically over 1–3 days) and autumn rainfall (September and October) explain 28% of icing density interannual variation overall. Interannual icing variation and significant meteorological forcing variables differ among ecoregions where varied geological settings and permafrost conditions influence the hydrological regime. Future icings may develop less frequently due to decreasing winter warming intervals, but increasing autumn rainfall may increase icing density where Canadian Shield leads to strong threshold-mediated runoff generation processes.

Spatiotemporal impacts of wildfire and climate warming on permafrost across a subarctic region, Canada†

Field observations show significant impacts of wildfires on active layer thickness and ground temperatures. However, the importance of fires to permafrost conditions at regional scales remains unclear, especially with climate warming. This study evaluated the regional impacts of fire on permafrost with climate change from 1942 to 2100 using a process-based model in a large subarctic region in the Northwest Territories, Canada. Climate warming is shown to be the dominant factor for permafrost reduction. The warming trend of climate reduces permafrost extent in this region from 67% at present to 2% by 2100. For burned areas, fire increases the reduction of permafrost extent by up to 9% on average, with up to 16% for forest, 10% for tundra and bogs, and 4% for fens. Fire accelerates permafrost disappearance by 5 years on average. The effects of fire on active layer thickness and permafrost extent are much larger in forest areas than in tundra, bogs, and fens. Since active layer is thicker after a fire and cannot recover in most of the areas, the fire effects on active layer are widespread. On average, fires thickens active layer by about 0.5 m. The fire effects on active layer increased significantly after 1990 due to climate warming.