Development Of Biological Soil Crusts Research Articles

Carbon fixation by biological soil crusts following revegetation of sand dunes in arid desert regions of China: A four-year field study

Abstract Biological soil crusts (BSCs) are important sources of carbon input to ecosystems in arid and semiarid regions, where vascular plants are restricted by the rigorous environment and limited soil water. Sand dune stabilization by revegetation can enhance colonization and development of BSCs on sandy soil surfaces, and convert BSCs from the early successional stage dominated by cyanobacteria and algae to the later stage dominated by lichens and mosses. This study estimated the carbon fixation by two successional stages of BSCs based on four years of field observations. Carbon fixation by two BSCs has been compared via estimating daily carbon fixation using measuring net photosynthesis with ten sampling plots per crust stage in situ, combining with calculating the wet daytime of crustal cryptogam thallus. However, diurnal carbon fixation of both BSCs was largely determined by crustal water content rather than photosynthesis photon flux and temperature. The range of optimal gravimetric water content for early BSCs was 1–3.5%, and 1–5% for the later BSCs. The annual carbon fixation was 11.36 g C m − 2 yr − 1 for cyanobacteria–algae dominated crusts and 26.75 g C m − 2 yr − 1 for lichen–moss dominated crusts. The latter had a higher carbon input due to a higher water-holding capacity, prolonging wet daytime and higher chlorophyll content, as well as higher light capture. These findings indicate the recovery of BSCs is expected to significantly increase carbon input into sandy desert ecosystems.

Biological soil crust development affects physicochemical characteristics of soil surface in semiarid ecosystems

Water and nutrients are scarce resources in arid and semiarid ecosystems. In these regions, biological soil crusts (BSCs) occupy a large part of the soil surface in the open spaces surrounding patches of vegetation. BSCs affect physicochemical soil properties, such as aggregate stability, water retention, organic carbon (OC) and nitrogen (N) content, associated with primary ecosystem processes like water availability and soil fertility. However, the way BSCs modify soil surface and subsurface properties greatly depends on the type of BSC. We hypothesised that physicochemical properties of soil crusts and of their underlying soils would improve with crust development stage. Physicochemical properties of various types of soil crusts (physical crusts and several BSC development stages) and of the underlying soil (soil layers 0–1 cm and 1–5 cm underneath the crusts) in two semiarid areas in SE Spain were analysed. The properties that differed significantly depending on crust development stage were aggregate stability, water content (WC) (at −33 kPa and −1500 kPa), OC and N content. Aggregate stability was higher under well-developed BSCs (cyanobacterial, lichen and moss crusts) than under physical crusts or incipient BSCs. WC, OC and N content significantly increased in the crust and its underlying soil with crust development, especially in the first centimetre of soil underneath the crust. Our results highlight the significant role of BSCs in water availability, soil stability and soil fertility in semiarid areas.

Chemical composition of volatile oil from Artemisia ordosica and its allelopathic effects on desert soil microalgae, Palmellococcus miniatus

Plants have been used to restore vegetation in desert region in Shapotou, where naturally biological soil crusts (BSCs) have formed after planting for several years. However, few works have been done on the allelopathic effects between the plants and soil microalgae in BSCs currently. In this study, we investigated the chemical compositions of volatile oil of Artemisia ordosica and its allelopathic effects on photosynthetic system II (PSII) and antioxidant system of Palmellococcus miniatus, a green algae isolated from BSCs. 37 components, consisted of 17 terpenoids, 14 alcohols, 2 esters, 2 ketones and other 2 components were identified in the volatile oil from A. ordosica by GC-MS analysis. High concentration of volatile oil could significantly inhibit the growth and photosynthetic activity (Fv/Fm), and decreased the photosynthetic parameters by affecting photon absorption, electron transport and the reaction center of PSII of P. miniatus, and also cause the significant increase of superoxide dismutase (SOD; EC 1.15.1.) activity, peroxidase (POD; EC 1.11.1.7) activity, reactive oxygen evolution (ROS) and malondialdehyde (MDA) contents of P. miniatus through the combined effects of components in volatile oil. The results indicated that the emission of volatile oil of A. ordosica could inhibit the growth, photosynthesis of P. miniatus through the oxidative damage, and thus might negatively affect the development of BSCs.

Characteristics of surface runoff in a sandy area in southern Mu Us sandy land

To better understand the key role of biological soil crusts (BSCs) in redistribution processes of limited water resources in semi-arid sandy ecosystems, surface runoff over BSCs in a sandy area in the southern Mu Us sandy area was observed in this study. The results indicated that runoff occurred twice among six rainfall events during the experimental period. Runoff yields varied among types of BSCs, and that they increased in the following order: light algae crusts, dark algae crusts and moss crusts. These findings showed that runoff yields were increasing with the development of BSCs. The percentage of runoff in individual rainfall events increased from light algae crusts to moss crusts, which indicated that the redistribution ratios of different types of BSCs differed. Surface runoff of BSCs may be influenced by rainfall, rain intensity, degree of water saturation of the BSCs before the rain and degree of development of BSCs. Formation of BSCs on the surface of sand dunes changed the spatial distribution pattern of water in this semi-arid sandy ecosystem, which increased the heterogeneity of resources such as water and nutrients. Therefore, making appropriate disturbances of BSCs during storm season is beneficial to maintaining the balance of natural resources in the semi-arid sandy ecosystem.

Composition of photosynthetic organisms and diurnal changes of photosynthetic efficiency in algae and moss crusts

In Dalateqi region of Inner Mongolia (at eastern edge of Qubqi Desert), man-made algae crusts were constructed with two filamentous cyanobacteria. Two to three years later, the algae crusts began to succeed to moss crusts under some microterrain conditions. In order to characterize the community structure and compare the photosynthetic characteristics in different successional stages of biological soil crusts (BSCs). This paper studied composition of photosynthetic organisms and diurnal changes of photosynthetic efficiency, with microscopic observation and chlorophyll fluorescence monitoring techniques. Compared with the early successional algae crusts, later moss crusts had a higher photosynthetic efficiency, which was closely related with the different community structure of photosynthetic organisms. Further, moss crusts had a higher total photosynthetic biomass, although the algal biomass decreased along the successional sequence. Similar to higher vascular plants, a midday depression of photosynthetic efficiency was detected in BSCs, although the depression extents were different in different successional stages. Analysis demonstrated the depression was mainly caused by non-stomata limitations, and photosynthetically active radiation (PAR) was a significant influencing factor in the down-regulation of photosynthetic efficiency. We conclude, cyanobacterial inoculation effectively promotes the formation, development and seccession of BSCs. Furthermore, the later successional BSCs have a better photosynthetic performance and thus are expected to play a more effective role in desertification control.

Effects of biological soil crusts on soil nematode communities following dune stabilization in the Tengger Desert, Northern China

Biological soil crusts (BSCs) play an important role in the dune fixation and maintaining soil biota in arid desert systems. Free-living soil nematode communities could be used as significant bioindicators to reflect soil recover regime after sand burial. However, the relationship between BSCs and nematodes is rarely known. To examine the effects of BSCs on soil nematodes, 72 soil samples under cyanobacteria–lichen and moss crusts were collected to analyse nematode communities in the different aged vegetated areas at the southeastern edge of the Tengger Desert. Our results showed the colonization and development of BSCs significantly enhanced nematode diversity. Nematode abundances, generic richness, H′, MI, EI and SI were greater under crusts than those under noncrust. In particular, nematode abundances, generic richness, H′, MI, EI and SI were positively correlated with crust ages. The differences in nematode communities were also dependent upon crust types. Nematode abundances and generic richness under moss crusts were higher than those under cyanobacteria–lichen crusts. This can be contributed to the present and succession of BSCs that increased thickness of topsoil after dunes have been stabilized, namely, creating suitable habitats and providing an essential food source for nematodes.

CARBON FIXATION OF CYANOBACTERIAL–ALGAL CRUSTS AFTER DESERT FIXATION AND ITS IMPLICATION TO SOIL ORGANIC CARBON ACCUMULATION IN DESERT

ABSTRACTEnhanced carbon fixation in soil crusts may facilitate the restoration of damaged ecosystems, but this requires greater knowledge of carbon fixation patterns and mechanisms. We measured the net photosynthetic rate (Pn) and estimated annual carbon fixation (ACF) in cyanobacterial–algal crusts after desert fixation in the Tengger Desert, northwestern China. The accumulated carbon fixation since the establishment of a restoration site was also calculated. In addition, stepwise regression analysis was used to study the relation between Pn and ACF and the physicochemical properties of crusts. Results showed that Pn was significantly higher at a more established 51‐year‐old restoration site (1·57 µmol m−2 s−1) than at a younger 15‐year‐old site (0·92 µmol m−2 s−1). The ACF also increased significantly with restoration time, but in two temporal phases, a slower ACF phase between 15 and 26 years of restoration (0·28–0·7 gC m−2 y−1) and a high ACF phase after 43–51 years of restoration (3·3 gC m−2 y−1). Stepwise regression analysis revealed that Pn was significantly correlated with chlorophyll a and crust cover, whereas ACF was only correlated with crust cover. Accumulated carbon fixation increased from 2·9 gC m−2 after 15 years to 35·4 gC m−2 at 51 years following establishment of the restoration site. The accumulated carbon fixation was positively correlated with soil organic carbon content. This study demonstrated that carbon fixation by cyanobacterial–algal crusts increased progressively after desert fixation. Artificial measures, like the establishment of these restoration zones, can facilitate the colonization and development of biological soil crusts and are an effective biological tool for desert soil restoration. Copyright © 2011 John Wiley & Sons, Ltd.

Successional stages of biological soil crusts and their microstructure variability in Shapotou region (China)

In order to investigate succession of biological soil crusts (BSCs) and their microstructure variability, we conducted this work in Shapotou revegetation region at the southeast edge of Tengger Deser. The results showed that BSCs generally succeeded as a pathway of “Algae crusts, algae–lichen crusts, lichen crusts, lichen–moss crusts and moss crusts”. Occasionally mosses directly occurred on algae crusts, and BSCs succeeded from algae crusts to moss crusts. Crust vertical stratification was a common phenomenon, from top to bottom an inorganic layer, algae-dense layer and algae-sparse layer were divided in algae crusts; a thallus layer, rhizoid layer and sub-rhizoid layer in lichen crusts; a “stem-leaf” layer, rhizoid layer and sub-rhizoid layer in moss crusts, respectively. The main crust binding organisms varied from filamental cyanobacteria (dominated by Microcoleus) in algae crusts to lichen rhizoids, free-living cyanobacterial filaments and fungal hyphaes in lichen crusts, and to moss rhizoids and fungal hyphaes in moss crusts. The dominant phototrophic organisms varied from Microcoleus (algae) in algae crusts to Collema (lichens) in lichen crusts, and to Bryum (or Didymodon and Tortula; mosses) in moss crusts. Total phototrophic biomass increased while the free-living algal biomass decreased with the succession of BSCs. In addition, exopolysaccharides and fine particles accumulated in the course of development and succession of BSCs, all of which lead to a gradual increase in crust thickness and porosity, while decrease in the bulk density.

Biological topsoil crusts at early successional stages on Quaternary substrates dumped by mining in Brandenburg, NE Germany

The influence of biological soil crusts in natural ecosystems on structures and processes is well investigated. However, in South-Brandenburg (Germany), it is possible to study the development of biological soil crusts (BSC) during initial ecosystem genesis on two artificial water catchments with well-known ages and under differing starting conditions. The two experimental sites are located in the recultivation area of the lignite open-cast mining district of southern Brandenburg with a distance of approximately 1 km between them. Two different topographies were constructed at the experimental sites: the experimental plot at the catchment Neuer Lugteich was shaped like a dune, whereas the artificial water catchment Hühnerwasser was modelled as an inclined slope. The catchment Neuer Lugteich is four years older than Hühnerwasser. The original substrate at Neuer Lugteich is more sandy and carbonate-free compared to the original substrate dumped at Hühnerwasser. At both sites geomorphological differentiation and crust development were compared and the importance of substrate-dependent water availability and crust type clarified. Once settled, the crusts influenced the water regime of the soils by delaying infiltration through enhanced water repellency, and by limiting water infiltration. Chlorophyll analysis revealed that all crusts were at early stages of development. At Neuer Lugteich, the establishment of the biological soil crusts was closely associated with the vegetation succession, whereas no clear succession of the crusts could be observed at Hühnerwasser. The mosaic-like pattern of the biological soils crusts is associated with the distribution of fine-grained material here.

Association of ant nests with successional stages of biological soil crusts in the Tengger Desert, Northern China

Biological soil crusts are important cover in arid desert landscapes, yet their importance as habitats to secondary producers such as ants is relatively unknown. This study was conducted to determine if the presence and development of biological soil crusts on dune surfaces stabilized by revegetation facilitates ant establishment and survival. We measured topsoil properties and crustal features during different successional stages, which were characterized by cyanobacteria and algae, lichens, and mosses, respectively. The species richness and abundance (nest density) of ants were closely associated with silt content, soil organic matter, nitrogen and soil moisture, as well as topsoil temperature. However, ant nest distribution was largely dependent on the biomass and thickness of crusts and topsoil. These findings provide evidence that the recovery or development of biological soil crusts on dune surfaces in the Tengger Desert could favor and maintain higher ant species diversity. Therefore, the disturbance of crusts would result in a reduction of ant species richness and abundance in desert systems.

The effects of plantation development on biological soil crust and topsoil properties in a desert in northern China

Afforestation was one of most successful measures to rehabilitate desertified lands and to improve regional eco-environments in the Horqin Sand Land. A field experiment was conducted during 2005 and 2006 in the Horqin Sand Land to investigate the effects of Populus simonii plantation growth on soil crust formation and topsoil properties. The results showed that plantation establishment promoted the development of biological soil crust and improved properties of topsoil 0–5 cm under the crust in sand dunes. As the plantation aged, soil crust developed gradually from a physical crust, to a lichen crust and then to a moss crust and crust coverage, thickness, hardness, very fine sand content, clay, silt, organic carbon and nutrients increased significantly. Clay, silt, organic carbon and nutrients in topsoil 0–2.5 cm and 2.5–5.0 cm under the soil crust increased with plantation age and crust development, but the magnitude of changes decreased with increasing soil depth. Changes in soil physical and chemical properties in the crust had a significant positive correlation with plantation age, and changes in physical and chemical properties in the 0–5.0 cm layer of topsoil had a significant positive correlation with soil crust development and plantation age. Soil crust development and changes in topsoil properties under the soil crust can be attributed primarily to the reestablishment of vegetation and the duration that the vegetation is in place.

Spatial gradients in ecohydrologic properties within a pinyon‐juniper ecosystem

AbstractThe influence of woody vegetation and biological soil crusts on infiltration capacity is one of the several uncertainties associated with the ecohydrologic effects of woody plant encroachment into arid and semi‐arid land systems. The objective of this study was to quantify the effects of Utah juniper (Juniperus osteosperma) and pinyon pine (Pinus edulis) on subcanopy and intercanopy ecohydrologic properties. We measured soil sorptivity, unsaturated hydraulic conductivity [K(h)], soil water content (SWC), and water repellency along radial line transects from under Juniperus and Pinus trees into the centre of the intercanopy space between trees. In the subcanopy, litter mounds, hydrophobic soils, and roots all appear to contribute to preferential flow to below‐surface soils via wetted patches. For both Juniperus and Pinus, K(h) was significantly lower in the subcanopy than the intercanopy average; however, line transect measurements did not show distinct boundaries in K(h) between the two regions. K(h) increased by eight‐fold across a gradient extending outward from near the edge of the canopy to approximately 2 times the canopy radius (CR). This suggests that the influence of these species on hydrologic properties extends significantly into the intercanopy region. Analysis of biological soil crust cover within the intercanopy showed that beyond the gradient zone, increasing structural development of biological soil crust was associated with increased K(h). Furthermore, these results indicate that the distance from the canopy and direction should be considered in the assessment and modelling of woody plant and biological soil crust influence on infiltration capacity. Copyright © 2008 John Wiley & Sons, Ltd.

Southern African Biological Soil Crusts are Ubiquitous and Highly Diverse in Drylands, Being Restricted by Rainfall Frequency

Biological soil crusts (BSCs) are found in all dryland regions of the world, including the polar regions. They are also known to occur in the southern African region. Although there were a number of case studies on BSCs from that region, we did not know if they are a normal part of the vegetation cover or just a phenomenon that occasionally occurs here and there. In order to investigate diversity, distribution patterns, and the driving factors of both, we followed a random sampling system of observatories along a transect, stretching from the Namibian-Angolan border down south to the Cape Peninsula, covering seven different major biomes. Biological soil crusts were found to occur in six out of seven biomes. Despite the fact that soil-dwelling algae occurred in the Fynbos biome, crust formation was not observed for hitherto unknown reasons. Seven BSC types were distinguished on the basis of morphology and taxonomic composition: three of them were cyanobacteria-dominated, one with additional chlorolichens, two with bryophytes, one hypolithic type restricted to quartz gravel pavements, and the unique lichen fields of the Namib Desert. Besides 29 green algal species in 21 genera, one heterokont alga, 12 cyanolichens, 14 chlorolichens, two genera of liverworts, and three genera of mosses, these crusts are positioned among the most diverse BSCs worldwide mainly because of the unusual high cyanobacterial species richness comprising 58 species in 21 genera. They contribute considerably to the biodiversity of arid and semi-arid bioregions. Taxonomic diversity of cyanobacteria was significantly higher in the winter rain zone than in the summer rain zone (54 versus 32 species). The soil photosynthetic biomass (chlorophylla/m2), the carbon content of the soil and the number of BSC types were significantly higher in the winter rain zone (U27, 29=215.0, p=0.004 [chla]; U21, 21=135.0, p=0.031 [C]; U27, 29=261.5, p=0.028 [BSC types]; excluding the fog-dominated Namib biome). The winter rain zone is characterized by a lower precipitation amount, but a higher rain frequency with the number of rainy days more evenly distributed over the year. The dry period is significantly shorter per year in the winter rain zone (U8, 9=5.0, p=0.003). We conclude that rain frequency and duration of dry periods rather than the precipitation amount is the main factor for BSC growth and succession. Nitrogen content of the soils along the transect was generally very low and correlated with soil carbon content. There was a weak trend that an increasing proportion of silt and clay (<0.63 mm) in the soil is associated with higher values of BSC chlorophyll content (Pearson correlation coefficient=0.314, p=0.237). Furthermore, we found a significant positive correlation between silt and clay and the number of BSC types (Pearson correlation coefficient=0.519, p=0.039), suggesting that fine grain-size promotes BSC succession and their biomass content. Lichens and bryophytes occurred in BSCs with lower disturbance frequencies (e.g. trampling) only. Crust thickness and chlorophyll content increased significantly from crusts of the early successional type to the late successional crust types. From our results, we conclude that BSCs are a normal and frequent element of the vegetation in arid and semi-arid southwestern Africa, and that rain frequency and duration of dry periods rather than the precipitation amount are the key factors for the development, differentiation and composition of BSCs.

Revisiting classic water erosion models in drylands: The strong impact of biological soil crusts

Soil erosion and subsequent degradation has been a contributor to societal collapse in the past and is one of the major expressions of desertification in arid regions. The revised universal soil loss equation (RUSLE) models soil lost to water erosion as a function of climate erosivity (the degree to which rainfall can result in erosion), topography, soil erodibility, and land use/management. The soil erodibility factor ( K) is primarily based upon inherent soil properties (those which change slowly or not at all) such as soil texture and organic matter content, while the cover/management factor ( C) is based on several parameters including biological soil crust (BSC) cover. We examined the effect of two more precise indicators of BSC development, chlorophyll a and exopolysaccharides (EPS), upon soil stability, which is closely inversely related to soil loss in an erosion event. To examine the relative influence of these elements of the C factor to the K factor, we conducted our investigation across eight strongly differing soils in the 0.8 million ha Grand Staircase-Escalante National Monument. We found that within every soil group, chlorophyll a was a moderate to excellent predictor of soil stability ( R 2 = 0.21–0.75), and consistently better than EPS. Using a simple structural equation model, we explained over half of the variance in soil stability and determined that the direct effect of chlorophyll a was 3× more important than soil group in determining soil stability. Our results suggest that, holding the intensity of erosive forces constant, the acceleration or reduction of soil erosion in arid landscapes will primarily be an outcome of management practices. This is because the factor which is most influential to soil erosion, BSC development, is also among the most manageable, implying that water erosion in drylands has a solution.

The Influence of Biological Soil Crusts on Soil Characteristics along a High Arctic Glacier Foreland, Nunavut, Canada

ABSTRACTThis study explores the physical, chemical and microclimatological properties of soils along a High Arctic glacier foreland and adjacent moraine in relation to the development of biological soil crusts. We examine various edaphic properties: soil temperature, volumetric water content, organic carbon content, and texture in surface samples (∼1 cm) with and without a cover of biological soil crust as well as changes in nitrogen, phosphorus, potassium, organic carbon, pH, volumetric water content, bulk density, and texture in crusted surfaces (<1 cm) and soil cores (5 cm) along a chronosequence following deglaciation. Soil crusts developed within four years of deglaciation and subsequent peaks in crust cover and thickness coincided with an accumulation of nitrogen and organic carbon in the crust. Crusted surfaces had significantly higher volumetric water content, organic carbon, a greater silt and clay fraction, and lower temperature compared to uncrusted soils. A steady supply of water from glacier melt promoted rapid development of biological soil crusts, creating an edaphic environment with enhanced moisture and nutrient properties which contributed to the high rate of vascular plant succession previously observed on this foreland. Results presented in this study are compared with edaphic conditions at other circumpolar sites and glacier forelands.

Biological soil crust development and its topsoil properties in the process of dune stabilization, Inner Mongolia, China

This study was undertaken at Horqin Sand Land, Inner Mongolia, Northern China. Field samples of biological soil crusts (BSCs) and underlying topsoil (0–5 cm under BSC) were taken in areas of different dune stabilization stages, and their physicochemical properties were analyzed, including particle size distribution, bulk density, organic matter, nitrogen, phosphorus, electrical conductivity (EC), pH, and CaCO3 content. The results revealed that semi-mobile dunes, semi-fixed dunes and fixed dunes had developed a physical crust, algae crust and moss crust, respectively. The thickness, hardness, water content, fine fraction and nutrient contents of BSCs were gradually increasing along the dune stabilization gradient. Meanwhile, BSC establishment and development enhanced the bulk density, silt and clay content, and nutrients of the topsoil under it, in an increasing tend from semi-mobile dune to fixed dune. Organic matter concentrations and other nutrients in the 0–5 cm topsoil layer under BSCs were significantly higher compared to unconsolidated soil (control). Moreover, there were strong significant positive correlations between topsoil and BSCs’ organic matter, total nitrogen, available nitrogen, available phosphorus, CaCO3, and <0.05 mm particle content, suggesting that BSCs have an influence on some of the properties of the underlying topsoil.

Composition and function of biological soil crust communities along topographic gradients in grasslands of central interior British Columbia (Chilcotin) and southwestern Yukon (Kluane)

Grasslands in the rainshadow of the Chilcotin (British Columbia) and St. Elias (Kluane, Yukon) mountain ranges of western Canada are characterized by widely spaced clumps of bunchgrass and sage, between which can be found lichen-dominated biological soil crusts (BSC). Our examination of Chilcotin and Kluane grasslands showed differential BSC development along topographic gradients, favoring those sites with lower levels of soil disturbance. Lichen species richness was greatest in upper topographic positions, that is, on valley side terraces in the Chilcotin and esker slopes in Kluane. Common BSC lichens in both grasslands included Acarospora schleicheri, Caloplaca tominii , Collema tenax , Diploschistes muscorum , Fulgensia bracteata , Phaeorrhiza nimbosa , Placidium squamulosum , and Psora decipiens . Chilcotin BSC additionally contained many Cladonia species (e.g., C. carneola , C. chlorophaea , C. pyxidata ), particularly where vegetation had encroached on BSC. The potential for nitrogen fixation by Collema -dominated crusts in Kluane was examined using acetylene reduction assays (ARA) and soil surface microclimate monitoring. ARA activity was highly dependent upon the duration of wetting events, reaching C2H4 levels up to 63 μmol·m–2·h–1after 40 h of hydration. Given the abundance of Collema-dominated crusts in Kluane and the optimal conditions for ARA activity that are reached during wetting–drying transitions, we hypothesized that BSC communities potentially make an important contribution to ecosystem nitrogen budgets. Enrichment in total and mineralizable N, as well as 15N natural abundance values, was consistent with N fixation making an important contribution to soil N pools in these ecosystems. Both Chilcotin and Kluane BSC had similar spongy microstructures that contrasted with the platy microstructures of the underlying surface mineral soils, but only the latter site showed micromorphological evidence of burial of mosses and other BSC components by continuing loess deposition. BSC may have performed similar roles in analogous steppe-like ecosystems that existed under full-glacial conditions in the unglaciated areas of eastern Beringia in Alaska and Yukon.