Plant Succession in Glacier Forelands Controlled by Site Age Alone?—A Case Study at Schwarzenbergferner, European Alps

1. Introduction

Succession, one of the most fundamental concepts in plant community ecology [1,2], describes the development of a plant community over time. Classical ecosystem studied include sand dunes [3], old fields [4,5], volcanic ash [6], and glacial till [7,8]. A distinction is made between primary and secondary succession [9], the former occurring on newly created ground without any diaspores, the latter on disturbed ground, where pre-existing plant communities or at least seedbanks to initiate succession exist. A common approach for investigating plant succession is the employment of so-called chronosequences. It is a "space-for-time substitution" in the sense of Pickett [10], deriving a temporal sequence by spatially different sites of various site age. Despite some shortcomings [11], chronosequences are commonly employed to investigate vegetation developments, that exceeds the lifespan of an investigator. According to Walker et al. [12] succession can be successfully and sufficiently accurate explored, if plant communities are following convergent successional trajectories and chronosequences are interpreted carefully.

Key locations for the study of primary succession are in front of a retreating glacier [13]. The renowned ecologist Ellenberg [14] (p. 440) aptly states: “Nowhere can succession be studied more profitably than in the valley below the front of a large glacier“. Since the end of Little Ice Age (LIA) around 1850 the glaciers of the Alps are retreating, accelerated in recent decades. During the last one and a half to two centuries alpine glaciers lost more than 50% of their LIA area [15,16,17,18,19], continuously enlarging the “glacier forelands”, a term introduced by Kinzl [20] for the ground between the recent glacier termini and the terminal moraines left by the LIA glaciers. Studies on vegetation development in glacial forelands of the Alps have a long tradition. The earliest accounts date back to the 19^th and early 20^th century [e.g. 21-31]. Glacier foreland studies remain highly relevant today in light of accelerated climate warming and extreme changes in the ice balance [16,18,19].

In glacier forelands of the European Alps, chronosequence studies are widespread [e.g. 32-41]. The reconstruction of site age (i.e., time since deglaciation) relies on datable traces of the former glacier extent (moraines) as well as other length change records, e.g. by historical maps, aerial photographs or satellite imagery [e.g. 38, 40]. The assumption is, that the increasing distance to the recent glacier terminus represents an earlier date of deglaciation in the past, and thus locations in different successional stages are encountered. This method is well suitable for revealing differences in species numbers, ground cover values, and vegetation structure across study sites. However, as different areas with potentially different biotic and abiotic site factors (e.g., slope, elevation, aspect, substrate, disturbance history, seed sources in the surrounding, …) are investigated, not only the time since deglaciation may be in control for the vegetation. In glaciated high mountain environments, the time since deglaciation often is accompanied by a change in elevation, which in turn goes hand in hand with a change in microclimatic conditions (changes in air and soil temperatures, duration of snow cover, length of the growing season), factors relevant for the vegetation development observed, too. This paper presents results of a chronosequence study in the foreland of Schwarzenbergferner (SBF, “ferner” is a Tyrolian toponym for glacier) in the Eastern European Alps. Main objectives of the paper are:

(1)

How is the plant composition, vegetation structure and phytodiversity along the chronosequence covering roughly one and half century?

(2)

As there is a pronounced vertical distance between highest (=youngest) and lowest (=oldest) sies, what is the elevational gradient of bioclimatic factors?

(3)

Can vegetation development in the glacier foreland of SBF accurately explained by the time since deglaciation, as it is done in many glacier forelands.

Figure 1. “Aerial” view from Schrankogel (3497m asl) into the glacier foreland of SBF, with the approximate location of the sample sites A to K and geomorphic surface features shown (Photograph: Thomas Fickert, 08/2015).

Figure 1. “Aerial” view from Schrankogel (3497m asl) into the glacier foreland of SBF, with the approximate location of the sample sites A to K and geomorphic surface features shown (Photograph: Thomas Fickert, 08/2015).

2. Materials and Methods

2.1. Study Area

SBF descends from the southeast side of Schrankogel (3497 m asl, UTM 32T E: 659431, N: 5212216) towards the Sulz valley, a tributary on the East of the Ötz valley in the Eastern Alps (“Stubai Alps”) of Tyrol (Austria). The terminus at the time of vegetation sampling have been on the southwest-facing slope at approximately 2830 m asl (see Figure 2).

From here, the glacier foreland, deglaciated since LIA around 1858, extends for over 2 kilometers and more than 600 vertical meters to a distinctive terminal moraine, which marks the LIA maximum extent of the SBF, presumably united with the glacier tongue of Sulztalferner at that time. Geologically Schrankogel belong to the East Alpine crystalline complex (“Ötztal-Stubai Complex”) [42]. It is considered part of the Upper East Alpine nappe system. Crystalline paragneisses and mica schists, predominate

From a macroclimatic perspective, the Alps are transitional from the temperate Central European climate in the North to a Mediterranean type climate in the South. As a prominent topographic barrier, the Alps cause orographic precipitation on the outer slopes, both in the North and in the South. The more continental central Alps experience significantly lower precipitation and higher solar radiation at all elevations. The Inn Valley and the lower sections of its tributaries are typical inner-Alpine dry valleys, showcased by the climate diagram of Umhausen (1031 m asl) in the lower Ötz valley (Figure 3). The station Obergurgl (1907 m asl) in the upper Ötz valley is characteristic of the climatic conditions at middle elevations, virtually at the lower end of the SBF chronosequence (Figure 3). The station Sonnblick (3106 m asl) in the Hohe Tauern, located approximately 140 km east of the Ötz valley, represents the climatic conditions near the upper end of the chronosequence, i.e. in the recently deglaciated glacier foreland. The average annual precipitation at Hoher Sonnblick is 1800 mm. The average annual temperature on the Sonnblick is -4.6 °C, with positive mean temperatures occurring only for a few weeks in summer (Figure 3). A large proportion of the precipitation at higher elevations falls as snow, resulting in thick and long-lasting snowpacks. Since the end of the LIA, a warming trend has been observed in the Eastern Alps. With 2 °C the Alpine region is experiencing twice the warming of the global average due to albedo-driven feedback mechanisms [44,45,46,47]. Particularly in the summer months, a decline in snowfall at high elevations in the Alps has been observed over the last few decades.

The vertical climate gradient (temperature, precipitation snow cover, …) create a pronounced vertical differentiation of vegetation, soils, and morphodynamics. Concerning vegetation, the study area is mostly within the alpine (dwarf shrubs and meadows) and subnival zone (scree slopes with cushion plants and pioneer species with low ground cover). The lowest sample sites close the LIA terminal moraine are already located in the potential subalpine coniferous forest zone. The current lack of trees at this elevation is likely due to past anthropogenic influences, as elsewhere in the Alps. Permafrost is also widespread, as evidenced by numerous rock glaciers in the upper Sulz Valley (see also [42] for the upper Ötz valley).

2.2.Data Sampling and Analyyses

Samling design in general follows [48]. Vegetation sampling was performed along a chronosequence, extending from the recent glacier terminus down to the LIA terminal moraines (Table 1). The chronosequence measures 2.1 km in length with a vertical distance of roughly 600 m between highest and lowest sample sites. In total 10 different locations (sites A to J) were sampled, deglaciated at different points in time (A: 4 yrs.; B: 5 yrs.; C: 15 yrs.; D: 20 yrs.; E: 40 yrs.; F: 60 yrs.; G: 80 yrs.; H: 110 yrs.; J: 130 yrs.; K: 155yrs; I as site nomination is missing to avoid confusion with roman numbers I, II & III). The date of deglaciation was assumed by a combined use of dateable geomorphological structures visible in the field, historical maps showing the location of past glacier termini and the annual glacier reports provided by the Austrian Alpine Club (OeAV)[49].

At each location A to K (see Figure 1 and Figure 2) vegetation sampling was conducted on three sample sites (I, II & III) of 10 m² (2x5 m) (Figure 4). Based on the assumption that more important changes occur during the early stages of succession compared to later ones, the time intervals between the earlier surveys are shorter. Site selection was not randomly but deliberately to avoid disturbed sites (as far as visually assessable), and to get hold of sites representing “mean” conditions, i.e. alluvial areas, wet hollows with above-average snow cover duration or dry, wind exposed knolls were disregarded. Sites were considered typical for the respective site age. Sampling occurred square-meter-wise by visual estimation of the ground cover of each vascular plant species (taxonomy acc. to [50]) as well as terricolous lichen species and moss (sampled as undifferentiated species group) with the smallest unit being 0.01% ground cover (i.e. 1 cm x 1 cm on a 1 m²-subplot). The amount of coarse debris (coverage of rocks > 6 cm) was visually determined in % ground cover.

Raw data are subsequently converted to mean ground cover values as well as total number of species, first per sample site (i.e. 10 m²), finally for sample location (i.e. I, II and III = 30 m²). Plots A to F were sampled in August 2009, and plots G to K in August 2010. All species were assigned the corresponding life-form type of the species (acc. to [51]).

To detect relationships between the species inventory and underlying site conditions, multivariate ordination methods are a common and well-suited tool. They are aimed to visualize gradients in vegetation datasets and (if available) the responsible ecological factors [52,53,54,55,56,57,58]. For this study, vegetation data were analyzed by a Correspondence Analysis (CA), i.e. an indirect unimodal ordination procedure. Graphically, ordinations are depicted as two-dimensional scatter plots, with the sample sites represented as symbols. The position of each symbol in the ordination space indicates its similarity to each other. Symbols that are close together in the ordination space are similar in their species composition (if analysis is based on species inventory, as done here). Environmental variables are shown as arrows. Their length indicates the variable's significance for the separation of the sample sites within the ordination space. The arrows point from the origin, corresponding to the mean value of a variable, to sample sites with above-average values of that variable, sample sites with below-average values are located in the opposite direction. Ordination analysis was performed by the program Canoco 4.5 [59].

Figure 4. Vegetation sampling occurred on 10 m²-plots (2x5 m) with three sample sites per location A to K.

Figure 4. Vegetation sampling occurred on 10 m²-plots (2x5 m) with three sample sites per location A to K.

The vegetation studies are complemented by a one-year-measurements (September 1, 2010, to August 31, 2011) on each location of hourly soil temperatures at a depth of 10 cm, which is the main root horizon of most plants [60]. Soil temperatures have been proven to be easily measurable and biologically significant climate elements [61,62,63]. Körner [62,63] has repeatedly highlighted the decoupling of alpine plants from free atmosphere temperatures. He emphasizes that for low-growing plants in alpine habitats with a high proportion of underground biomass, soil temperatures are probably even more important for plant physiology than air temperatures.

According to Körner [63], a soil depth of 10 cm represents a good compromise between placements, that are too shallow (resulting in strong short-term fluctuations) and those, that are too deep (resulting in a long response time). Soil temperature loggers of the GeoPrecision model M-Log 5W were used, which can be read out via an infrared interface. The loggers were placed at the central out of the three sample plots at each sample location, i.e. A II to K II, see Fig. 2, Figure S1). Since the individual sample plots hardly differ in terms of their topographical site conditions (slope, aspect, shading, microtopography, etc.), the values are considered representative for the respective elevation. With the exception of Logger B, which failed shortly before the end of the measurement period (July 11, 2011), all measurement series are complete. The missing values from Logger B were supplemented using linear regression with the neighboring Logger A (R² = 0.9814). Next to the soil temperature values, the duration of the snow cover (= isothermal conditions between day and night), temperature sums (sum of degree-hours above 5°C) and the vegetation period (= number of days with a mean > 5°C), as well as the number of ice days (days with 24 h below 0°C), freeze-thaw-days (alternating temperatures below and above 0°C within a day,) and frost-free days (days with 24 h above 0°C) can also be derived from the logger data. In general, caution is advised when evaluating one-year measurement series, as they may be characterized by exceptional weather conditions and thus deviate considerably from the average climatic conditions to which the plants at their locations are adapted. Nevertheless, even such short-term measurement series are valuable for ecological interpretation, particularly when used for the comparison of individual sites, rather than for describing average site conditions [63].

3. Results

3.1. Vegetation

Within the 30 phytosociological samples in the glacier foreland of SBF a total of 90 vascular plant species, 5 terricolous lichens, and moss as species group (Table S1) were encountered. Table 2 gives key figures for the ten sample locations of the SBF chronosequence.

The A samples (Photo S1a), located at an elevation of 2750 m asl, are characterized by 37% coarse debris content. According to the annual glacier report of 2004 provided by the OeAV [49], it can be assumed that these areas became deglaciated in this year, i.e. five years prior to vegetation sampling. In addition to mosses, 19 different species of vascular plants have meanwhile established, with very low ground cover, though (~0,4 % on average). Early colonizers include Arabis alpina, Cerastium uniflorum, Oxyria digyna, Sagina saginoides, and Saxifraga oppositifolia, as well as the grasses Poa alpina and P. laxa – common pioneer species, that also dominate early colonization in other glacier forelands of the siliceous Central Alps [35,37,39,41].

The B plots (Photo S1b) are located approximately 15 meters away from the A plots at roughly the same elevation. The average proportion of coarse debris is higher (Table 2). On the plots, deglaciated approximately 6 years prior to sampling, 20 different vascular plant species and moss were recorded; the species composition in general resembles that of the A plots (see Table S1), as does the ground cover with an average of 0.38%.

The C plots (Photo S1c), located approximately 75 to 80 meters in front of the B plots at an elevation of 2730 m asl, were deglaciated 15 years earlier as indicated by data from the OeAV glacier measurement reports [49], and field markers from the Austrian Glacier Monitoring program close to the sample plots. The proportion of coarse debris is about 20%. A total of 22 different vascular plant species were recorded, at large identical to those in plots A and B. New species include the rare Artemisia mutellina and Cardamine resedifolia, which is also present in later stages (Table S1). The average ground cover is still very low with 1.3%.

Figure 5. CA-ordination of the vegetation plots (n = 30) along the SBF chronosequence. The pie charts show the life-form composition and total cover of the study plots; the pie charts for plots A through E are enlarged due to their low total cover for better reading.

Figure 5. CA-ordination of the vegetation plots (n = 30) along the SBF chronosequence. The pie charts show the life-form composition and total cover of the study plots; the pie charts for plots A through E are enlarged due to their low total cover for better reading.

The D plots (Photo S1d) are located at an elevation of 2720 m asl behind a prominent rampart left by the glacial advance during the cool, wet 1970s and 1980s. Based on the OeAV glacier measurement report [49], the area likely was deglaciated a few years after the most recent phase of glacial growth, suggesting an age of approximately 20 years at the time of the survey. The number of vascular plant species is 19 and the species composition is similar to sites A to C, the average amount of coarse debris is slightly above 20%. Several of the early pioneer species are present, with Achillea moschata and Veronica alpina appearing the first time on the D sites. With increasing distance from the recent glacier terminus and thus a longer period of time since deglaciation and for plant colonization, the vascular plant cover slightly increases, reaching an average of 2.7% here. With a virtually unchanged species composition (see Table S1), all areas that are deglaciated for approximately two decades (i.e. plots A to D) can be assigned to a "pioneer stage" in plant succession. Besides the pioneers mentioned above, Leucanthemopsis alpina, Ranunculus glacialis, Saxifraga bryoides, and Geum reptans, as well as the grass Agrostis rupestre are other common species. In the ordination plot shown in Figure 5, all sites representing the pioneer stages appear in a unique and widespread cluster, underscoring the irregular and highly random species composition during early colonization. A striking feature of this early stage is the high proportion of woody subshrubs (chamaephytes) (see Figure 5).

The E plots (Photo S1e) are located on a flat area in front of the 1980s moraine at an elevation of 2700 m asl. The plots were deglaciated for about 40 years at the time of sampling. The average cover of coarse debris is roughly 16 %. A total of 20 vascular plant species were recorded here, making up for average ground cover of 2.5%, while cover values can be as high as 10%+ with above-average moss content (see plot E I). In general, moss cover is highest on the E plots, decreasing both towards younger and older sites (Figure 5). Cryptogam cover probably declines in the later successional stages due to increased competition for light in the progressively denser vegetation cover. The terricolous lichen Streocaulon alpinum is also represented in the E plots for the first time.

Approximately 250 meters further away on another flat area at an elevation of 2650 m asl are the F-plots (Photo S1f) located. These plots were assumed to be deglaciated for approximately 60 years. Poor in coarse rocks (7.2%), the F-plots harbor a total of 21 vascular plants species. The total ground cover of vascular plants slightly increases to an average of 6.8%. Plots E and F (40 to 60 years ice-free) represent an early successional stage in which most of the aforementioned pioneer species are still present associated with later successional species. This early successional stage exhibits a mixture of dwarf shrubs, grasses, and herbs in varying proportions. Characteristic features include the first appearance of the ground-dwelling lichen Stereocaulon alpinum, herbs such as Gnaphalium supinum, Hieracium cf. alpinum, and the dwarf willow Salix herbacea. The replacement of the pioneer species Saxifraga oppositifolia and S. exarata indicates signs of competition. Regarding the total number of species per plot (20 and 21 vascular plants), this early successional stage does not differ much from the pioneer stage (plots A to D), but the cover values are somewhat higher (Figure 5).

Plots G (photo S1g) are located approximately 300 meters away from plots F at an elevation of 2600 m asl. They were deglaciated roughly for 80 years. Almost half of the sample sites are covered by coarse debris. With a total of 38 species of vascular plants and two new terricolous lichens (Cladonia rangifera and Cetraria islandica), this stage is significantly more species-rich than the younger sites. While some initial colonizers disappear, including Ranunculus glacialis, Minuartia gerardii, Linaria alpina, and Androsace alpina, several new species appear, absent in the pioneer and early successional stage (see Table S1). Among these, Cirsium spinosissimum, Euphrasia minima, and Festuca halleri are particularly noteworthy in terms of frequency and/or cover. Total ground cover now reaches 20 to 25%, with the average cover of vascular plants at 13.4%. Terricolous lichens and mosses locally cover larger areas, but in general the proportions of the life forms are rather balanced (Figure 5). This later successional stage can be classified as an initial grassland rich in herbs and grasses, although it has not yet developed into its typical form [39].

The H plots (photo S1h) are located approximately 700 meters away from the G plots at an elevation of 2450 m asl, and were estimated to be about 110 years old. The coverage of coarse debris is approximately 35%. With 48 different vascular plants, as well as mosses and two terricolous lichens, the plots are species-rich, and the total ground cover increases sharply to values exceeding 60%. Important species here include the herbs Campanula scheuchzeri, Leontodon hispidus, and various clover species (Trifolium pallescens, T. pratense ssp. nivale, T. badium), which may play an important role in nitrogen enrichment in the poorly developed soils [64]. Among the grasses, Poa alpina, Festuca halleri, and especially Trisetum spicatum stands out with higher ground cover. Shrubs, particularly Salix helvetica and Rhododendron ferrugineum, become more important, than in earlier stages, making these areas representative for a shrub-rich phase of the later successional stages.

The J plots (Photo S1i) are located at an elevation of approximately 2300 m on a steep slope and are assumed to be deglaciated for about 130 years. The ground cover of coarse debris is roughly 30%. With 55 different vascular plants, these are the most species-rich plots along the chronosequence. The total ground cover of vascular plants reaches 75%, about the half is represented by phanerophytes (Figure 5). In addition to the shrub species mentioned above, Salix glaucosericea and Empetrum hermaphroditum are also present. These are well developed shrub communities, characteristic for the late successional stages.

The K plots (Photo S1j) are located at an elevation of 2150 m just behind the LIA terminal moraine. For these plots a site age of approximately 150 years can be assumed. The plots are flat with an average cover of coarse debris of 25%. With 32 different vascular plant species, the species richness is considerably lower than that of the J plots, and also the ground cover of vascular plants decreases in favor of the terricolous lichens. They are well represented here with five different species and high cover (Ø 22.7%). Mosses reach mean ground cover values of 10.5% (Figure 5). Grasses and herbs are almost entirely absent, and hardly any of the early colonizers is left (see Table S1). Instead, several subalpine elements are present, including the first small Picea abies individuals and Arctostaphyllos uva-ursi. Due to their unique species composition, in Figure 5 the K plots are clearly distinct from the other samples. The decreasing species numbers and ground cover values are likely due to specific microclimatic site conditions, as the valley location favors both the formation of cold air settling and a longer persistence of the snowpack due to the steep terrain and low angle of the winter sun.

3.1.Microclimate

To assess microclimatic differences between sample sites concerning factors such as soil temperature, snow cover duration (indicated by isothermal conditions between day and night), length of vegetation period (daily mean > 5°C), temperature sums, as well as the number of ice days (ID), freeze-thaw-days (FTD), and frost-free days (FFD) the soil temperature measurements are used. Figure 6 shows the annual course of the ten temperature loggers installed in the foreland of the SBF [from 60].

There is a pronounced vertical microclimatic gradient, finding its expression in the soil temperatures, snow cover duration, temperature sums, as well as the freeze-thaw-conditions (Figure 7). The onset of winter snow is synchronous, while the date of snowmelt in spring and early summer is gradual at the individual sites (Figure 6). Between the earliest and latest day of snow cover removal (logger J II and logger C II, respectively), indicated by the restart of diurnal temperature fluctuations, is a period of time of almost eight weeks (53 days). As expected, snow melt at lower elevations is earlier than at higher elevations; however, due to topographical conditions, the lowest logger is not the earliest to become snow-free and the highest is not the latest. Particularly at logger location K II, the low-angled winter sun and the formation of cold air settling appears to favor a longer persistence of the snowpack. The proximity to the glacier terminus of the highest sites, the possibility of dead ice in the subsurface and cool katabatic glacier winds, create "supercooled" conditions close to the glacier (i.e. A, B & C, Fig. 7c). Moisture saturated soils after snowmelt may intensify this effect, as energy is required for evaporation, energy which is therefore unavailable for warming the soil.

Figure 6. Annual courses of soil temperatures (1 September 2010 to 31 August 2011) at the ten logger sites in the glacier foreland of SBF and of air temperature at the station Obergurgl (Ötz valley, Tyrol) in (a); panel (b) shows a zoom for the period of the spring snow melt for all ten sites, covering a 53-day interval [modified from 60].

Figure 6. Annual courses of soil temperatures (1 September 2010 to 31 August 2011) at the ten logger sites in the glacier foreland of SBF and of air temperature at the station Obergurgl (Ötz valley, Tyrol) in (a); panel (b) shows a zoom for the period of the spring snow melt for all ten sites, covering a 53-day interval [modified from 60].

Finally, an upward temperature gradient is also evident for temperature sums (Figure 7d). Especially in climatically extreme alpine locations, warmth is crucial for plant life processes. According to Körner [63], 5°C represents a critical thermal threshold for plant activity. Therefore, the heat sums, i.e., the degree-hours above this threshold, are an important indicator of the period of favorable conditions for plant growth in alpine regions – the higher the heat sums, the better. Here, too, the areas in the immediate glacier foreland and location K in the valley are cooler than expected by elevation and negatively diverge from the trend. The values range from over 1000°h in the immediate glacier foreland to over 3000°h in the lower elevations of the SBF glacier foreland.

Figure 7. Vertical gradients of selected (bio)climatic parameters at the ten sites in the glacier foreland of SBF (Stubai Alps): (a) duration of the snow-free period in days; (b) number of ice days, freeze-thaw days, and frost-free days; (c) soil temperatures (mean, minimum, and maximum) during the snow-free period; (d) temperature sums in °h > 5°C (= sum of all degree values >5°C).

Figure 7. Vertical gradients of selected (bio)climatic parameters at the ten sites in the glacier foreland of SBF (Stubai Alps): (a) duration of the snow-free period in days; (b) number of ice days, freeze-thaw days, and frost-free days; (c) soil temperatures (mean, minimum, and maximum) during the snow-free period; (d) temperature sums in °h > 5°C (= sum of all degree values >5°C).

4. Discussion

Along the SBF chronosequence different successional stages can be identified, each expressed by a specific set of plant species and vegetation structure, as well as certain statistical parameters such as species numbers or ground cover values. The environmental variables in the CA-ordination (Figure 5) show, that the factor "time since deglaciation" is highly correlated to the first ordination axis and has a high explanatory power for the separation of the vegetation samples within the ordination space. However, next to varying dates of becoming deglaciated, the chronosequence is characterized by a pronounced vertical distance of 600 meters, too, which is – as shown above – associated with an elevational gradient in bioclimatic parameters. The comparable lengths of the arrows representing site age and bioclimatic variables in Figure 5 indicate that the latter are equally important for arrangement of the sample sites within the ordination space. The almost opposite orientation to the variable "elevation" implies a high correlation between site age and the complex variable elevation and its associated bioclimatic variables (see Table 3). This is hardly surprising, since in a topographically highly structured regions like the Alps, glacier forelands are commonly associated with an elevational gradient. Primary succession in glacier forelands is generally characterized by an increase in species richness and ground cover with site age. At SBF, too, a strong non-linear relationship of increasing species numbers and ground cover values with site age exists (see Figure 8).

However, if species richness and coverage are related to the complex variable “elevation” used as bioclimatic proxy, the explanatory power is virtually identical (see Figure 8). So, the site age is a good predictor for successional development of vegetation in glacier forelands, if one individual chronosequence is considered. If two or more chronosequences are compared (e.g. [37,39]) other factors (e.g. at SBF bioclimatic ones summarized under “elevation”) comes into play. This illustrates site-specific, differences in vegetation development of glacier forelands, that cannot be explained by the time since deglaciation alone [8,13,37,39,65]. From case to case factors such as grain size, moisture, snow cover duration, inclination, solar radiation (shade vs. sun), thermal conditions, geology (siliceous vs. calcareous) geomorphological processes and frequencies and/or magnitudes of disturbances as well as biotic drivers and interactions such as competition, facilitation, allelopathy affect and modify the successional trajectory, that might be responsible for patterns observed [13,33,39,41,66,67]. In addition, the position in regard to the treeline has been proven to be crucial for successional patterns [37,39]. Proglacial areas of large glaciers extending close to or even below the treeline ecotone are colonized more rapidly and with higher diversity—due to a richer seed pool and generally more favorable growing conditions [27,36]—than those of smaller glaciers terminating in the species-poor upper alpine or subnival zones [14].

Figure 9 visualizes those site-specific differences in vegetation development by contrasting photographs of two different glacier forelands of roughly the same age: Morteratsch Glacier in the Engadin (Switzerland), located just below 2,000 m asl in the treeline ecotone, and SBF at 2,650 m asl. While the 60 year-old site at Morteratsch harbors tall larches (Larix decidua) and alders (Alnus viridis), the higher site on SBF of the same age has not yet moved beyond a sparsely vegetated early successional stage.

Figure 8. Trends in species richness and total vegetation cover in the glacier foreland of SBF, mean per sample location A to K, in relation to time since deglaciation (left) and elevation (right). The explanatory power (R²) is high and virtually identical for both variables.

Figure 8. Trends in species richness and total vegetation cover in the glacier foreland of SBF, mean per sample location A to K, in relation to time since deglaciation (left) and elevation (right). The explanatory power (R²) is high and virtually identical for both variables.

Next to the variables “time since deglaciation" and “elevation” other factors might locally be significant for vegetation development in glacier forelands:

• Grazing, which promotes the zoochorous dispersal of species and can lead to the selection of certain species [e.g. 37]
• High-mountain-specific morphodynamics with its numerous disturbances, which can repeatedly throw back succession to earlier stages [e.g. 67, 68]
• Grain size composition and resulting differences in the water balance of sites [e.g. 36]
• Differences in exposure, causing different species compositions at different sites (e.g., shady vs. sunny slopes), even though the areas are of the same age [e.g. 33]
• Characteristics of and distance to potential seed sources, from which primary succession originates [e.g. 69]

Figure 9. Comparison of the glacier foreland of the Morteratsch Glacier in the Engadine (Switzerland) located just under 2,000 m asl in the treeline ecotone (left; the signpost of the glacier path reads “glacier terminus in 1950, glacier retreat since 1900 741m”) and the glacier foreland of SBF (right) in the Stubai Alps (Tyrol) at 2,650 m asl.

Figure 9. Comparison of the glacier foreland of the Morteratsch Glacier in the Engadine (Switzerland) located just under 2,000 m asl in the treeline ecotone (left; the signpost of the glacier path reads “glacier terminus in 1950, glacier retreat since 1900 741m”) and the glacier foreland of SBF (right) in the Stubai Alps (Tyrol) at 2,650 m asl.

Individual chronosequences are appropriate to identify trajectories of primary succession of plants in glacier foreland (i.e. changes in species richness, ground cover, plant functional traits, and community structure) as a function of site age and local environmental conditions. However, shifts in species composition and general patterns in species assemblages other than the existence of various successional stages with time since deglaciation are hard to deduce, as many different factors next to site age control succession in glacier forelands

5. Conclusions

Chronosequences allow for studying vegetation succession over periods of time much longer than a researchers life span. They are appropriate when there is evidence that differently aged sites are following the same trajectory [12]. In addition, they are more reliably used for developments truly related to site age (e.g. species richness, plant cover, vegetation structure) than for those related to something else (e.g. species composition). Chronosequences can provide insights into general successional patterns as well as differences in species composition and vegetation structure at sites with varying site age. However, a critical evaluation of the temporal vegetation development must take into account whether factors other than site age become equally or even more important. Old field succession, in this respect, provides a rather clear picture of the temporal development on abandoned fields. Characterized by rather uniform site conditions, early successional stages on both dry and mesic fields developed at first from open annual to perennial herbal vegetation. Dry fields persist for decades in a perennial grass stage with scattered shrubs and trees, while the mesic sites transition to woodlands after two to three decades. Those general chronosequence results could be replicated by semi-permanent plots, proving that site age is indeed the responsible factor for succession on old fields [5,12].

However, glacier forelands are far more complex than abandoned fields. Vegetation dynamics and plant succession here is influenced by substrate, conditions (both physical and chemical) geomorphological disturbance, hydrology, microclimate, microtopography and snow cover, seed sources and dispersal pathways of available speciss, in addition to terrain age [8,33,35,39,68,70]. So, in glacier forelands chronosequences have both their values and their limitations. They allow for identifying the successional trajectory of an individual proglacial area, but for the deduction of broad successional patterns, judiciousness should be applied especially if environmental heterogeneity is great.