Mound Microtopography Research Articles

Microtopography and ecology of pit-mound structures in second-growth versus old-growth forests

Pit and mound microtopography is an important structural component of most forests, influencing soil processes and habitat diversity. These features have diminished greatly in northeastern U.S. forests since European settlement, asa result of the history of repeated logging, land-clearance followed by reforestation, and the smaller size of trees (and therefore windthrow features) comprising the prevailing second-growth forests. Despite the potential importance of this region-wide shift in forest microtopography on ecosystem structure and function, the differences in pit and mound size, distribution, and longevity between second-growth and old-growth forests are unexplored. Likewise, although many studies demonstrate that mounds and/or pits are hotspots for tree regeneration there is scant information about whether location on a mound or pit affects tree survival and growth beyond the seedling stage, or whether microtopographic regeneration patterns differ in old-growth and second-growth forests.We compare a simulated hurricane experiment initiated in 1990 in second-growth forest (the pulldown) and an old-growth forest that was blown down by a hurricane in 1938 (Pisgah) to examine differences in pit-mound microtopography and ecology between second-growth and old-growth forest. At Pisgah, fewer, larger mounds comprised a similar areal coverage as at the pulldown. Repeated measurements of individual pit-mound structures in the pulldown revealed that pit infill proceeded more rapidly than mound erosion. Mound area increased but height decreased over time as soil from the mound tops eroded and spread around the mound base. Although 40% of mounds in the pulldown were>1m tall immediately after the manipulation (maximum of 2.9m), after 25years, maximum mound height was 0.9m. In contrast, 11% of mounds at Pisgah remained>1m tall in 1989, 50years after blowdown. At both sites, trees, especially Betula spp., were disproportionately found on mounds. Fewer trees than expected grew in pits at Pisgah. Tree mortality was somewhat higher on mounds and pits than on other substrates. Asa mechanism to increase stand-level tree diversity, windthrow may be more critical in old-growth forests, in which niches for early-mid successional species are few, than in second-growth forest, in which early-mid successional species already comprise the majority of the trees. Pit-mound structures are a diminished component of second-growth forest, and silvicultural techniques designed to restore old-growth characteristics could include measures to preserve and enhance pit-mound features, and to cultivate large-diameter trees that will eventually create the large, long-lasting pit-mounds of the future.

Transformation of iron forms during pedogenesis after tree uprooting in a natural beech-dominated forest

Treethrow dynamics was revealed to be a key biomechanical effect of individual trees in soil formation in mountain temperate forests. The resulting pit–mound microtopography represents a specific pattern of microsites with a potential influence on the course of pedogenesis (Šamonil et al., 2010a). The aim of this study was to investigate the influence of tree uprooting on the transformation of Fe forms, as these forms indicate the degree of pedogenesis in Cambisols.Soil samples originated from a Haplic Cambisols region in a natural fir–beech forest in the Western Carpathians, Czech Republic. Four pit–mound pairs of different ages – 20, 28, 180 and 191years – as well as control, undisturbed profiles were sampled.Iron forms were studied using the following two different approaches: (i) Fe forms (exchangeable, crystalline, and amorphous together with organically complexed Fe) were extracted by three specific extracting agents then subsequently determined by ICP-OES; and (ii) non-destructive methods such as Voltammetry of Microparticles (VMP) and Diffuse Reflectance Spectroscopy (DRS) were used.The following Fe species were detected by VMP within individual microsites: ionic Fe(III), ferrihydrite, and poorly crystalline and crystalline Fe(III) oxides. Mn(III,IV) oxides were also detected. Goethite, hematite, Fe2+–Fe3+ pairs in Fe-bearing aluminosilicates (biotite, chlorite) and octahedral Fe3+ (total Fe3+ oxides and silicates) were quantified by means of the DRS technique.Ferrihydrite and Fe2+/Fe3+ ratio were higher in soils from the pits compared to samples originating from the mounds and this ratio increased with increasing depth. Linear mixed effect (LME) models fitted by restricted maximum likelihood (REML) determined the relation between iron forms and other soil characteristics. Based on the development of Fe forms with time, we can conclude that tree uprooting significantly accelerates pedogenesis in the natural forest.

Ice storms, tree throw, and hillslope sediment transport in northern hardwood forests

AbstractIn December 2008, 694 trees uprooted within a 108 ha (1·08 km2) watershed in central Massachusetts due to a severe ice storm, resulting in the displacement of ~1300 m3 of root material, unconsolidated sediment, and fractured bedrock. Overall, we find that uprooting and tree throw is often grouped in clusters and cascades; conifers displace more material than deciduous trees; areas with abundant mature hemlock and steep slopes are more susceptible to tree throw, with clusters as dense as 125 per hectare; and failure is predominantly downhill, suggesting that ice storms promote efficient downslope hillslope sediment transport in northern hardwood forests. Combining the recurrence interval of severe storms in New England (20–75 years) with the forest response presented here, we calculate a sediment transport rate of 2–5 × 10−5 m3 m−1 a−1 averaged over the entire watershed. Forest susceptibility to tree throw differed based on location in the watershed; some areas experienced up to ~30× higher than average sediment transport rates, while others experienced no tree throw. Two severe storms following the 2008 ice storm (hurricane in 2011; snow storm in October 2012) did not result in significant tree throw within the study area, highlighting that the coupling of storm severity and forest susceptibility controls the amount of tree throw during a given forest disturbance. In addition to recent tree throw from the 2008 ice storm, widespread pit and mound microtopography in the study area indicates that tree throw is a recurrent process in this landscape. Two factors emerge that will influence future ice storms related hillslope sediment transport in the steep forested hillslopes of New England: regional climate gradients and changing climate determine the size, intensity and recurrence of ice storms; forest management practices and health control the tree age and type. Copyright © 2015 John Wiley & Sons, Ltd.

Disturbances can control fine-scale pedodiversity in old-growth forests: is the soil evolution theory disturbed as well?

Abstract. Biota–soil interactions in natural ecosystems are the subject of considerable research. Our hypothesis is that individual trees play a significant role through biomechanical and biochemical disturbances affecting soil formation in temperate forests, resulting in a complex spatial pattern of disturbance regimes and a close relationship between disturbance histories and soil units. In Žofínský Prales (Czech Republic) – the fourth oldest, continuously protected reserve in Europe and the first site of global research network SIGEO (Smithsonian Institution Global Earth Observatories) in continental Europe – we compared extensive dendrochronological, soil and pit–mound microtopography data both temporally and spatially from an area of anthropogenically unaffected 42 ha collected from 2008–2012. These data sets differ in terms of information complexity and length of memory: tree cores contain complex information about the disturbance history of the past 350 years, footprints of disturbances from the uprooting of a specific tree can persist 1700 years, and soils represent an extensive composite phenotype that has been developing for at least the entire postglacial period (10 500 years). On average, 6.18–13.41% of the canopy on individual soil units was disturbed per decade. Even though the "backbone" of key events in the development of the forest ecosystem remained the same (e.g. the 1870s, 1880s and 1980s), the internal structure of disturbance history often differed among soil units; the most exceptional were Gleysols and Histosols, where important feedback from soil to trees was expected. However, the characteristics of treethrow dynamics as well as the frequencies of stronger releases in core series also significantly differed along a gradient of terrestrial soil weathering and leaching (Haplic Cambisols – Dystric Cambisols – Entic Podzols – Albic Podzols). These results suggest the existence of several disturbance regimes within the forest, controlling fine-scale pedodiversity.

Dynamic of Plant Composition and Regeneration following Windthrow in a Temperate Beech Forest.

The effects of soil pedoturbation (i.e., pit and mound microtopography, PM) on development of herbaceous plant species and woody species regeneration were examined in a temperate beech forest (Fagus orientalis Lipsky) in northern Iran. We recorded the vegetation in 20 pairs of disturbed and adjacent undisturbed plots and established a chronosequence of PM ages to study the effect of time since microsite formation on cover percent of herbaceous plants and woody regeneration status. According to our findings, Carex acutiformis L., Sambucus ebulus L., Brachypodium pinnatum L., and Cyclamen coum L. are found only in the PM microsites, whereas the Equisetum ramosissimum L. is recorded only under closed canopy. The coverage percent of Rubus caesius L. increased in PM microsites compared to closed canopy intensively. In addition, Albizia julibrissin Durazz. is detected in PM microsite, whereas the Acer cappadocicum B. and Prunus persica L. species were recorded only under closed canopy. We found significant differences in understory species diversity between different ages of PM, and disturbed and adjacent undisturbed plots. Our study supports that the PM complex will create a mosaic of environmental conditions. This environmental heterogeneity could be responsible for the diversity of herbaceous plant species and regeneration of woody species.

Importance of bird seed dispersal in the development of characteristic vegetation on termite mounds in north-eastern Namibia

Mound-building termites function as ecological engineers in tropical savanna landscapes. Large termite mounds support a greater diversity of plants than off-mound areas, but little is known about the process by which diverse mound vegetation develops. This study examined the dispersal mode of woody plants on termite mounds and assessed the process of plant assemblage formation on mounds. This study focused on mound status (termites active vs. inactive) and mound microtopography (mound cones vs. pediments) to develop a chronology of vegetation development. The frequencies of occurrence of woody plants were lower on active mounds (67 %), especially on cones (46 %) compared to inactive mounds (95 %). Species richness and the abundance of woody plants increased in the order of cones, pediments of active mounds and inactive mounds. The proportion of bird-dispersed plant species was much higher on mounds (>40 %) than in off-mound areas (3 %). Salvadora persica was the main bird-dispersed plant and occurred preferentially on active mounds. A schematic plant succession model incorporating the process of endozoochory was developed to explain vegetation development on active mounds.

Variation in savanna vegetation on termite mounds in north-eastern Namibia

Abstract:In savanna, termite mounds support more diverse vegetation than off-mound areas, but little is known of the patterns in plant assemblages on mounds. To explain vegetation differentiation between (1) component structures of termite mounds (conical centre vs. pediment), (2) active and inactive mounds (termites present vs. termites absent), and (3) sites on and off mounds (on mounds vs. surrounding savanna), species composition, richness and abundances of woody plants were recorded on 70 mounds and in 13 savanna plots (each 20 × 20 m) in north-eastern Namibia, focusing on soil hardness, mound status (active or not) and mound micro-topography as explanatory factors. Woody plants were absent from 33% of active mounds (54% of active cones) but were absent from only 5% of inactive mounds. Species richness and abundance per mound (mean ± SD) were lower on active mounds with (2.0 ± 1.8 and 4.6 ± 6.6, respectively) and without pediments (0.6 ± 0.6 and 0.9 ± 1.1, respectively) than on inactive mounds (4.4 ± 2.7, 19.4 ± 18.8, respectively). Despite the lower woody plant cover, some characteristic species, such as Salvadora persica, occurred preferentially on active mounds; this species occurred on 42% of active mounds. Mean soil hardness (± SD) was higher on conical parts of active mounds (4300 ± 2620 kPa) than on adjacent pediments (583 ± 328 kPa) and inactive mounds (725 ± 619 kPa). This study suggested that mound status, mound micro-topography, and soil hardness promote variability in the vegetation on mounds.

Effects of post‐windthrow salvage logging on microsites, plant composition and regeneration

AbstractQuestionsHow does windthrow influence plant diversity and composition as well as regeneration and microsite characteristics? What are the consequences of post‐windthrow salvage logging on these key attributes?LocationEastern black spruce–moss forest,Quebec,Canada.MethodsA total of 92 plots were sampled, each with a radius of 11.28 m; 49 of these plots were salvaged while 43 were unsalvaged. Regeneration density, plant diversity and seedbeds were characterized. We tested the effect of microtopography and windthrow severity on species richness and Shannon diversity index for salvaged and unsalvaged windthrows using a mixed model. Partial redundancy analysis (RDA) determined which environmental and stand characteristics were most important in explaining differences in plant species and forest floor types among the treatments. The effects of treatments (salvaged and unsalvaged windthrows), microtopography attributes, windthrow severity and regeneration species on seedling and sapling abundance were tested using a linear mixed model.ResultsSalvaged windthrow, with a large proportion of skid trails, dead mosses andSphagnum, had a lower degree of seedbed heterogeneity. Also, some understorey species present in the unsalvaged ecosystem were absent from the salvaged windthrow.Sphagnumand other moss species were clearly associated with the unsalvaged treatment. White birches were positively associated with mound microtopography in the unsalvaged windthrow.ConclusionFrom an ecosystem‐based forest management perspective, natural post‐windthrow understorey conditions and microsite heterogeneity can be in part maintained in salvaged cut blocks by incorporating retention patches that include downed and standing dead wood and living trees of diverse sizes. These steps should favour plant regeneration and augment diversity for salvage logging after wind disturbance.

Microenvironmental heterogeneity and Quercus michauxii regeneration in experimental gaps

Canopy gaps are foci for forest regeneration; the gap environment promotes establishment and tree growth into the canopy. In bottomland hardwood forests, microtopography and light are modified simultaneously in natural gaps. Their combined effects may influence seedling establishment and growth. Desirable red and white oaks are often regenerated in bottomland hardwood forests through group selection cutting that mimics the gap environment. We tested effects of canopy openness and microtopography on regeneration of a moderately shade-intolerant oak, Quercus michauxii, in experimental canopy gaps in a bottomland hardwood forest on the Atlantic Coastal Plain. Gap sizes were centered on the median opening size of natural gaps in an old-growth bottomland hardwood forest. Acorns were planted at 100 random level locations in and around each of two replicate 7, 10, 14, 20, 29, and 40 m radius gaps in early spring. Acorns were also planted on pit and mound microtopography complexes created in the center, at the edge, and under intact canopy surrounding two 29 m and three 40 m radius gaps. Over all gaps, 21–37% of the randomly located acorns emerged and survived one summer; 8–26% survived two summers. Survival was influenced more by seedling size than by gap size or distance from gap center. Seedlings on level locations that survived two seasons were taller in the first year, had greater maximum leaf area, and tended to be farther from the center of large gaps than seedlings that died after 1 year. Height of second-year seedlings was related more to first-year plant size than to distance from gap center or gap size. Seedling emergence from acorns planted on microtopography complexes was greater on mounds than in pits at gap center and beneath intact canopy; no seedlings emerged in pits in the gap center. Once established, seedling survival and growth on the microtopography complexes was higher at the gap center. Management of bottomland hardwoods for Q. michauxii regeneration from acorns may be most successful if practices that create conditions favorable for formation of a seedling bank, i.e. raised topography beneath a thin canopy, are followed by light harvests that promote seedling growth and ascent into open canopy.

Breeding bird assemblages of hurricane-created gaps and adjacent closed canopy forest in the southern Appalachians

We studied breeding bird assemblages in forest gaps created in 1995 by Hurricane Opal at the Bent Creek Experimental Forest in Asheville, NC. We hypothesized that forest gaps and adjacent closed-canopy forest would differ in bird density, richness, diversity, and relative abundances of some species. To test this hypothesis we censused breeding bird assemblages for 2 years in 12 gaps (0.1–1.2 ha) and 12 adjacent closed canopy controls using strip transects. Gaps had more coarse woody debris, shrub cover, brushpiles, and pit and mound microtopography than controls. Canopy cover was lower in gaps than controls, but remained high ( 69.4 ± 2.1% versus 89.6 ± 1.7% ). Bird assemblage similarity was high. Total density and species richness of birds were higher in gaps than in controls, but species diversity did not differ between treatments. Shrub (primarily Carolina Wrens) and bark-foragers, and cavity shrub and canopy-nesters were more abundant in gaps than in controls. Densities of gap-associated (Indigo Bunting, Hooded Warbler, Carolina Wren) and edge (Eastern Towhee) species were more abundant in gaps. Abundance of interior species including Red-eyed Vireo and Scarlet Tanager were about equal in gaps and controls. Only Ovenbirds were more abundant in controls than gaps. Species that require larger patches of young second-growth forest, such as Prairie Warbler, and Yellow-breasted Chat, did not occur in gaps; but neither are they abundant in the Asheville basin. No Brown-headed Cowbirds were observed in gaps or controls. Unpublished data indicate that parasitism of artificial ground nests did not occur, and predation rates did not differ between gaps and controls. Juvenile birds and other evidence of breeding were observed more often in gaps than in controls, suggesting that gaps attract bird families for foraging and provide microsites that attract breeding pairs. Gap size was positively correlated with bird density, species richness, and diversity. This study suggests that small openings and interior edge habitat created by treefall gaps within a forested matrix do not adversely affect breeding birds as measured by the abundance of individual species or community indices. We suggest that canopy gaps increase avian diversity at a landscape scale by providing habitat patches for some species that require young, second-growth forest, and serve as magnets for recruitment and foraging.

Bucket Mounding as a Mechanical Site Preparation Technique in Wetlands

Abstract This article summarizes the information in the literature concerning site preparation in wetlands with special emphasis on bucket mounding. Mounding as a site preparation technique has been used since the 18th century for reestablishing tree species on wet sites, and it is commonly used in parts of Canada and Scandinavia. In the Lake States, a version of mounding called bucket mounding is coming into use for regenerating cutover wetland sites. Bucket mounding differs from other mounding operations in that it is used exclusively in wetlands and uses a tracked excavator to create the mounds, rather than equipment towed behind or attached to a skidder or bulldozer. In wet areas, bucket mounding creates a raised planting site, resulting in more aerated soil above the water table, warmer soil temperatures during the growing season, greater nutrient availability, and a small degree of vegetation control. Bucket mounding mimics the natural pit and mound microtopography that naturally occurs as a result of wind storms across the Great Lakes Region. This microtopography is important for natural regeneration establishment and growth. This article provides an overview of natural pit and mound formation, types of mounds, mounding equipment, the effects of mounding on the seedling environment, and planted species survival. Additional considerations for Lake States conditions are also discussed. North. J. Appl. For. 18(1):7–13.

The influence of organic litter on the erosive effects of raindrops and of gravity drops released from desert shrubs

This paper reports on a laboratory study of the effects of water drop impacts on litter and sand splash beneath desert shrubs. Individual drops of 5.7 mm diameter were released from heights of 0.5, 1.0, and 1.5 m (selected to encompass the height range of typical desert shrubs) onto targets of bare or partially litter-covered, saturated fine sand. The natural litter, largely derived from the saltbush Atriplex vesicaria, was collected from desert shrubland sites in western New South Wales (NSW). The drop impacts caused both sand and litter particles to undergo splash displacement. The mass of sand splashed was found to increase with drop fall height, while mass of litter particles splashed did not vary significantly with fall height. Weights of sand moved by airsplash were significantly diminished by surface litter applied at the rate of 200 g/m 2. These findings indicate that gravity drops released from desert shrubs may provide both an erosive force beneath these plants, and a means for dispersing litter from the plant base into the surrounding landscape, where litter may continue to affect hydrologic and erosional processes. By restricting splash of mineral particles, litter acts to limit soil splash from beneath shrubs, and in this way may contribute to the persistence of plant mound microtopography that is common in desert shrublands. Under open-field conditions, large raindrops delivered in convective showers must cause similar airsplash transport of litter particles, thus playing a role in the distribution of litter within the landscape.

Seasonal comparison of soil temperature and moisture in pits and mounds under vine maple gaps and conifer canopy in a coastal western hemlock forest

In this study we attempted to determine if vine maple priority gaps show similar trends in temperature and moisture status to those reported in the literature for treefall gaps and whether temperature and moisture status differed between microtopographic positions (pits and mounds). Biweekly measurements of mid-day soil and air temperature, moisture contents at 30-, 50- and 80-cm depths, and depths to the groundwater table were made in pit and mound locations within six vine maple priority gaps paired with six conifer canopy sites. Trends did not follow those found in treefall gaps: vine maple gaps had similar mid-day temperature and moisture status to the surrounding conifer forest. Larger gaps had higher mid-day air temperatures in the summer, higher mid-day soil temperatures in the spring and summer, and greater amounts of throughfall in the spring and summer than smaller gaps. Trends in mid-day soil temperature and moisture status for pit and mound microtopography followed those reported in the literature. Pits were significantly cooler in summer and warmer in winter than mounds and pits were wetter than mounds in all seasons. This study suggests that soil microtopography has an effect on soil climate that overwhelms the influence of vine maple gaps. Key words: Vine maple, canopy gap, soil moisture, soil temperature, microtopography, pits and mounds

Comparison of Sampling Methods for Estimating Pit and Mound Microtopography

AbstractWe developed a method for making rapid estimates of pit and mound microtopography for characterizing research sites, soil map units, and ecological land types. We conducted this study in the drumlinized uplands of northeastern Wisconsin, where pit and mound microtopography is extensive. We established one 10 by 30 m grid and one 300‐m transect on each of four drumlins and identified a pit, a mound, or neither at nodes 1 m apart (300 observations per grid or transect). Pit and mound microtopography was estimated by calculating the proportion of pits, mounds, and points lacking evidence of pit and mound microtopography at nodes within each grid or transect. The optimal node spacing for grid and transect sampling methods was determined by repeatedly subsampling data derived from each method and identifying the lowest mean square error. The precision of grid sampling vs. transect sampling was determined by comparing the standard errors estimated with each method. Transects were more precise and time‐efficient than grids for estimating pit and mound microtopography percentages. Satisfactory results were obtained with 300‐m‐long transects with 3‐m node spacing. Transect methods appear applicable to the quantification of other types of microtopography.

Do Appalachian Herbaceous Understories Ever Recover from Clearcutting?

Abstract: Life history characteristics of many herbaceous understory plants suggest that such species recover slowly from major perturbations such as clear cutting. We examined herbaceous cover and richness in the understories of nine primary (“old‐growth”) forests in the southern Appalachian Mountains and of nine comparable secondary forests, ranging in age from 45 to 87 years since clear cutting. Neither cover nor richness increased with age in the secondary forests. This suggests three possibilities: (1) that recovery is so slow or variable among sites that 87 years is insufficient time to detect it; (2) that such forests will never recover to match remnant primary forests because climatic conditions are different today than when the forests became established; or (3) that herbaceous plants colonize pit and mound microtopography caused by the death of trees, so that recovery must await the growth, death, and decomposition of the trees of the secondary forest. Whatever the mechanism, herbaceous understory communities in the mixed‐mesophytic forests of the Appalachians appear unlikely to recover within the present planned logging cycles of 40–150 years, suggesting a future loss of diversity of understory herbaceous plants.

Hydraulic roughness of earth covers at a cold‐desert waste burial site

Abstract A kinematic wave model of overland flow was used to calculate hydraulic roughness coefficients for earth covers and native hillslope surfaces at a waste burial site located in a cold‐desert region of southeast Idaho. Manning n roughness coefficients were greater on earth cover plots planted to crested wheatgrass [Agropyron desertorum (Fisch, ex Link) Schult.] (a bunchgrass, average n = 0.076) than on those planted to streambank wheatgrass [Elymus lancelolatus (Scribner & J. G. Smith) Gould] (a sodgrass, average n = 0.030). Mound and intermound microtopography strongly influenced overland flow geometry on the bare native hillslope plots resulting in low apparent roughness values (average n = 0.013). Time‐related changes in hydraulic roughness appeared to be caused by development of a raininduced crust on exposed soil surfaces that reduced infiltration and increased plot smoothness.

TREE UPROOTING: REVIEW OF TYPES AND PATTERNS OF SOIL DISTURBANCE

This paper reviews the processes of tree uprooting, examines the classification of pit and mound microtopography and assesses the effects of tree uprooting on soil mixing and genesis. The processes by which soil-horizon clasts are mixed as they slump off the root plate, and the ultimate patterns of soil horizonation within mounds, are primary foci of the paper. Longevity of treethrow mounds can exceed 2000 years, making these landforms more lasting features than is often assumed. Because of their great longevity, the pits and mounds formed by uprooting have lasting effects on soil morphology. Soils of these microsites often classify in different soil orders or suborders than do adjacent, less disturbed soils. The importance of tree uprooting to mass movement processes is examined. In some areas uprooting may be the primary mass wasting mechanism. Nonetheless, estimates of the amount of sediment moved and the net distance of transport vary greatly and may in some cases be overestimated. [Key words: pedotur...

Effects of treethrow microtopography on the characteristics and genesis of Spodosols, Michigan, USA

Summary This study examined the pedogenic effects of pit and mound microtopography, formed by tree uprooting, in a Spodosol (Haplorthods) landscape. Most soils in treethrow pits were Entic Haplorthods or Spodic Upidsamments whereas mound soils usually classified as Typic Udipsamments, suggesting that degree of profile development is: Undisturbed ≥ Pit > Mound. Given that pit and mound soils are substantially younger than pedons on “undisturbed” sites, rates of pedogenesis are thought to be: Pit > Undisturbed > Mound. Strong pedogenesis in pits was explained by: 1. greater water contents in the upper sola, which may facilitate weathering processes, 2. thicker O horizons, which may lead to increased production of organic acids, and 3. greater insulation by thick litter and snow cover, which reduces the incidence of soil freezing. In winter, mound soils may develop impermeable layers of concrete soil frost that impede infiltration of snowmelt waters, whereas pit soils remain unfrozen or acquire only a porous, granular frost layer. Thus, saturated flow of snowmelt within pits is relatively unrestricted, resulting in maximal leaching and profile differentiation.