Bedrock Cover Research Articles

Thermal conduction in karst terrains dominating cave atmosphere temperatures: Quantification of thermal diffusivity

The relatively stable temperature of most cave interiors is caused by the transfer of the surface atmosphere temperature signal to the karst underground by thermal conduction. Transferring underground thermal signals by conduction implies the amplitude attenuation of the external thermal anomaly as well as a signal delay. The magnitude of these attenuation and delay effects is proportional to the thickness of bedrock cover above the cave and is controlled by the bedrock thermal diffusivity. Here we present 5-year temperature record of Los Pilones Cave, in central Spain, where advection has a limited control on thermal anomalies all year round. Thus, thermal conduction dominates the thermal variability in the cave, with simulations of a 1-D thermal conduction model explaining up to 94% of the variability observed. Thermal attenuation and signal delays are highly correlated with the thickness of bedrock cover (r2 = 0.95 in both cases), enabling the calculation of a thermal diffusivity of 5.07*10−7 ±1.27*10−7 m2/s. The calculated thermal diffusivity is in the range of available values measured in laboratory for carbonate lithologies, although differs beyond uncertainty from the limited number of studies that calculated this value from field observations in karst. Thus, local rock properties significantly impact thermal diffusivities, and more experimental studies are required to show a complete distribution of this value in karst settings. The reported uncertainty, that was calculated using different time windows within the studied period, represents a variability of 23% on the thermal diffusivity. Karst terrains are mostly composed of three phases, rock, air and water, that have different thermal properties. Thus, variability of the water content in the rock porosity through time is likely a significant control on the measured uncertainty. Since temperature in the studied cave has a minor component affected by advection, this cave represents a paradigmatic location to observe the important role of thermal conduction in caves. Therefore, conclusions from this study can be useful to better understand thermal regimes and interannual trends of most cave interiors despite occurrence of variable impacts of seasonal ventilation dynamics in cave temperature records. In addition, the mechanism of thermal conduction results in a thermal decoupling between the surface and cave atmospheres affecting cave microclimate, which has important implications for multiple disciplines interested in karst underground environments.

Capturing the complexity of soil evolution: Heterogeneities in rock cover and chemical weathering in Montana's Rocky Mountains

We investigate the relationship between chemical weathering, persistence of soil cover, and topography in two neighboring mountain ranges in the northern Rockies of western Montana, USA. We augment existing tools for measuring chemical weathering with adjustments for both local and landscape-scale contributions from unweathered rock fragments, boulders, and bedrock exposure. Adjusted weathering intensities recognize that quantifying weathering in mountainous systems should account for rock exposure, rather than focusing solely on fine-grained soil mantles. Our study systems' distinct morphologies are shaped by their unique climate histories. The previously glaciated Bitterroot Mountains consist of steep hillslopes with abundant rock cover, while the neighboring unglaciated Sapphire Mountains display convex, soil-mantled hillslopes. Over 380 soil thickness measurements, 118 analyses of soil and rock geochemistry, and digital terrain analysis reveal that patchy soils in the bedrock-rich system are roughly half as thick as those in the continuously-soil-mantled landscape, and ~45% less weathered, despite wetter conditions that would be expected to enhance weathering. These disparities increase when accounting for coarse rock fragments in soils and bedrock cover across the study catchments. The near continuously soil-mantled Sapphire system experiences ~1.5 times greater weathering intensity at a catchment scale compared to the bedrock-rich Bitterroot system. Rock exposure across the mountainous study system increases with increasing slope gradient. However, we find no clear threshold at which soils decrease in abundance or weathering intensity, and soils are surprisingly resilient even at the steepest hillslopes (comprising ~60% of the landscape area at slopes >30°). Our new data quantify soil abundance and chemical weathering intensity at both local and landscape scales. This work highlights how measurements of soil and rock cover need to be incorporated into studies quantifying chemical weathering, as traditional approaches may significantly overestimate and mischaracterize weathering regimes in mountain environments.

One of the world’s largest regreening programs promotes healthy tree growth and nutrient accumulation up to 40-years post restoration

Mining and smelting degraded landscapes are characterised by heavily eroded, acidic soils that are contaminated with toxic metals and depleted of essential nutrients. Increasing forest cover through regreening of degraded landscapes has been highlighted to support carbon (C) mitigation measures and protect biodiversity. One of the worlds largest regreening programs in the City of Greater Sudbury, Ontario has been ongoing since 1978 and involves the liming and fertilization of selected areas followed by planting of primarily coniferous trees. In this study, we assessed how aboveground biomass (AGB) and aboveground nutrient (calcium (Ca), magnesium (Mg), nitrogen (N), phosphorous (P), potassium (K), and C) pools changed using a space-for-time approach. We established a series of sites ranging from 15 to 42 years since treatment. To determine the potential effects of erosion on AGB and AGB nutrient pools, each site was categorized as “stable” (<10% bedrock cover) or “eroded” (greater than 30% bedrock cover). Both AGB and AGB nutrient pools increased with time since regreening at rates similar to conifer plantations grown in undisturbed regions. Individual tree growth and nutrient accumulation did not differ between stable and eroded sites; however, stable sites had a higher stem density leading to overall higher per hectare AGB and AGB nutrient pools. Future N limitation of the regreening forests does not appear to be a concern as aboveground N pools were six times larger than applied N. Conversely, aboveground P concentrations decreased with time since tree planting and the 40-year-old study sites had aboveground P concentrations below values for “healthy” trees. This study shows that the regreening efforts have led to a massive addition of 1,144,588 Mg of AGB (550,547 Mg C) onto the landscape, and capable of sustaining healthy tree growth up to 40-years post regreening. However, as the regreening stands age, nutrient limitation may impact future tree growth and warrants further study.

World's Largest Regreening Project Promotes Healthy Tree Growth and Nutrient Accumulation Up to 40-Years Post Restoration

Mining and smelting degraded landscapes are characterised by heavily eroded, acidic soils that are contaminated with toxic metals and depleted of essential nutrients. Restoring forests on these landscapes has been highlighted to support carbon (C) mitigation measures and protect biodiversity. Understanding how tree growth and aboveground nutrient accumulation changes following restoration will be essential to planning future forest restoration projects. In this study, we assessed aboveground biomass (AGB) and aboveground nutrient (calcium (Ca), magnesium (Mg), nitrogen (N), phosphorous (P), potassium (K), and C) pools across as series of sites ranging in age from 15 to 42 years old; to determine the effects of erosion on AGB and AGB nutrient pools each site was categorized as 'stable' (less than 10% bedrock cover) or 'eroded' (greater than 30% bedrock cover). Both AGB and AGB nutrient pools increased with time since restoration at rates similar to coniferous plantations grown in areas unimpacted by centuries of mining and smelting practices. Individual tree growth and nutrient accumulation did not differ between stable and eroded sites; however, stable sites had a higher stem density leading to overall higher AGB and AGB nutrient pools. Future N limitation of the regreening forests does not appear to be a concern as aboveground N pools are six times larger than applied N, indicating additional N is entering into the system whether through residual soil organic matter or the establishment of N-fixing species. Conversely, aboveground P concentrations are decreasing with time since tree planting and the 40-year-old study sites have aboveground P concentrations below values for 'healthy' trees. This study shows that the regreening efforts have led to a massive addition of 1, 144, 588 Mg of AGB (550, 547 Mg C) onto the landscape, and capable of sustaining healthy tree growth up to 40-years post restoration. However, as the regreening stands age, nutrient limitation may impact future tree growth. Future studies should continue to investigate nutrient cycling within these remediated forests particularly nutrients of concern such as P.

Multivariate geostatistical modeling of the spatial sediment distribution in a large scale drainage basin, Upper Rhone, Switzerland

There is a notable discrepancy between detailed sediment budget studies in small headwater catchments (<102km2) focusing on the identification of sedimentary landforms in the field (e.g. talus cones, moraine deposits, fans) and large scale studies (>103km2) in higher order catchments applying modeling and/or remote sensing based approaches for major sediment storage delineation.To bridge the gap between these scales, we compiled an inventory of sediment and bedrock coverage from field mapping, remote sensing analysis and published data for five key sites in the Upper Rhone Basin (Val d'Illiez, Val de la Liène, Turtmanntal, Lötschental, Goms; 360.3km2, equivalent to 6.7% of the Upper Rhone Basin). This inventory was used as training and testing data for the classification of sediment and bedrock cover. From a digital elevation model (2×2m ground resolution) and Landsat imagery we derived 22 parameters characterizing local morphometry, topography and position, contributing area, and climatic and biotic factors on different spatial scales, which were used as inputs for different statistical models (logistic regression, principal component logistic regression, generalized additive model). Best prediction results with an excellent performance (mean AUROC: 0.8721±0.0012) and both a high spatial and non-spatial transferability were achieved applying a generalized additive model. Since the model has a high thematic consistency, the independent input variables chosen based on their geomorphic relevance are suitable to model the spatial distribution of sediment.Our high-resolution classification shows that 53.5±21.7% of the Upper Rhone Basin are covered with sediment. These are by no means evenly distributed: small headwaters (<5km2) feature a very strong variability in sediment coverage, with watersheds drowning in sediments juxtaposed to watersheds devoid of sediment cover. In contrast, larger watersheds predominantly show a bimodal distribution, with highest densities for bedrock (30–40%) being consistently lower than for sediment cover (60–65%). Earlier studies quantifying sedimentary cover and volume focus on the broad glacially overdeepened Rhone Valley that accounts for c. 9% of our study area. While our data support its importance, we conservatively estimate that the remaining 90% of sediment cover, mainly located outside trunk valleys, account for a volume of 2.6–13km3, i.e. 2–16% of the estimated sediment volume stored in the Rhone Valley between Brig and Lake Geneva. Furthermore, our data reveal increased relative sediment cover in areas deglaciated since the Little Ice Age, as compared to headwater regions without this recent glacial imprint. We therefore conclude that sediment storage in low-order valleys, often neglected in large scale studies, constitutes a significant component of large scale sediment budgets that needs to be better included into future analysis.

Semi-automated classification of exposed bedrock cover in British Columbia's Southern Mountains using a Random Forest approach

Knowledge of the spatial distribution of exposed bedrock (EB) is essential for natural resource inventories, environmental monitoring, and landscape evolution modelling. This paper presents a method for the use of a Random Forest (RF) classifier and legacy land data to locate areas of EB in a mountainous landscape of southern British Columbia, Canada. EB map accuracy increased from 48% to 88% with the use of RF models in comparison to the legacy land cover maps. Reducing the total number of predictor variables from 43 to 17 had a negligible effect on prediction accuracy. Meaningful relationships between EB predictions and important predictor variables were observed in partial dependence plots. These findings emphasize that substantial improvement in EB map accuracy is possible with a limited number of predictor variables, and interpretation of such maps is improved where the relationships between specific predictor variables and predicted classes can be observed and evaluated.

Current denudation rates in dolostone karst from central Spain: Implications for the formation of unroofed caves

Rock tablets of known weight were buried in the soil of a karst region in Central Spain to evaluate the carbonate weathering during a period of a year. The experiment was conducted at two different soil depths: 5–10 and 50–55cm from the surface. The parental rock used in the experiment is composed of dolomite and magnesite with variable proportion of accessory minerals and minor elements. Soil mineral and chemical composition as well as its texture was also characterized. Meteorological conditions at the site together with temperature and CO2 in both soil levels were monitored. Sets of tablets were retrieved after 6 and 12months of the start of the experiment to account for seasonal weathering. Different lithologies do not exhibit significant differences in weathering, although a large inter-sample variability is attributed to variable size and distribution of the porosity. Results show an enhanced weathering during the wet and cold season that accounts for 78±1% of the total annual weathering. Rock tablets examined under scanning electron microscopy prior and after exposure to natural environment show that most of the material lost occurred along cracks, edges or large pores. Although dissolution is a common process, most of the weathering is due to crystal detachment. Rock tablets at the depth of 5–10cm were weathered 68±1% more than those set at 50–55cm from the surface. Higher soil moisture and concentration of CO2 were found deeper in the soil, which likely enhanced the dissolution of carbonate. However, physical weathering dominated weight loss of rock tablets at both soil depths; especially at the 5–10cm level where soil thermal and moisture cycles were more frequent and greater. Denudation rate calculated from the 12months set provides values of 2.48±1.07μm/yr and 1.75±0.66μm/yr at the depths of 5–10 and 50–55cm, respectively. Since the conditions at the average contact between soil and bedrock are similar to those at the 50–55cm depth, we consider that this is a more reliable denudation rate for the studied location during the studied period. The calculated weathering rate suggests that denudation has a limited contribution to the thinning of bedrock over caves at this site. Therefore, we consider that the formation of unroofed caves in this region most likely results from the thinning of bedrock cover over caves due to collapse of blocks from their ceilings.

Evaluating susceptibility of karst dolines (sinkholes) for collapse in Sango, Tennessee, USA.

Dolines or sinkholes are earth depressions that develop in soluble rocks complexes such as limestone, dolomite, gypsum, anhydrite, and halite; dolines appear in a variety of shapes from nearly circular to complex structures with highly curved perimeters. The occurrence of dolines in the studied karst area is not random; they are the results of geomorphic, hydrologic, and chemical processes that have caused partial subsidence, even the total collapse of the land surface when voids and caves are present in the bedrock and the regolith arch overbridging these voids is unstable. In the study area, the majority of collapses occur in the regolith (bedrock cover) that bridges voids in the bedrock. Because these collapsing dolines may result in property damage and even cause the loss of lives, there is a need to develop methods for evaluating karst hazards. These methods can then be used by planners and practitioners for urban and economic development, especially in regions with a growing population. The purpose of this project is threefold: 1) to develop a karst feature database, 2) to investigate critical indicators associated with doline collapse, and 3) to develop a doline susceptibility model for potential doline collapse based on external morphometric data. The study has revealed the presence of short range spatial dependence in the distribution of the dolines' morphometric parameters such as circularity, the geographic orientation of the main doline axes, and the length-to-width doline ratios; therefore, geostatistics can be used to spatially evaluate the susceptibility of the karst area for doline collapse. The partial susceptibility estimates were combined into a final probability map enabling the identification of areas where, until now, undetected dolines may cause significant hazards.

Macro-roughness model of bedrock–alluvial river morphodynamics

Abstract. The 1-D saltation–abrasion model of channel bedrock incision of Sklar and Dietrich (2004), in which the erosion rate is buffered by the surface area fraction of bedrock covered by alluvium, was a major advance over models that treat river erosion as a function of bed slope and drainage area. Their model is, however, limited because it calculates bed cover in terms of bedload sediment supply rather than local bedload transport. It implicitly assumes that as sediment supply from upstream changes, the transport rate adjusts instantaneously everywhere downstream to match. This assumption is not valid in general, and thus can give rise to unphysical consequences. Here we present a unified morphodynamic formulation of both channel incision and alluviation that specifically tracks the spatiotemporal variation in both bedload transport and alluvial thickness. It does so by relating the bedrock cover fraction to the ratio of alluvium thickness to bedrock macro-roughness, rather than to the ratio of bedload supply rate to capacity bedload transport. The new formulation (MRSAA) predicts waves of alluviation and rarification, in addition to bedrock erosion. Embedded in it are three physical processes: alluvial diffusion, fast downstream advection of alluvial disturbances, and slow upstream migration of incisional disturbances. Solutions of this formulation over a fixed bed are used to demonstrate the stripping of an initial alluvial cover, the emplacement of alluvial cover over an initially bare bed and the advection–diffusion of a sediment pulse over an alluvial bed. A solution for alluvial–incisional interaction in a channel with a basement undergoing net rock uplift shows how an impulsive increase in sediment supply can quickly and completely bury the bedrock under thick alluvium, thus blocking bedrock erosion. As the river responds to rock uplift or base level fall, the transition point separating an alluvial reach upstream from an alluvial–bedrock reach downstream migrates upstream in the form of a "hidden knickpoint". A tectonically more complex case of rock uplift subject to a localized zone of subsidence (graben) yields a steady-state solution that is not attainable with the original saltation–abrasion model. A solution for the case of bedrock–alluvial coevolution upstream of an alluviated river mouth illustrates how the bedrock surface can be progressively buried not far below the alluvium. Because the model tracks the spatiotemporal variation in both bedload transport and alluvial thickness, it is applicable to the study of the incisional response of a river subject to temporally varying sediment supply. It thus has the potential to capture the response of an alluvial–bedrock river to massive impulsive sediment inputs associated with landslides or debris flows.

Weathering of plagioclase across variable flow and solute transport regimes

The study area is situated in a fault zone with fractured granites and metasediments. In a conceptual model, infiltrating water first passes the bedrock cover of soil and saprolite and then partly enters the fractures. Weathering reactions of minerals occur in small pores and fissures in the bedrock cover zone to continue in the larger fractures. Pumping tests were carried out in a number of boreholes to measure the drawdown as a function of pumping time. From the results, values of transmissivity (T) could be derived. In combination with the storage coefficient (S) for similar fault zones, the hydraulic diffusivity (D = T/S) could be computed. Water samples, collected from the boreholes, represent fluid packets with a history of weathering reactions in the bedrock cover and in the larger fractures. The major element composition of these samples was used by means of the SiB mass balance algorithm (Pacheco and Van der Weijden, 1996) to calculate the moles L−1 of dissolved plagioclase (oligoclase with An ≈ 0.20) and the moles L−1 of secondary phases (gibbsite, halloysite, smectite) precipitated along the flow paths of the samples. These results were then used to calculate the net dissolved silica concentrations ([H4SiO40]) related to dissolution of plagioclase followed by precipitation of each of the secondary phases. An interpretation of a plot of each of these [H4SiO40] ’s versusD is that at D < 0.7 m2 s−1, dissolution of plagioclase is followed by precipitation of halloysite in the large fractures of the fault zone (open system), whereas at D ⩾ 0.7 m2 s−1 precipitation of both halloysite and smectite occurs in the rock matrix with small fissures and pores (semi-open system). Before being pumped, the percolating fluids travelled 0.01–13.7 years. During these periods, plagioclase weathered at rates (WPl) of 10−(12.9±1.1) moles m−2 s−1, which are approximately 2.2 orders of magnitude higher than solid-state weathering rates reported in various field studies. In this study, it is suggested that part of the apparent discrepancy between the results is due to changes in hydraulic diffusivity of the weathering environments occurring over the geologic times.

Pre‐ and Post‐Mined Bauxite Soils of Jamaica: Physical and Chemical Properties

Earthy bauxite (aluminum ore) deposits over limestone bedrock cover over 20% of Jamaica's surface area. With the shortage of arable land in Jamaica, there is great interest in using post‐mined bauxite lands for agriculture. The purpose of this study was to compare physical and chemical properties of pre‐ and post‐mined bauxite soils that are relevant in assessing suitability of these lands for small‐scale agriculture. Pedons were described and sampled for characterization at two pre‐mined sites and four post‐mined sites near Mandeville, Jamaica. Post‐mined soils were generally steeper, shallower, and higher in limestone rock fragments than pre‐mined soils. Pre‐mined soils consisted of Ap and Bo horizons making up clayey Oxisols with low charge and low bulk density. The new caret (^) symbol, which indicates horizons formed from human transported material, was helpful in describing pedons of post‐mined soils. Post‐mined soils consisted of ^Ap horizons (replaced topsoil) and 2^C horizons (fill material) over limestone. Some post‐mined pedons also had 2Bob or 3Bob horizons (Bo in situ buried by replaced topsoil and/or fill material). Post‐mined soils had higher bulk density in the ^Ap horizons than pre‐mined Ap horizons and higher pH throughout due to the incorporation of limestone rock fragments. In ^Ap horizons, organic matter and granular structure increased with time after reclamation and establishment of grass cover.

HoistEM data processing for discovery of high grade manganese ore under regolith cover

A time-domain airborne electromagnetic (AEM) survey was flown with the HoistEM system over the Woodie Woodie manganese mine corridor in the east Pilbara of Western Australia. Conductivity Depth Image (CDI) processing helped to discriminate shallow, regolith-related responses from conductive ore. About half of the known manganese ore zones correlate with elevated conductivities on the CDIs, and recent drilling of several new EM targets in areas of regolith and bedrock cover has discovered over six large-tonnage manganese ore bodies to date. EMFlow software has made it practical to process data gathered by HoistEM to separate the ore body response from the responses of conductive palaeochannels and other conductive features related to bedrock geology or the regolith. Experimentation with EMFlow input parameters helped to produce CDI results that were more reliable. The CDI results were further refined by calibrating conductivity values from CDI processing to correlate with borehole conductivity results. Borehole-calibrated CDI results have lower noise, line levelling problems are suppressed, and depths of conductive horizons are more accurate. The HoistEM survey-design tests and processing results demonstrate that HoistEM is a cost-effective method for exploration of podiform, high-grade manganese ore bodies, as long as the host rocks are comparatively resistive, and a flight line spacing of 80 m or less is used.

HoistEM Data Processing for Discovery of High-Grade Manganese Ore Under Regolith Cover

A time-domain airborne electromagnetic (AEM) survey was flown with the HoistEM system over the Woodie Woodie manganese mine corridor in the east Pilbara of Western Australia. Conductivity Depth Image (CDI) processing helped to discriminate shallow, regolith-related responses from conductive ore. About half of the known manganese ore zones correlate with elevated conductivities on the CDIs, and recent drilling of several new EM targets in areas of regolith and bedrock cover has discovered over six large-tonnage manganese ore bodies to date. EMFlow software has made it practical to process data gathered by HoistEM to separate the ore body response from the responses of conductive palaeochannels and other conductive features related to bedrock geology or the regolith. Experimentation with EMFlow input parameters helped to produce CDI results that were more reliable. The CDI results were further refined by calibrating conductivity values from CDI processing to correlate with borehole conductivity results. Borehole-calibrated CDI results have lower noise, line levelling problems are suppressed, and depths of conductive horizons are more accurate. The HoistEM survey-design tests and processing results demonstrate that HoistEM is a cost-effective method for exploration of podiform, high-grade manganese ore bodies, as long as the host rocks are comparatively resistive, and a flight line spacing of 80 m or less is used.

High‐resolution seismic traveltime tomography incorporating static corrections applied to a till‐covered bedrock environment

A major obstacle in tomographic inversion is near‐surface velocity variations. Such shallow velocity variations need to be known and correctly accounted for to obtain images of deeper structures with high resolution and quality. Bedrock cover in many areas consists of unconsolidated sediments and glacial till. To handle the problems associated with this cover, we present a tomographic method that solves for the 3D velocity structure and receiver static corrections simultaneously. We test the method on first‐arrival picks from deep seismic reflection data acquired in the mid‐ late to 1980s in the Siljan Ring area, central Sweden. To use this data set successfully, one needs to handle a number of problems, including time‐varying, near‐surface velocities from data recorded in winter and summer, several sources and receivers within each inversion cell, varying thickness of the cover layer in each inversion cell, and complex 3D geology. Simultaneous inversion for static corrections and velocity produces a much better image than standard tomography without statics. The velocity model from the simultaneous inversion is superior to the velocity model produced using refraction statics obtained from standard reflection seismic processing prior to inversion. Best results using the simultaneous inversion are obtained when the initial top velocity layer is set to the near‐surface bedrock velocity rather than the velocity of the cover. The resulting static calculations may, in the future, be compared to refraction static corrections in standard reflection seismic processing. The preferred final model shows a good correlation with the mapped geology and the airborne magneticmap.

The use of Landsat mss imagery to determine plant characteristics and community spectral separability from sub‐tropical to arid zones in northern Australia

Abstract Plant communities along a vegetation gradient comprising sub‐tropical, semi‐arid and arid regions differed in terms of their green canopy cover, percent count (number of species present) and species richness per survey site. Analysis of green vegetation cover relative to background on Landsat MSS resulted in least distinctiveness for the subtropical region, where higher green vegetation percentages and a senesced herbaceous layer tended to homogenise the mostly near‐infrared reflective response. In the arid zone, plant spectral distinctiveness was minimised in areas of stony or bedrock cover but enhanced due to darkening in the dune areas despite low cover percentages. Due to vegetation darkening in the semi‐arid area, the spectral separability of green plant communities was enhanced within the 25% cover range. This range corresponded mostly to upper threshold levels of darkened vegetation prevalent globally in semi‐arid rangelands and in sandy areas of arid environments.

Winter Habitat Selection by Mountain Sheep

During winter 1975-76, 197 observations of groups of wintering mountain sheep (Ovis canadensis) provided data on habitat selection near Thompson Falls in northwestern Montana. Sheep selected against upper elevations, drainage bottoms, upper slopes on ridges, areas with a slope steepness of 10-35%, east and southeast aspects, areas greater than 320 m from steep terrain, and closed forests. Preferences (P < 0.1) were shown for cliffs, areas with slope steepness greater than 80% and closer than 320 m to steep terrain or cliffs, and shrubland-grassland and open forest vegetation types. Adult ram groups were observed at higher elevations (P < 0.05) than ewe-juvenile or young ram groups. Sheep apparently sought warmer elevations throughout the winter. There was general agreement on the selection of habitat as estimated from direct observations and fecal group counts. J. WILDL. MANAGE. 46(2):359-366 The importance of winter range in the welfare of mountain sheep has been expressed by several investigators (Honess and Frost 1942, Smith 1954, Buechner 1960, Oldemeyer et al. 1971, Rutherford 1972). However, the mixture of meterological, topographic, and biotic variables which provide adequate winter range has received limited study. Knowledge of variables selected would provide criteria for selecting transplant sites containing adequate winter range. Rutherford (1972) observed that evaluation of potential transplant sites is complicated because mountain sheep are selective and their habitat requirements are not well understood. Our objectives were to quantitatively describe the habitat used by mountain sheep in winter, identify the selection of sites among those available, and evaluate the preference of mountain sheep for biotic and abiotic environmental variables on winter range. STUDY AREA The Thompson Falls mountain sheep winter range is along the southeast end of the Cabinet Mountains just north of the Clark Fork River midway between Plains and Thompson Falls, Montana (Brown 1974). Poorly developed soils and numerous talus slopes emanating from rugged crags of exposed bedrock cover much of the 1,660-ha study area. The valley floor is at 730 m, with the mountains rising to over 1,800 m in less than 2.4 km with south-southwest exposures predominating. Climate is Pacific maritime and January is the coldest month with temperatures averaging -3.1 C (range -37.8 to 13.3 C). Mean annual precipitation is 57.2 cm with greatest amounts falling during November-January. The winter of 1975-76 was slightly milder than average in severity. Plant communities were bryoids on talus slopes, shrubland-grassland complexes, low to mid-elevation Douglas-fir (Pseudotsuga menziesii)-common snowberry (Symphoricarpos albus) communities, and higher Douglas-fir-mallow ninebark (Physocarpus malvaceus) communities. Major grass species present were bluebunch wheatgrass (Agropyron spicatum), pine reedgrass (Calamagros1 Funded by the National Rifle Association, Wildlife Management Institute, Montana Forest and Conservation Experiment Station, and Montana Cooperative Wildlife Research Unit. 2 Present address: Soil Conservation Service, Roseburg, OR 97470. J. Wildl. Manage. 46(2):1982 359 This content downloaded from 207.46.13.111 on Sun, 22 May 2016 04:21:07 UTC All use subject to http://about.jstor.org/terms 360 WINTER HABITAT SELECTION BY MOUNTAIN SHEEP' Tilton and Willard tis rubescens), and rough fescue (Festuca scabrella). Other important shrubs were Rocky Mountain maple (Acer glabrum), Saskatoon serviceberry (Amelanchier alnifolia), snowbrush ceanothus (Ceanothus velutinus), Lewis mockorange (Philadelphus lewisii), antelope bitterbrush (Purshia tridentata), and common chokecherry (Prunus virginiana).