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- Study sites and chronologies
- Sampling methods
Global change models predict that the speed and magnitude of climate warming will be greatest at the highest latitudes, with raised mean temperatures, particularly during the winter, and altered patterns of precipitation (IPCC 2001). Current evidence suggests that these predicted changes are well under way in parts of the high Arctic, as evidenced by the decreasing extent and thickness of sea ice during the northern summer and by other physical and biological parameters (Serreze et al. 2000; Walsh & Chapman 2001; Wang & Ikeda 2001). There is also convincing evidence that in some areas of the Arctic this is leading to the rapid melting of glacial ice, resulting in the exposure of new land surfaces for colonization by the Arctic biota. This process of glacial ‘retreat’, however, is not purely a recent phenomenon: data for glaciers on the west coast of Spitsbergen (Svalbard) reveal a history of rapid glacial regression extending back over 150 years or more (Liestøl 1988; Dowdeswell 1995; Lefauconnier et al. 1999) and proglacial areas have been increasing as new ground surface is revealed.
It is particularly surprising, given the significance of Arctic systems for climate change models, that information on the time-scales of ecosystem development following glacial regression in the high Arctic remain both speculative and anecdotal and that the chronology and sequence of community assembly remain largely undocumented or assumed. These successions differ fundamentally from those elsewhere, being highly constrained by low temperatures, short growing seasons, limited moisture and nutrient availability, cryoturbation of soils and the effects of permafrost in restricting developing soil depth. The severe climate limits the growth form of plants present, such that the most mature vegetation type rarely exceeds 0.1 m in height. Because the plant communities are simple and of low species diversity, coupled with the fact that high Arctic land masses are often geographically isolated, the diversity of input of allochthonous plant propagules into established communities is restricted (Stocklin & Baumler 1996). Thus, studies that model the impact of climate change on the interface between the boreal forest and low Arctic tundra ecosytems (Starfield & Chapin 1996), and which suggest that major vegetation changes may occur within 150 years as trees begin to invade tundra areas, are not immediately applicable to the high Arctic.
The importance of allochthonous inputs of wind-blown detritus and dispersing insects as primary sources of energy and nutrients during the very early stages of community development in the high Arctic and elsewhere have already been stressed (Hodkinson et al. 2001, 2002). These transfers between and among the terrestrial and aquatic habitats are undoubtedly a continuing feature of ecosystem development. This paper details the subsequent development of the soil and plant communities along a 2000-year chronosequence on and adjacent to the foreland of the Midtre Lovénbre glacier near Ny-Ålesund, Kongsfjord on the west coast of Spitsbergen, Svalbard. Comparison is made with community development on a nearby chain of small islands, the Lovén Islands lying in Kongsfjord, which were successively released from beneath the ice during the regression of the main valley glacier, the Kongsbre. These islands are about 3 km from the mainland shore and separated by at least 1 km of open water.
The two chronosequences, developing initially on slightly alkaline substrates, provided contrasting systems in which to investigate community assembly. The Midtre Lovénbre communities developed on bare nutrient-poor moraines in close proximity to the cooling effect of a large glacier but with free access to colonizing organisms from adjacent areas. The Lovén Islands communities, by contrast, developed on glacial diamicton and currently experience a slightly more temperate thermal environment (Brossard et al. 1993). Lying some distance from the main valley glacier, they are surrounded by the relatively warm waters of Kongsfjord, which also establish a significant potential barrier to plant and animal colonization. The islands are now important nesting sites for seabirds, and while sampling locations were selected away from prime nesting areas, they undoubtedly have received significant nutrient enrichment from the surrounding marine ecosystem, including some deposition of sea shell material. Birds may also have transported plant propagules into the sites from surrounding mainland areas.
Our primary objective was to produce detailed chronologies and descriptions of the process of community assembly on typical high Arctic glacier forelands, with the intention of establishing a platform for a broader understanding of the time-scales and rules of assembly for the developing plant and animal communities. This is intended to provide a clearer insight into the possible effects of changing climate on these inherently fragile systems and a better appreciation of their likely rates of recovery following disturbance. Comparison between the contrasting mainland and island chain chronosequences provided a unique opportunity to evaluate the relative importance of chance and determinism in establishing the direction of community assembly in similar low diversity ecosystems with differing degrees of isolation (Ward & Thornton 1998, 2000; Young et al. 2001).
Study sites and chronologies
- Top of page
- Study sites and chronologies
- Sampling methods
Study locations (Table 1) were situated close to Ny-Ålesund, Kongsfjord, West Spitsbergen, Svalbard, where the general vegetation has been mapped from the air using scanned infrared photography and GPS linked to ground survey (Brossard et al. 1998; Nilsen et al. 1999a,b). The Midtre Lovénbre chronosequence comprised a series of seven sites of different chronological age arranged along a linear transect across the glacier foreland. The Lovén islands chronosequence comprised four sites on different islands within Kongsfjord, exposed sequentially during the retreat of the main valley glacier, the Kongsbre. Only three main sites were subsequently analysed in detail (Table 1). The Midtre Lovénbre and Lovén Island sites were sampled in mid-July to early August 2000 and 2001, respectively.
Table 1. Study sites on Kongsfjord, Spitsbergen, Svalbard. Sites were aged in 2000 by either aerial/ground photography (P) or radiocarbon dating (RC). Radiocarbon dates are calibrated age ranges based on 1σ (see text for publication codes)
|Site||Location||Altitude (m a.s.l.)||Dating method||Age (years)|
| Midtre Lovénbre 7||N 78°54.534′||12||RC||BP 1926–1820|
|E 12°04.537′|| || || |
| Midtre Lovénbre 6||N 78°54.200′||15||RC and estimation||150|
|E 12°06.763′|| || || |
| Midtre Lovénbre 5||N 78°54.082′||35||P||100|
|E 12°06.339′|| || || |
| Midtre Lovénbre 4||N 78°53.984′||40||P||60|
|E 12°06.085′|| || || |
| Midtre Lovénbre 3||N 78°53.816′||48||P||37|
|E 12°05.590′|| || || |
| Midtre Lovénbre 2||N 78°53.704′||58||P||16|
|E 12°05.262′|| || || |
| Midtre Lovénbre 1||N 78°53.653′||60||P||2|
|E 12°04.797′|| || || |
| Storholmen||N 78°55.860′||30||RC||BP 968–926|
|E 12°13.617′|| || || |
| Midtholmen||N 78°55.914′||22||RC||BP 1174–1058|
|E 12°18.156′|| || || |
| Leirholmen||N 78°55.215′||14||P||100|
|E 12°20.285′|| || || |
The forelands of high Arctic polythermal glaciers, such as the Midtre Lovénbre, remain relatively unstable following their release from the surface ice. Polythermal glaciers gather a large reservoir of meltwater beneath the glacial mass during winter. This water is released abruptly as a surging breakout in spring. The timing and location on the foreland of this breakout is unpredictable, ensuring that over time a significant proportion of this area is subjected to reworking of the substrate, with vegetation being washed away and the succession regressing to time zero. The precise chronological age of vegetation on these disturbed areas is thus unknown. Methods developed for describing vegetation change on stabilized glacier forelands elsewhere are therefore less appropriate to the high Arctic. As our intention was to describe, as far as possible, the chronological development of undisturbed animal and plant communities, a directed method of sampling was employed. Thus an initial survey of the whole glacier foreland was conducted to identify areas of obvious disturbance. The least-disturbed continuous transect line across the foreland, along which sampling sites were subsequently chosen, was then identified. Individual sites of different age were then chosen by selecting visually typical areas within what appeared to be the most representative of the mature vegetation type, as measured by percentage vegetation ground cover and number of vascular plant species present. As far as possible the sites were of equivalent size, relatively flat surfaced, with a gentle north-facing slope and with early snowmelt.
On the Midtre Lovénbre seven sampling plots (c. 20 × 30 m), were established at roughly equal distances along a transect of 1.7 km across the proglacial area. Starting at the glacier snout, sites 1–5 lay on the moraine, site 6 on the sandur below the terminal moraine and site 7 a little to one side on a raised rock ridge beyond the sandur. Plot 7 was suspected of having been unaffected by very recent glaciation. The transect line was surveyed using a Leitz Total Station and site locations were fixed using GPS. Chronology of the seven site plots was established from vertical and oblique aerial and ground-based photographs held by the Norsk Polarinstitutt Archive, Tromsø, which show the position of the glacier front in 1995, 1990, 1977, 1971, 1969, 1966, 1948 and 1936. In addition, photographs in Hamberg (1894) show the position of the glacier snout at the end of the 19th century, allowing site 5 to be dated at around 100 years. Based on linear extrapolation between known glacier front positions we estimate that sites 1–4 were exposed 2, 16, 37 and 60 years before the sampling date (AD 2000). Organic matter (humic material with living roots removed) from the lowest organic horizon at site 7 was radiocarbon dated to 1923 BP (ref. sample SSR-6663, Site 7) (Table 1), indicating a much older vegetation history. However, radiocarbon analysis of similar material from site 6 suggested a recent origin and we estimate this site to be around 150 years old, based on its position on the sandur below Site 5. It should be noted, however, that the radiocarbon dates set a minimum age for the earliest residual organic matter and that sites may well have a longer history of vegetation development.
Four of the Lovén Islands, Breskjera, Leirholmen, Midtholmen and Storholmen, were originally selected to represent a suspected chronosequence. Breskjera, the most recent island, exposed for about 60 years, was subsequently shown to be devoid of soil and vegetation and was not sampled further. Historical records indicate that Leirholmen was ice covered around 1837 (see Chydenius 1865) and was being released from its ice cover in 1895 (see Garwood in Conway 1897, 1898 and Isachsen 1912; photographs by Garwood in the Scott Polar Research Institute, Cambridge (ref. P49/11/6,14 and 15 and P51/19/2)). By contrast, early charts (e.g. Scoresby 1820) show Midtholmen and Storholmen as discrete islands. Radio carbon analyses of samples from the lowest organic horizon date the initial stages of vegetation development on the Storholmen plots to 998 (ref. sample AA-46470(GU-9626)) and Midtholmen to 1116 BP (ref. sample AA-46471(GU-9628)) (Table 1). The island samples for vegetation and soil analyses were taken within representative plots of approximately 30 m diameter on the shallow down slope below the crest of each island.
- Top of page
- Study sites and chronologies
- Sampling methods
The general trends in soil development and community assembly followed the classic sequence recorded for glacier foreland chronosequences elsewhere (see Matthews 1992) but the time-scales were significantly extended and the most mature vegetation types comprised low prostrate communities. There was a general trend over time of increasing soil organic matter depth, percentage organic matter, nitrogen and soil moisture but decreasing trends in soil pH and mean particle size. On undisturbed sites a period of around 100 years was required for the establishment of 100% ground cover by vegetation, even at the apparently more favourable sites on the Lovén Islands. Substrate disturbance through frost heave, slumping of ice-cored moraine or erosion by meandering drainage streams on the Midtre Lovénbre ensured that only a limited proportion of the proglacial area developed the expected most mature vegetation type. Disturbed areas regress to earlier stages of successional development of unknown provenance, rendering some of the analytical techniques widely employed elsewhere, such as canonical correspondence analysis of samples from throughout the whole moraine (see Matthews & Whittaker 1987) inappropriate.
Initial colonization of bare moraines was by a cyanobacterial crust that served to stabilize the soil surface and probably, through nitrogen fixation, to raise the nutrient status of the surface soil horizon and thereby facilitate the establishment of vascular plants (Liengen & Olsen 1997; Bliss & Gold 1999; Liengen 1999; Dickson 2000). For 60 years cyanobacterial crust formed the dominant ground cover (up to 34%), after which time it declined, but still formed a significant component of the most mature vegetation types at the older sites.
Vascular plants were slow to establish and represented only a minor component of ground cover for the first 100 years. The vascular plants could be subdivided into species groups that appeared at different stages of succession. The earliest colonisers were S. oppositifolia and to a lesser extent S. polaris. Both species are recorded as having mycorrhizal associations that may aid nutrient capture and retention (Miller & Laurensen 1978; Read & Haselwandter 1981; Kohn & Stasovski 1990; Titus & Moral 1998), although S. oppositifolia is also reported as being non-mycorrhizal on Svalbard (Väre et al. 1992). These were followed by a group of mid-successional species, including B. purpurescens, S. nivalis, A. humifusa, M. rubella and C. arcticum, that exploit the open nature of the vegetation but tended to disappear from the community as total cover increased. Several species that became important elements of the mature vegetation at the oldest sites, including S. acaulis, P. hirsuta, B. vivipara*, D. octopetala*, C. tetragona*, L. arctica and O. digyna, did not appear until 60 years. Some of these later species (indicated by *) may also be capable of obtaining nitrogen through association with mycorrhizal or free-living soil bacteria (e.g. Nosko et al. 1994; Barmicheva 1998) but again Väre et al. (1992) record B. vivipara as non-mycorrhizal on Svalbard. The contribution of these associations to the nitrogen economy of the soil, however, is thought to be small (Solheim et al. 1996; Robinson & Wookey 1997). Others, such as P. hirsuta, are parasitic on the roots of other species. The process of facilitation during community assembly (Chapin et al. 1994; McCook 1994; Callaway & Walker 1997) can be recognized in C. tetragona, a frost-susceptible calcifuge species that tends to occur at stable, thermally favourable moist sites with significant humus accumulation (Acock 1940). Acidification of the initially alkaline substrate, together with associated humus development, appeared to facilitate its entry into the community.
Species richness of the vascular plant communities, despite the loss of mid-successional species, tended to increase gradually for around 100 years, beyond which time only the very occasional species was added to the community. Bryophytes became increasingly important components in all communities with the progression of time, with R. lanuginosum becoming a codominant species on the Midtre Lovénbre and bryophytes as a group becoming the dominant taxon on the Lovén Islands.
Comparison of the Midtre Lovénbre and Lovén Island chronosequences revealed many similarities but some intriguing differences. After 100 years, soil development on the two chronosequences showed broad similarities in the depth of organic horizon, percentage organic matter, water content and pH, although vegetation ground cover was higher on Leirholmen. However, on the older sites (150+ years) differences became more accentuated, with the organic horizon depth, percentage organic matter and water content on Storholmen and Midtholmen significantly greater than at Midtre Lovénbre sites 6 and 7. These deeper organic soils were accompanied by a greatly increased bryophyte cover. Vascular plant species richness, however, was lower.
What are the reasons for these differences? There are several possible explanations. The present microclimate, at least on Midtholmen and Storholmen, is marginally more favourable for plant growth (Brossard et al. 1993) but this in itself is unlikely to produce such a marked difference in a nutrient-limited environment. Inputs of nutients, particularly nitrogen and phosphorus from nesting seabirds and wildfowl, could explain both the increased growth rate and the shift in species composition towards the bryophytes. Fertilization of ground beneath bird cliffs, leading to enhanced plant growth, particularly of mosses, is a well-known phenomenon in the high Arctic. Robinson et al. (1998) conducted nutrient and temperature enhancement experiments on polar semi-desert sites at mainland sites close to Ny-Ålesund and noted that nutrient treatments had the strongest effect, leading to markedly increased growth of bryophytes, S. polaris and B. vivipara and greater accumulation of organic matter. Other species, such as D. octopetala and S. oppositifolia, declined as nutrient levels increased, again consistent with trends seen in our chronosequences. They concluded that after 5 years their nutrient treatment plots were coming to resemble the vegetation below bird cliffs. Similar nutrient treatments on tundra heath vegetation produced enhanced growth in S. polaris but little response in C. tetragona (Havström et al. 1993; Baddeley et al. 1994). It is interesting to note the highest rate of cellulose degradation was recorded at site 7 on Midtre Lovénbre and was associated with a lower soil moisture content and a smaller accretion of organic matter than on the islands, despite the older age of the site. However, across the Midtre Lovénbre transect, the decomposition results were consistent with Robinson et al. (1995), who demonstrated enhanced nutrient mineralization and an increased rate of litter decomposition with increased soil moisture levels at polar semi-desert and tundra heath sites, respectively. Comparable decomposition data were not collected for the islands in the current study.
One factor that is almost invariably overlooked when studying glacial chronosequences is the impact of additional external factors such as animal influences on ecosystem development (Cooper & Wookey 2001; van der Wal et al. 2001; Wookey et al. 2002). Nesting birds in the high Arctic suffer constant predation from Arctic foxes and therefore congregate at isolated nesting sites on cliffs or remote islands, with the resulting nutrient fertilization. Thus, exposed sites like the Midtre Lovénbre are rarely used by nesting birds and largely avoid the eutrophication effects observed on the Lovén Islands. Isolation, however, works two ways. The island plant communities are largely excluded from the grazing pressure exerted by reindeer, although the wetter areas may be subjected to goose grazing (Alsos et al. 1998). By contrast, the Midtre Lovénbre sites are grazed by reindeer and this can impact on both the standing crop and reproductive success of some constituent species such as D. octopetala (Wada 1999). It remains a point of debate whether the addition/removal of grazing on nutrient rich/poor sites might influence significantly the direction of community development. Might the higher diversity of vascular plants on the Midtre Lovénbre be maintained by grazing?
There is little evidence that community development on the Lovén Islands is or has been constrained by the ability of plant propagules to cross the intervening water barrier from the mainland. Generally, the same common species were present and it is always difficult to demonstrate conclusively the absence of a given species. For example, C. tetragona, a late successional species on the Midtre Lovénbre, despite its absence from our plots on Midtholmen and Storholmen, occurs locally on parts of both islands as a subdominant species (Brossard et al. 1993).
In the low diversity ecosystems of the high Arctic, with a limited pool of widely distributed plant species available, it is hard to separate the effects of chance and determinism in governing the direction of community development. There was a tendency for community assembly on both Midtre Lovénbre and Lovén Island chronosequences to follow the same general trend and largely involve the same key species at the same stages. This suggests a measure of determinism by default, where assembling communities progressively recruit from a limited pool of effectively dispersed species, each with their particular ecological requirements that determines their point of entry into the community. Thus, the early colonizers such as S. oppositifolia exploit the bare substrate; the later entries such as C. tetragona require facilitation through modification of the environment before they make their entry. Some mid-successional species disappear as vegetation cover begins to increase, presumably as a result of competition from established and later establishing species. There are, however, notable exceptions that suggest an important element of chance in community assembly. S. caespitosa was absent from the Midtre Lovénbre transect but was common at every stage of the Lovén Islands succession. This implies that early arrival, possibly coupled with higher nutrient availability, allowed this species to establish a place within the developing community and that 1000 years of competition, within a closed vegetation structure, has not led to its elimination.
Comparative data for the two chronosequences give some glimpse of what might happen under conditions of climate warming. They suggest that in the absence of significant nutrient enrichment community development on glacial chronosequences will accelerate but will be constrained, at least in the shorter term, by nutrient limitations and a limited pool of potential invasive species. Where nutrients are less limited there will initially be accelerated development towards a moss-dominated community, potentially with a lower species richness of vascular plants. This accords with observations that mosses are a unique feature of many low Arctic sites and that their importance increases with the passage of time (Hobbie et al. 2000; Beringer et al. 2001).