Distribution of Biological Soil Crusts on a Young Glacial Foreland in Southern Iceland and Their Role in Primary Succession

Abstract

This work examines the occurrence of biological soil crusts (BSCs) on glacial foreland moraines and their relationship to other vegetative components of the post-glacial landscape. BSCs on moraines of all ages are biologically complex composites of cyanobacteria, mosses, lichens, liverworts, and fungi. The amount of surface cover by BSCs and other components of the successional communities vary approximately with the ages of the surfaces. During the pioneer successional stage, BSCs are more abundant than other community components and consist primarily of filamentous cyanobacteria. On the youngest moraines, vascular plants, with the exception of graminoids, occur exclusively where BSCs are present. On successively older moraines, the coverage by mosses and vascular plants generally increases while that of BSCs decreases, although substantial variations occur that are attributed to exposure to environmental factors, primarily wind. Overall successional patterns suggest an essential role of BSCs in facilitating vascular plant colonization mainly during the pioneer stage, likely through enhancement of soil moisture and nutrient availability. The importance of facilitation by BSCs appears to decrease on older moraines as BSCs are replaced or subsumed by vascular plants and mosses.

1. Introduction

Biological soil crusts (BSCs or biocrusts) are millimeter to centimeter-thick surface coatings that form initially by the adhesion of soil particles to the organic residues of microbial (cyanobacteria and green algae) communities on surfaces unsuitable for the growth of vascular plants. Typically, the microbial communities of the incipient BSC, which contain organisms capable of fixing atmospheric nitrogen, gradually incorporate macroscopic components, primarily bryophytes (mosses and liverworts) and lichens [1]. Fungi often contribute to BSCs through decomposition of the organic matter of other BSC components, making nutrients available [2]. Consequently, BSCs are essentially a consortium of various organisms that create a living surficial soil layer. These communities are the pioneering colonizers in many Arctic and subarctic habitats where they influence numerous ecosystem processes by stabilizing the soil surface against erosion, enhancing the rate of soil formation, increasing moisture retention, and improving nitrogen (N) and carbon (C) cycling [3,4]. In cold environments where decomposition rates are low, BSCs may be the main source of nutrients for vascular plant colonizers [5]. Conversely, nonvascular plants, such as mosses, are much less dependent on soil nutrients than most vascular plants and so are able to proliferate independently of BSCs.

Much of the current research literature investigating cold-climate BSCs is focused on identifying the microbial components at the molecular or genetic level [6,7,8,9,10,11] and their role in nitrogen and/or carbon cycling during soil development [5,10,12,13]. However, relatively little research considers BSCs in cold climates as landscape components or examines the role of BSCs in primary ecological succession on glacier forelands [4].

It is known that as the development of vascular plant cover on forelands increases with time, these plants store more nitrogen that can be recycled through their death and decomposition, thereby decreasing the importance of BSCs. Potentially, the development of vascular and nonvascular plant cover results in spatial competition between the plants and the organisms comprising BSCs [1,4]. In some glacier foreland settings, as soils develop and succession proceeds, BSCs appear to be replaced by vascular plants [1,2,14]. However, other studies have suggested that as BSCs develop, their thickness and resistance to physical penetration increases, potentially impeding the germination and establishment of vascular plant seedlings [12].

The Skaftafellsjökull, an outlet glacier of the Vatnajökull ice cap in southern Iceland, has retreated about 3 km over the past 135 years as part of the global trend of modern glacial recession resulting from anthropogenic climate change [15,16,17,18,19,20,21]. This recession has exposed the initially non-vegetated deposits of glacial moraines and outwash to colonization by living organisms, of which vascular and nonvascular plants are the most visible. The members of the biotic community on this landscape, the glacial foreland, are subject to various changes over time due to autogenic and allogenic changes in their physical environment and intra-community competition, a suite of processes broadly termed primary ecological succession [22,23,24,25,26,27].

Multiple studies have been conducted on the foreland of the Skaftafellsjökull, examining changes in species diversity [28], soil properties [29,30], and primary ecological succession [31,32]. Of these, Glausen and Tanner [31] suggested that following the pioneer stage, species richness and vegetation coverage both increased on moraines through the mid-successional stages, but species richness decreased on the oldest portions of the landscape, even as total vegetation increased. In a general sense, total vegetative cover on the foreland increases with distance from the current glacial terminus. As noted in ref. [33], the mature successional community is a shrub-heath assemblage dominated by low shrubs, common mosses, and dwarf trees (birch and willow). Importantly, all of the previous studies here have noted the presence of BSCs, but none have attempted to characterize them or examined their relationship with the plants that comprise the vegetative communities at various stages of succession.

This study examines BSCs as important components of the post-glacial landscape and attempts to clarify their role in primary ecological succession on the glacial foreland, i.e., BSCs are either facilitative—aiding the colonization and succession of higher plant communities—or inhibitive—retarding the temporal changes of the vascular and nonvascular plant communities. This objective is undertaken through detailed examination of the distribution and physical characteristics of the BSCs and their relationship with the surrounding plant community on the foreland of the Skaftafellsjókull.

2. Methods

2.1. Study Location

The Skaftafellsjökull, within Vatnajökull National Park, is one of many outlet glaciers of the Vatnajökull ice cap (see inset of Figure 1). Many of these glaciers reached their Little Ice Age maximum extent in the late nineteenth century and receded consistently through the 1930s and into the 1940s, responding to climate warming. A period of glacial readvance coincided with temporary cooling in the 1960s and 1970s, but recession resumed in the 1980s and is continuing [18,19,20,21]. Changes in the positions of the termini of these glaciers have been recorded by a combination of scientists and local residents. The data are collected by the Hydrologic Service of the National Energy Authority (located in Reykjavik). Consequently, the ages of the recessional moraines of these glaciers are known with reasonable accuracy.

The study site is the foreland of the Skaftafellsjökull, in Vatnajökull National Park, which is located at latitude N 64°1.0′ and has a mean elevation of about 100 m above sea level. The southeastern coast of Iceland experiences a cool maritime climate with a mean annual precipitation of approximately 1800 mm [34] and the mean annual temperature is 4 °C to 6 °C, with a winter (January) mean of near 0 °C and a summer (July) mean of 10 °C. The land surface closest to the glacier is subject to glacial winds of variable strength.

The most distal moraine of the Skaftafellsjökull foreland is dated to the position of the ice front in 1890, with the next most distal moraine identified as the ice position in 1904. A very pronounced topography is formed by a set of moraines that date from the position of the glacial front in 1939. The more proximal moraines date to the positions of the glacial front in 1954, 1965, 1980, and 2003 [4,5,6,28]. Consequently, the foreland of the Skaftafellsjökull forms a chronosequence, a land surface on which adjacent areas are different ages, with the distance from the modern ice-front corresponding approximately to the time of exposure of the surface [35]. Notably, between some of the moraines are lower, flatter areas floored by outwash deposits from meltwater streams. Consequently, the age approximations apply to the ice-contact glacial deposits (i.e., the moraines) and not to the glacial outwash deposits whose ages are not well constrained.

2.2. Field Techniques

BSCs were studied by establishing study sites on the moraines located between the current front of the glacier and the outwash plain distal to the position of the glacier at the end of the Little Ice Age. Measurements were made on moraine surfaces estimated to have been exposed by ice retreat in 1890, 1904, 1939, 1945, 1954, 1965, 1980, and 2003 (Figure 1) and location coordinates recorded by a handheld GPS device (model GPSMAP67) manufactured by Garmin Ltd. (Olathe, KS, USA) (

Table 1. Age and GPS coordinates of the measurement sites.

Site Number	Moraine	Station Coordinates
One	2003	(A) N 64°1.300′, W 16°55.814′ (B) N 63°1.298′, W 16°55.762′
Two	1980	(A) N 64°1.253′, W 16°55.836′ (B) N 64°1.262′, W 16°55.848′
Three	1965	(A) N 64°1.251′, W 16°56.225′ (B) N 64°1.244′, W 16°56.202′
Four	1954	(A) N 64°1.218′, W 16°56.341′ (B) N 64°1.219′, W° 16°56.343′
Five	1946	(A) N 64°1.190′, W 16°56.314′ (B) N 64°1.188′, W 16°56.305′
Six	1939	(A) N 64°1.053′, W 16°56.516′ (B) N 64°1.100′, W 16°55.553′
Seven	1904	(A) N 64°1.132′,W 16°56.994′ (B) N 64°1.207′, W 16°57.027′
Eight	1890	(A) N 64°1.017′, W 16°57.175′ (B) N 64°1.134′, W 16°57.143′

). On each moraine, measurement sites were chosen near the crest for optimum age control. Specific site selections were made to ensure a neutral aspect (i.e., approximately horizontal surfaces) for measurements to eliminate bias from the environmental impact of directional winds, such as katabatic winds from the glacier. Nevertheless, differences in elevation and the surrounding topography do lead to differences in wind exposure between sites. These elevation differences are more pronounced near steeper, recent moraines than near topographically subdued older moraines.

Near the crest of each moraine, two plots (designated A and B) with neutral aspect were established, each measuring 100 m² (10 m by 10 m). Within each plot, measurements of the relative percent of surface cover by BSCs, mosses, vascular plants, lichens, and exposed ground (rock and soil) were made within a 0.5 m × 0.5 m quadrat that was rotated spatially to provide 1.0 m² of contiguous coverage. Measurements were made for 1 m² at each corner and in the center of each plot, for 5 m² of measurement per plot, or 10 m² per moraine. BSCs were described by surficial appearance, thickness above soil layer or glacial substrate, color, using Munsell soil color chart codes (by Munsell Color, a division of X-Rite Inc., Grand Rapids, MI, USA), and macroscopic constituents such as cyanobacterial mats, mosses, liverworts, lichens, and fungi. Because mosses are a major constituent of the landscape, but also a common component of the biocrust, the appropriate category for mosses, i.e., as a BSC component or as a vegetation functional group, was evaluated on a case-by-case basis. If the moss consisted of thick mats of mature gametophytes with no underlying visible BSC, it was counted as the moss vegetative functional group. Conversely, a visible BSC with immature moss gametophytes and/or protonema was classified as BSC cover. Similarly, lichens were also recorded as either a BSC component if they appeared to be a component of an underlying BSC or a vegetative community component if growing independently of a BSC. The most prominent groups of vascular plant species at each site were noted but not measured separately, as specific taxonomic groups (e.g., forbs and low shrubs) were not measured separately, although prominent representative species were noted. The collected data were analyzed statistically by linear regression comparisons of the major biotic components of the landscape, i.e., BSCs, mosses, and vascular plants. The significance of the regression was tested by paired two-tailed t-analysis, and the significance of the temporal trends for each component were tested by one-way ANOVA. All statistical tests were performed using the SigmaStat 4.0 software package published by Systat Software Inc. (San Jose, CA, USA).

3. Results

The results of the surveys of the landscape components measured in each 100 m² plot are summarized in

Table 2. Surface coverage mean percent (composite of stations A and B with standard deviation in parentheses) for landscape components for each moraine plus BSC characteristics. Each mean represents 20 m² of surface cover.

Moraine	BSC	Mosses	Vascular Plants	Lichens	Bare Ground
2003	26.4 (21.7)	16.1 (6.5)	1.8 (0.4)	0	56.0 (19.5)
1980	12.0 (5.7)	20.2 (15.1)	2.2 (2.6)	1.0 (2.2)	64.6 (15.8)
1965	5.0 (4.7)	50.0 (33.9)	5.0 (4.7)	0	40.0 (20.4)
1954	3.5 (6.8)	30.0 (23.3)	11.5 (9.3)	5.0 (2.7)	52.5 (33.2)
1946	27.5 (9.9)	40 (21.6)	7.5 (8.5)	3.9 (4.9)	25.0 (12.1)
1939	10.0 (3.1)	40.0 (31.6)	17.5 (16.4)	1.1 (2.1)	30.1 (36.9)
1904	11.8 (9.3)	44.2 (17.1)	14.1 (8.0)	0.8 (1.5)	29.8 (23.2)
1890	17.8 (16.3)	31.5 (23.1)	9.2 (4.3)	2.5 (2.1)	39.3 (14.5)

. The primary physical characteristics (thickness and color) of the BSC at each moraine are summarized in

Table 3. Mean thickness and color of the BSC of each moraine.

Moraine	BSC Color (Munsell Color)	BSC Thickness (cm)
2003	very dark gray (5Y 3/1)	1 to 2
1980	black (5Y 2.5/1)	0.5 to 1.5
1965	dark olive gray (5Y 3/2)	up to 2
1954	very dark gray (5Y 3/1)	1 to 2.5
1946	dark olive gray (5Y 3/2)	up to 4
1939	dark olive gray (5Y 3/2)	up to 2
1904	black (2.5Y 2.5/1)	2 to 3
1890	very dark brown (10YR 2/2)	2 to 3

.

3.1. BSC Features

Within a narrow range of variation, the BSCs are uniformly dark in color (Table 3), suggesting that the surface consists primarily of cyanobacteria, except where there is a veneer of lichens or mosses [36]. No attempt is made here to isolate and identify the specific cyanobacterial taxa. The surficial morphology of the BSCs is variable, ranging from nearly smooth to knobby, to rugose (sensu [37]), to rippled (Figure 2). On coarse gravel surfaces, the BSCs typically surround the rock clasts and extend vertically in the space between adjacent clasts, imparting a “squeeze-up” morphology. Ripple-like forms are elongated sections of uplifted crust surface, with mosses observed growing on the “ripple” crests in some locations and in the troughs between the “ripples” in others. These morphologies are in some cases due to frost heaving, i.e., the vertical displacement of the BSCs by the expansion of water below when freezing [37]. These features are not distributed uniformly by age. Biocrust surfaces that are knobby or rippled occur primarily on younger moraines, while rugose surfaces occur exclusively on older moraines, as described below. Similarly, the thickness of the BSCs display a temporal trend; BSCs on the youngest moraines are generally thinner than those on older moraines.

The biotic composition of the BSCs is also varied. Although the surfaces of most BSCs display the color and characteristics of filamentous cyanobacteria, locally the surface illustrates the incorporation of (most commonly) mosses, which impact the surficial morphology through the formation of ripples or rugose pockets. Various types of lichens, including foliose, squamulose, and less commonly fruticose varieties, occur on biocrust surfaces of all ages but are most prominent on older moraines. Common taxa include the taxa Peltigera and Stereocaulon sp. Liverworts are also a component of the biocrusts (although less abundant than lichens), particularly Anthelia juratzkana, a species that is widespread on moss heaths in Iceland [38]. Fungi do not appear to constitute a significant component of the BSC surfaces, but hyphae were observed within biocrusts below the surface at many locations.

3.2. Plant Communities

Mosses are the most common successional organisms on the glacial forelands and lava fields of Iceland, particularly members of the genus Racomitrium. Mosses on the Skaftafellsjökull foreland are primarily R. lanuginosum, with contributions by R. canescens. The work in ref. [31] examined differences in species richness on the Skaftafellsjökull foreland with a more thorough study of the vascular plant community than found here. For the present study, only the common elements of the plant community are noted. This community comprises low shrubs, commonly including Empetrum nigrum (crowberry), Calluna vulgaris (Scotch heather), Arctostaphylos uvaursi (bearberry), Vaccinium myrtillus (bilberry), Thymus praecox (wild thyme), and Saxifraga oppositofolia (purple saxifrage). Minor components include the dwarf trees Betula pubescens (downy birch), Salix lanata (wooly willow), and Salix phylicifolia (tea-leaved willow); graminoids, including various sedges and fescues; and common forbs such as Dryas octopetala (mountain avens), Bartsia alpina (alpine bartsia), Armeria maritima (thrift), and Pinguicula vulgaris (common butterwort). The mean percentage of cover by the various landscape components for each moraine is presented in Table 2.

3.3. Site Descriptions

• 2003 moraine (Site One)—On the broad ridged platform between the current lagoon and the front of a prominent push moraine dated to 1980. The site is fully exposed to wind from the glacier but shielded from wind from other directions. The surface consists mainly of boulders and gravel, with the BSC and moss subordinate and subequal (Figure 3a,b). The BSC typically fills the spaces between smaller clasts, but larger continuous patches of the BSC occur with areas of up to several square meters. Some of these have a “puffy” surface (Figure 2b) formed by cyanobacteria, which create ridges between adjacent rock clasts. The topography of the BSC surface is unrelated to the underlying rock topography. The BSC is very dark gray (5Y 3/1) with a thickness typically ranging from 0.2 cm to 0.5 cm, with a maximum of 1 cm. The surfaces of most of the BSC are mainly dominated by cyanobacteria, but large areas have veneers of mosses and/or lichens. In particular, where the BSC topography appears rippled, the ridges appear to have more moss than in the intervening flatter areas. In the cross-section, the uppermost layer of the BSC is porous and consists of a mixture of partially decomposed organic material and fine-grained soil particles sometimes containing fungal hyphae. Vascular plants at this site include graminoids, crowberry, wooly willow, wild thyme, and downy birch. The graminoids occur both within BSC patches but also independently. Notably, all other vascular plants are rooted within BSC patches.
• 1980 moraine (Site Two)—Crest of the moraine. This site is at a higher elevation and is more exposed to winds from multiple directions. The surface is dominated by gravel and boulders, with subordinate moss and BSC cover, with mosses slightly more abundant. The BSC is 0.5 to 1.5 cm thick, nearly black (5Y 2.5/1), and pervasively fills spaces between gravel clasts. Based on color, most of the BSC surface is dominated by cyanobacteria, but mosses and lichens make substantial contributions, particularly in depressions on the BSC surface. Vascular plants are sparse at this site. Those present include graminoids, both wooly willow and tea-leaved willow, bilberry, crowberry, and wild thyme. With the exception of graminoids, the vascular plants are limited to patches containing BSC and/or mosses.
• 1965 moraine (Site Three)—Crest of the moraine. The site is elevated and exposed to winds from multiple directions, but winds originating on the ice sheet are largely blocked by the 1980 moraine. The surface of this moraine is finer grained, i.e., it displays fewer large gravel and boulder-size clasts and more patches of sand-sized sediment than younger moraine sites. Surface cover by moss is slightly higher than for bare ground and BSC cover is minor at this site. The BSC is very dark olive gray (5Y 3/2) and up to 2 cm thick. The crust surface is irregular, forming knobby “squeeze-ups” between adjacent rock clasts. The BSC surface mainly appears dominated by cyanobacteria, but mosses and lichens are prominent locally. Vascular plants are more abundant at this site than at younger moraines but are still subordinate to moss and bare ground. Vascular plants include wooly willow, tea-leaf willow, birch, bilberry, and crowberry. The crowberry, birch, and willows mainly occur in discrete patches in close association with moss or the BSC.
• 1954 moraine (Site Four)—Flat distal to the 1965 crest, approximated as the 1954 ice front position. This site is exposed to winds from multiple directions. The surface is covered mainly by boulders and gravel, with mosses and vascular plants subordinate; the BSC is a minor surface component at this site. The BSC is very dark gray (5Y 3/1) and the thickness ranges from 1 cm to 2.5 cm. The BSC mainly appears as fill between gravel clasts rather than as continuous patches, and the surface is mainly dominated by lichens and mosses. Vascular plants are a significant component of the surface cover at this site and include patches of crowberry, wooly and tea-leaf willow, birch, heather, and bilberry. Cover by lichens is approximately equal to that by BSC.
• 1946 moraine (Site Five)—Base of the moraine approximated as the 1946 ice front position, upslope from a prominent kettle pond. The site is sheltered by the ridge at Site Four. The surface cover components are distributed irregularly, with pronounced m-scale variations in the landscape elements. Mosses are most abundant, with subordinate BSC and bare ground. Much of the site consists of raised mounds of mosses and BSC, in which the mosses consist of a thin veneer over a BSC base (Figure 4a). Many of the mounds appear eroded on the upslope side, exposing the underlying BSC (Figure 4b). The BSC is dark olive gray (5Y 3/2), up to 4 cm thick, and locally displays a prominent knobby surface morphology as well as large patches of rippled BSC with moss mantling the crests of the ripples. The vascular plant community is at its maximum diversity at this location and includes patches of wooly and tea-leaved willow, downy birch, bilberry, crowberry, and diverse forbs, including Alpine bartsia, common butterwort, and thrift.
  1939 moraine (Site Six)—The site is on the flat at the crest of the 1939 moraine. This is a recessional moraine with more subdued topography than younger moraines. The site is fully exposed to winds from various directions. The distribution of the main landscape elements is distinctly uneven with m-scale patches dominated entirely by either moss or rock, which are sub-equal components (Figure 5); BSC is subordinate. The BSC occurs primarily as fill between gravel clasts consisting of knobby cyanobacterial accumulations or rugose surfaces mantled by mosses and/or lichens. It is dark olive gray 5Y 3/2) and averages 2 cm in thickness. Vascular plants are a minor component consisting of a diverse assemblage, including downy birch, wooly and tea-leaved willow, crowberry, bilberry, mountain avens, heather, Alpine bartsia, and thrift.
  1904 moraine (Site Seven)—Crest of a boulder moraine identified as the position of the ice front in 1904. The location is fully exposed to the wind from multiple directions, but the distance to the ice front (currently over 2 km) reduces the strength of katabatic winds. Surface cover is dominated by subequal proportions of moss and rock, with subordinate contributions of the BSC and vascular plants. The BSC is mainly limited to small (<1 m²) patches where it fills the spaces between gravel clasts. The BSC surface is largely covered by a veneer of mosses, lichens, and liverworts, locally rugose (Figure 6a), with relatively little surface exposure of cyanobacterial-dominated crust surface. The thickness ranges from 2 to 3 cm and varies in color from nearly black to very dark brown (2.5Y 2.5/1 to 10YR 2/2). The vascular plant community is a mature successional assemblage consisting mainly of crowberry, bilberry, bearberry, heather, and sparse birch.
• 1890 moraine (Site Eight)—Crest of the most distal moraine dated to the ice front position in 1890. The landscape elements of this moraine are similar to those of the 1904 moraine (Site Seven). Surface cover is dominated by subequal proportions of moss and rock, with a major contribution of the BSC and lesser coverage by vascular plants and lichens. The BSC surface is dominated variously by mosses and various foliose and fruticose lichens (Figure 6b). The BSC ranges in thickness from 2 to 3 cm and the color is brown to dark grayish brown (10YR 4/2 to 10YR 4/3). The vascular plant community here is similar to that at Site 7.

3.4. Temporal Trends

The overall temporal trend for the biotic landscape elements is one of increasing coverage by mosses and vascular plants with time (i.e., on older moraines) and decreasing to near-constant cover by BSCs, although these trends are not strongly linear (Figure 7). Despite the constant aspect in the selection of measurement sites, there are differences in exposure to winds due to elevation and the surrounding topography. Previous studies of the Skaftafellsjökull foreland have identified distinct differences in general vegetative cover between sheltered and exposed locations [31,33]. This study finds the highest BSC coverage at the 2003 and 1946 moraines (Sites One and Five), which are topographically the lowest and most sheltered sites in the study.

Both mosses and vascular plant cover display a general trend of increasing coverage with moraine age in contrast to the trend of the BSCs (Figure 7). The significance of the temporal trends of the individual landscape components was tested by one-way ANOVA. This test finds that the variations in the populations of BSCs, mosses, and vascular plants, from the youngest to oldest moraines, are statistically significant (

Table 4. Statistical analyses (linear regression and one-way ANOVA) of the means for the major biotic components, BSCs, mosses, and vascular plants.

Regression	Equation		R2 Value	Two-Tailed p-Value
Moss v. BSC	y = −0.4793x + 40.83		R2 = 0.1347	0.014
Moss v. Vasc	y = 1.1031x + 24.513		R2 = 0.2769	0.0002
Vasc v. BSC	y = −0.2294x + 11.869		R2 = 0.1356	0.232
One-way ANOVA	f-value	p-value
Biocrust	6.118	0.001
Moss	2.6	0.019
Vascular	4.87	0.001

). Linear regressions of the data demonstrate that BSC coverage correlates negatively with both moss and vascular plant coverage (Figure 8). Paired t-test analyses establish that the moss–BSC negative correlation is statistically significant, while the vascular plant–BSC negative correlation is not (Table 2). Moss and vascular plant abundances correlate positively across landscape, and the correlation is statistically significant (Table 4).

4. Discussion

4.1. Site One BSC

Perhaps most notable among the results is the finding that the surface coverage by the BSC at Site One (the youngest moraine) is elevated compared to most of the older moraines. The BSC on this moraine is more abundant than mosses at a statistically significant level (Figure 7a,b), whereas mosses are more abundant than BSCs on all other moraines. The position of this site is most proximal to the proglacial lake of all sites in the study, and so may receive moisture from winds originating above the ice sheet. The extra moisture could stimulate the initial colonization and growth of cyanobacterial and algal organisms that form the BSC [7,39].

Interestingly, ref. [30] conducted a survey of vegetation in 2011 at sampling sites on the 2003, 1946, and 1890 moraines by measurement along transect lines oriented parallel to the moraine crests. While their measurements of mosses and vascular plant abundances are similar to those provided herein for the same moraines (within the limits of statistical significance), their measurements of BSC abundance are less, particularly on the 2003 moraine where the difference is more than an order of magnitude smaller. The passage of time (14 years) between the original measurements and those reported here might explain some part of the difference in BSC abundance simply by growth over that time. Utilizing Google Earth^® imagery from the years 2012 and 2024 allows measurement of approximately 800 m of recession of the Skaftafellsjökull ice front between the respective periods of study. The decrease in the intensity of katabatic winds as the glacier retreats, combined with growth of the proglacial lagoon, has made the landscape closest to the ice front more hospitable for biotic colonization and enhanced soil moisture.

4.2. Changes with Time

The decrease in BSCs with respect to both mosses and vascular plants may be largely a function of the overgrowth of BSCs by bryophytes and vascular plants on older moraines through primary succession. For example, at Site 5 (the 1946 moraine), erosion of mosses, possibly by water runoff from the nearby 1954 moraine, exposes the BSC that underlies the moss cover (Figure 4). Thus, a key element of primary succession on this foreland seems to be the growth of bryophytes and vascular plants at the expense of preexisting BSCs. The start of this process is observed at Site One, where vascular plants occur almost exclusively in association with the BSC. This interpretation is consistent with earlier suggestions that BSCs facilitate succession by enhancing soil moisture and providing bio-available N and C through the actions of N-fixing species and recycling of organic tissues [1,2,6,7,14,40]. However, in later successional stages on the oldest moraines, mosses continue to dominate the vegetative cover while vascular plant cover decreases. Ref. [33] suggested that the spread of mosses restricted available seeding sites for vascular plants, limiting their spread.

5. Conclusions

Colonization of the glacial foreland by BSCs occurs rapidly, within decades of exposure by glacial retreat. Retreat of the glacier over a multidecadal time scale enhances BSC formation through decreasing intensity of katabatic winds and possibly enhanced moisture as a proglacial lagoon forms. The youngest BSCs are dominantly simple cyanobacterial accumulations, but they also show initial stages of incorporation of mosses, lichens, and liverworts. Vascular plants on the youngest moraine occur almost exclusively in association with BSCs, suggesting that BSCs facilitate vascular plant colonization on an otherwise inhospitable substrate through enhanced soil moisture and nutrient availability. Coverage of the landscape by mosses and vascular plants concomitant with a decrease in BSC coverage on older moraines demonstrates that the progression of primary succession on the foreland involves the overgrowth of preexisting BSCs by the spread of mosses and vascular plants.