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Department of Biology, University of Oslo, P.O. Box 1066 Blindern, 0316 Oslo, Norway
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The relative abundance within a guild of six species of the lichen-forming fungal genus Umbilicaria was studied during primary colonization of rock surfaces in a chronosequence of ca. 245 y in an alpine glacier foreland in southern Norway. Although the density of the guild grew from zero to more than 1000 thalli/m2 and the species differ substantially in life history traits such as initial growth rate, maximal size, maturation rate and propagule types, the relative abundance among the species remained almost unchanged through those years. The relative abundance of species is correlated with their life history parameters, such as initial growth rate and size-related maturation. The pattern of relative abundance was also similar in the saxicolous communities outside the foreland, which are potentially several thousand years old. Outside the foreland however the density of the guild is only 1/10 of that in the oldest parts of the foreland, due to soil formation and vegetation growth that have covered many of the low profile rock habitats. Thus the areas affected by the disturbance of glacier expansions and retreats provide temporary opportunities for large increases in the population sizes of the members of the saxicolous community. The observations support the view that pre-emption of habitat rather than competitive exclusion is common in saxicolous lichen communities and that "succession" consists in the addition but rarely the loss of species.
Key words: community assembly, disturbance, glacier forelands, population dynamics, primary colonization
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). The communities of saxicolous lichen-forming fungi in alpine and arctic habitats at first glance may appear immutable; some of the slowest growing and longest living organisms on Earth are thought to live here (Frey 1959, Beschel 1963, Denton and Karlén 1973). Yet we know that rock slides and weathering, lava and ash flows, burial by river- or wind-deposited sediments, climatic fluctuations and glaciations, as well as human activities cause recurrent disturbances at different spatial and temporal scales in alpine environments (Reynolds and Tenhunen 1996, Crawford 1997, Komarkova and Wielgolaski 1999). After such disturbances the lichen communities reassemble, species by species. While the species composition of alpine and arctic lichen communities in the northern hemisphere is fairly well known (e.g. Creveld 1981, Thomson 1984–1997), our knowledge about the temporal scale of population and community dynamics in these habitats is limited (Frey 1933, 1959; Beschel 1957; Innes 1985; John 1989; Hestmark et al 2004, 2005), which is in contrast with our knowledge of the dynamics of assembly and succession of fungal communities on more ephemeral habitats such as a piece of dung, a rotting log or carcass, a dead leaf or conifer needle, which have been extensively studied by mycologists (Dix and Webster 1995, Dighton et al 2005).
The temporal scale of the dynamics in alpine habitats however easily may exceed the average research grant, research career or even human lifespan, and this poses a challenge to any study of lichen life histories (Beschel 1957, 1961; Hestmark et al 2004). Glacier forelands however do provide a natural laboratory for such studies (Cooper 1923, Fægri 1934, Matthews 1992, Chapin et al 1994, Rees 2001). By the stepwise retreat of glaciers, new areas are exposed in a temporal sequence, producing a temporal gradient in space, a chronosequence of habitats. The majority of data on lichen growth in alpine habitats have been provided by quaternary geologists and physical geographers using lichenometry to date recently glaciated substrates or rock falls by the estimated growth rates of certain crustose lichen species (e.g. Beschel 1957, Innes 1985). Questions of lichen biology have not been their primary concern.
In the present study we asked these questions: What is the population dynamics of the total Umbilicaria guild within the glacier foreland during 245 y of development? To what degree does this emerging guild exhibit succession or stability? Is there any relationship between the relative abundance of species and their life history traits such as initial growth rate and fecundity?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Jotunheimen is a remnant of the mountain range created by large tectonic overthrusts of Precambrian crusts and sediments during the Caledonian orogeny (Milnes and Koestler 1985). The bedrock in the study area consists of highly metamorphic rocks such as pyroxen-granulite, pyroxen-gneisses and ultramafic rocks. The quaternary geology of Jotunheimen is summarized by Holmsen (1982). The glacier Hellstugubreen is in a distinctly u-shaped valley and connects to a larger complex of glaciers associated with the alpine peak systems of Hellstugutindane and Memurutindane. The glacier front is presently at 1465 m altitude, and the lowermost parts of the foreland at 1418 m. The foreland forms an elongate area along the valley in front of the glacier. The maximum extension of the foreland is 1100 m.
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). Since then the glaciers have retreated, occasionally interrupted by minor advances. The geologist P.A. Øyen in 1901 marked the front of Hellstugubreen, and the dynamics and retreat of the glacier subsequently have been studied in detail by Liestøl (1962), the Norwegian Polar Research Institute and the Norwegian Water and Electricity Council (Norsk polarinstitutt 1962, 1969; Norges vassdrags og elektrisitetsvesen 1965; Norges vassdrags- og elektrisitetsvesen og Norsk polarinstitutt 1980). The outermost moraine in the foreland is assumed to have been deposited ca. 1740–1750. In the 150 y before 1901 there are no detailed recordings of the retreat of the glacier, but a comparison of the area exposed up to 1901 with that exposed during the 20th century indicates that glacier retreat up to 1901 was slow.
The lichens studied were all members of the genus Umbilicaria, which are among the first lichens to colonize glacier forelands (Fægri 1934, Storck 1963, Fahselt et al 1988, Hestmark 1991) and also dominate a number of more mature communities of saxicolous lichens (Frey 1933, Creveld 1981). The saxicolous community in the foreland roughly can be classified as belonging to the association Umbilicarietum cylindricae (Frey 1922, 1923; Klement 1959). Umbilicate lichens exploit the same class of environmental resources in a similar way and may thus be regarded as a guild (cf. Root 1967). The species studied were Umbilicaria aprina Nyl., U. cylindrica (L.) Del., U. hyperborea (Ach.) Hoffm., U. proboscidea (L.) Schrad, U. rigida (du Rietz) Frey and U. torrefacta (Lightf.) Schrad. Taxonomic details and species descriptions are found in Llano (1950), details regarding modes of reproduction and dispersal in Hestmark (1990, 1991). With the exception of U. aprina the species all reproduce by small (ca. 10 µ) sexually generated, single-celled, hyaline wind-dispersed ascospores, produced in black, cup-like apothecia on the upper side of the thallus. Umbilicaria aprina reproduces mainly by asexual thalloconidia developed patchily on the lower surface of the thallus (Hestmark 1990). None of the Umbilicaria species in the foreland reproduce the symbiosis intact (e.g. with soredia, isidia or similar structures). Data on the initial growth rate, apothecium production and population structure of U. cylindrica, U. hyperborea, U. proboscidea and U. torrefacta in the Hellstugubreen glacier foreland were presented by Hestmark et al (2004). In the present paper we use, as a comparative measure of fecundity among the species, the proportion of thalli up to 20 mm diam that had developed apothecia. Neither U. aprina nor U. rigida had developed apothecia. The initial growth rates of U. aprina and U. rigida used in the present study were calculated with the growth formulae presented in Hestmark et al (2004). Regression statistics were computed in the program StatView 4.0 (Abacus Concepts Inc., Berkeley, California), and plots and figures were generated in the program CricketGraph.
Fieldwork was carried out Jul–Sep 1993. Only the eastern half of the foreland was studied due to steepness and danger of rock falls in the western part. Eleven linear transects were fanned out from a large boulder marked "NP88" in red paint, lying midfront of the glacier (FIG. 1
). The boulder was marked by the Norwegian Polar Research Institute in 1988 indicating the ice front, which since has retreated a few meters more. For every fifth meter along the transects a sample plot of 0.5 x 0.5 m (0.25 m2) was searched. The quadrats were placed horizontally, and thus the surface area of the rocks under the quadrat might differ somewhat from plot to plot and be larger than 0.25m2. Transects were extended 20 m outside the foreland to sample the relative abundance of the species in the immediate "source communities". Altogether 1396 quadrats were examined. All Umbilicaria thalli present in the plots were identified to species, counted and their maximum diameter and reproductive status measured nondestructively. Altogether 131 193 individuals were examined.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). During the first 14 y after deglaciation few individuals are observed (0.66 per m2). Over the next 30 y the population grows linearly (r2 = 0.99 for a regression line on the first four data points), reaching 133 individual thalli per m2 approximately 45 y after deglaciation. Over the next 20 y the density increases rapidly to more than 800 individuals per m2. The density subsequently increases at a much slower rate, reaching an average of 1067 individual thalli pr. m2 in the oldest parts of the foreland.
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Guild structure.— – There were very distinct patterns of species commonness and rarity in the total sample. The most common species was U. cylindrica (n = 64696), constituting almost half (49.31%) of the total. Next came U. torrefacta (n = 36280) with a little more than one-quarter of the total (27.65%), followed by U. hyperborea (n = 29204) with a little less than one-quarter (22.26%). Thus these three species together constitute 96.8% of the guild and thus substantially dominate it. Among the three remaining species, together only representing 3.2% of the guild, the most common was U. proboscidea (n = 2911) (2.22%), followed by U. rigida (n = 306) (0.23%) and U. aprina (n = 96) (0.07%). The abundance rank series was thus U. cylindrica > U. torrefacta > U. hyperborea > U. proboscidea > U. rigida > U. aprina.
In the most recently exposed area (max. 14 y) only U. hyperborea and U. cylindrica were found. U. proboscidea and U. torrefacta then entered in the next 4 y. Within 32 y U. aprina and U. rigida also were present. Except for the first two stages, where low sample sizes create variation, the relative abundance rank of the four most frequent species remained remarkably stable through the chronosequence (FIG. 3). There was no clear pattern of species overturn or succession in the guild of the saxicolous lichen community constituted by the six Umbilicaria lichens.
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Life histories and relative abundance.— –
There was a significant correlation between the initial growth rate of the species and their relative abundance (FIG. 4). There was also a significant positive relationship between percent fecundity in thalli up to 20 mm and relative abundance (FIG. 5). Umbilicaria aprina was excluded from the regression because it only rarely produces apothecia. (Including it only will strengthen the relationship exhibited in FIG. 5). U. rigida did not produce apothecia in the specimens studied in the Hellstugubreen foreland but often does in other localities (cf. Gregersen et al 2006).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). A 40–50 y old Umbilicaria population in this glacier foreland is still a population consisting mainly of immigrants. The linearity of initial population growth suggests that the propagule rain is fairly constant and accumulative at this temporal scale; seasons and good or bad years of reproduction appear to be evened out within the years. The fact that the density in this linear growth phase reaches only 12% of the ultimate carrying capacity suggests that initial population growth is constrained by immigration, by propagule settlement rate. Such recruitment limitation previously has been shown to influence community development in several different ecosystems (Lewin 1986, Booth and Brosnan 1995). The subsequent exponential growth phase is comparatively short, ca. 30 y. It coincides with the maturation of the local population which now self recruits in addition to constantly receiving immigrants from the propagule rain.
The pronounced pattern of relative abundance between the six species accords well with observations from almost all other ecosystems that one or a few species tend to be abundant (dominant) and that there is a more or less exponential decrease in the relative abundance of the less dominant species (Preston 1948, 1962).
Explanations of such patterns are legion in the ecological literature. One explanation is that relative abundances reflect superior/inferior competitive ability. Another explanation is that different species have different and special preferences for particular habitats and that the relative abundance of species within a given landscape directly reflects the relative abundance of the preferred habitats present in that landscape. The significant correlations between relative abundance and the life history parameters of initial growth rate and fecundity seen in the present study might suggest that species-specific life histories influences relative abundance. Note, however, that the comparatively small differences in values for these life history parameters among the six species is not at all proportional to the differences observed in their relative abundance. Thus species-specific life histories seem to provide only limited explanatory power for the pattern of relative abundance.
More remarkable than the pattern of relative abundance in itself, is the maintenance of this pattern for 240 y when the populations of all the species go through a big population increase. The deviation from the pattern in the first time interval (up to max. 14) should not be accorded much significance because of the small sample size, a total of 33 thalli were found at a density of less than 1 per m2. The next interval is only 4 y older, and here the five most frequent species are present. The pronounced difference between the first and the second interval suggests that a time since habitat exposure of ca. 10–15 y marks a threshold for the emergence of the species into visible size. The elements of stochasticity and time lag in initial establishment and emergence then apparently are dampened in phase 3 where the stable abundance hierarchy is established. The subsequent stability is remarkable because it seems to preclude any significant effects of competitive interference as structuring the community, this in contrast to many other studies of community assembly and dynamics (Peet and Christensen 1980, Walker et al 1986, Huston and Smith 1987, McCook 1994, Chapin et al 1994). Three observations strongly suggest that the relative abundances within the guild are not determined by interspecific competition: First, the relative abundances do not change over at least a 220 y period; second, the relative abundances remain the same outside the foreland in habitats where interactions have had thousands of years to work under strongly density-dependent conditions; third, the relative abundances are established already during early colonization at low density where there is ample space for more establishments. The lichens of this study all share the same basic peltate body form and they are all rigid and compressed to the rock surface. If two species are closely matched in competitive ability, the rate of competitive displacement by the slightly dominant competitor can be slow, leading to almost indefinite coexistence (Huston 1979, Caswell 1982). Fast growing umbilicate lichens in coastal habitats have different abilities to get on top in overgrowth interactions but are still almost unable to out compete each other because a thallus that is overgrown from one side is able to grow in the other direction and the distance between the thalli and their maximum size often will preclude total cover (Hestmark 1997a, b). Several researchers have suggested that pre-emption of habitat rather than competitive exclusion is common in saxicolous lichen communities and that "succession" consists of addition but not loss of species (Frey 1933, Pentecost 1980, John 1989, Lawrey 1991). Saxicolous pioneer communities then also end up as climax communities. The pre-emption of habitat by the early occupants suggest that inhibition or "founder-control" would be more appropriate terms to characterize the mechanism of "succession" in the saxicolous community (cf. Connell and Slatyer 1977). After establishment their only substantial change might be their destruction, as indicated by the drop in the Umbilicaria population to a 10th of its size just outside the foreland. The density data also show however that within 100 y of disturbance the saxicolous community has bounced back 10-fold in comparison to the presumed initial conditions. The glacier foreland thus provides a temporary arena for a substantial increase in the saxicolous community which lasts at least 240 y. The development of the community in the subsequent hundreds and thousands of years is likely to be one of steady decline in population as more and more saxicolous habitats are drowned by soil and vegetation.
The absence of any clear pattern of succession also might be due to scale. In a model of forest succession Horn (1975) used the probabilities that an individual of a given species will be replaced by that of another species within a given interval of time to predict that the community will converge on an ultimately stationary abundance distribution of the species, regardless of the initial composition. The slow growth of Umbilicaria lichens suggests that few individual replacements might have taken place within the glacier foreland. On the rocks outside the foreland however replacement of individuals would seem probable. Yet the relative abundances remain the same.
If life histories and competitive interference do not explain the persistence of the abundance patterns, what does? Let us also exclude predation, a commonly invoked cause of community structure. The few reindeer in the Jotunheimen National Park evade the compressed saxicolous lichens because of the pain and teeth wear involved in scraping them of the rocks. Because the hierarchy of relative abundance is established already in the initial linear growth phase, and is also near identical to that in the source communities both within and outside the foreland, it is possible that the initial relative abundances simply reflect the relative abundances of species in the propagule rain deriving from the source communities. On the other hand the pattern might reflect the availability and extent of suitable habitat space (niches) for the different species over the landscape.
The dynamics of the saxicolous lichen community thus contrasts strongly with that of vascular plants inhabiting the patchy clay, sand and gravel habitats in a foreland. Most vascular alpine plants mature within a single or a few years, and thus the temporal scale of the life histories is different. Arabis alpina, Deschampsia alpina, Festuca vivipara, Oligotrichum hercynicum, Poa alpina, Ranunculus glacialis, Saxifraga oppositifolia, Saxifraga caespitosa and Trisetum spicatum were seen flowering in the areas closest to the glacier where almost no Umbilicaria was observed. The initial linear growth phase of a population of vascular plants is short, and local within foreland dispersal and recruitment much more influential on the population dynamics and the relative species abundances. The large seeds and leptocurtic seed shadows of most vascular plants also make local recruitment a much more powerful influence on population dynamics than in the light ascospores that travel freely over the landscape. Vascular plants in forelands also tend to exhibit distinct patterns of substitution of species with time, stages of succession (Cooper 1923, Fægri 1934, Matthews 1992, Chapin et al 1994). The pioneer species disappear as the vegetation cover develops and biotic interactions become increasingly important. The long term stability of community structure in the alpine saxicolous community observed in the present study contrast with the results of Fægri (1934) who at Nigardsbreen in Jostedalen, Norway, found a stage dominated by Umbilicaria lichens to last 40–60 y after deglaciation. The Nigaardsbreen foreland however is below the tree line, and the subsequent decline of the Umbilicaria guild is clearly due to shadowing by birches and the more vigorous growth of fruticose lichens and bryophytes in the moister, warmer climate. In contrast to woods or kelp forests, the saxicolous community in the alpine zone is and remains by and large a compressed one-layer photosynthetic cover of the rock substrate, a habitus mainly enforced by the strong winds that ice-blast all exposed surfaces and remove all litter.
The present study and conclusions concerns a select guild in the saxicolous community in an alpine habitat, that of the umbilicate lichens. Future studies will show whether the same conclusions apply to other lichen species present in these communities, such as members of the genera Rhizocarpon, Sporastatia, Pseudevernia Ophiopharma and others.
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FOOTNOTES |
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1 The authors have contributed equally to the paper. 
2 Corresponding author: Geir Hestmark, Department of Biology, University of Oslo, P.O. Box 1066 Blindern, 0316 Oslo, Norway. Fax: 47-22 85 46 64. E-mail: geir.hestmark{at}bio.uio.no
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