Colonisation and succession

Theoretical background

Existing vegetation has resulted from colonisation of bare surfaces by pioneer organisms, followed by the gradual and progressive displacement of pioneer species by others in a succession of communities which leads, in time, to the development of relatively stable, climax vegetation. Species in each successional or serai community are visualised as modifying edaphic and microclimatic conditions in such a way as to favour the establishment of species in the next community with which, as a result, they become unable to compete. The climax community is viewed as being essentially in equilibrium with its environment. Succession at a given site reflects the interaction of autogenic control, i.e. control by plants through competition and their influence on the environment, and allogenic control by abiotic factors (Tansley, 1935). Biogenic control by animals is a major influence locally, e.g. by bird perches and penguin breeding colonies. Muller (1952) considered some tundra vegetation to have developed by auto-succession, which he defined as 'a succession consisting of a single stage, in which pioneer and climax species are the same' due to the prevalence of allogenic control.

The climax concept exists in several forms. Its most strict adherents visualise the vegetation in each region as progressing, in the absence of disturbance, inexorably towards a single or monoclimax community, determined by climate which is regarded as exerting an overriding influence over the complex relationships between climate, soils and plant life form (Clements, 1916). Others, recognising that variation in bed rock, topography and other factors can permanently influence the interaction between soil and vegetation in a climatically uniform region, accept that such a region will support a range of climax vegetation types.

This polyclimax concept (Tansley, 1935) was modified by Whittaker (1953), who emphasised that vegetational change in space, as well as in time, can be continuous along environmental gradients and who thus preferred the concept of climax pattern. It is often expedient to refer to communities, or noda, while recognising the frequently continuous nature of variation in plant cover (Webber, 1978), as was done in Chapter 2. Further, it is now appreciated that climax vegetation is by no means static, showing cyclic and other changes at a particular point under uniform conditions, and directional change in response to any climatic change that is sustained over a period of years. Succession in polar regions is commonly so slow that it must inevitably be influenced by climatic change as well as autogenic processes. The time scale is such that the successional sequences are to a large extent inferred rather than observed, but strong if indirect evidence is provided by zonation of communities around retreating glaciers and on raised beaches of different age.

Much polar vegetation exists on terrain that has become available for colonisation only since the retreat of Wisconsin ice sheets or subsequent minor readvances, a period ranging from some 15000 years to a matter of months where glacial retreat is currently in progress. Other sites were subject to marine incursions during the Tertiary or Pleistocene. Therefore, it is not everywhere clear whether existing vegetation has reached a climax, and primary succession on rock, glacial moraines and other areas that have not previously supported plant cover is more prevalent in polar regions than in areas where vegetation has been present continuously for many millions of years. However, secondary succession is also widespread, where plant cover has been destroyed by frost action, fire or other causes.

The role of bryophytes and lichens

Bryophytes and lichens are effective colonisers during early stages of succession on dry land. They were traditionally believed to have an important influence on rock weathering, but this point is currently somewhat controversial. Both groups are likely to be beneficial in terms of increasing the availability of nitrogen in immature soils (Chapter 7). The role of lichens in colonising rock surfaces has been discussed by Syers & Iskandar (1973), Ugolini & Edmonds (1983), and by Topham (1977) who noted that the adaptations facilitating colonisation include tolerance of desiccation and extreme temperatures, longevity and low growth rates. Physical weathering is accelerated by rhizine penetration in foliose species, and by expansion and contraction of appressed, partially endolithic, crustose thalli in response to changes in water content. Lichens may induce chemical weathering by liberating oxalic acid, various lichen acids and carbonic acid formed when carbon dioxide released by respiration combines with water, but organic acids excreted by the mycobiont and acting as chelating agents appear to be more significant.

Scanning electron microscopy and other techniques have confirmed that chemical weathering takes place beneath crustose lichens on a range of rock types, creating irregular surfaces susceptible to physical weathering (Ascaso, 1985; Jones & Wilson, 1985). Penetration of lichen thalli into rock, with incorporation of rock fragments into the thalli, has been demonstrated on Signy I quartz mica-schist (Walton, 1985). In southern Victoria Land the surface of rocks containing cryptoendolithic lichens and other organisms has been observed to peel off periodically, apparently due to the cementing material between rock crystals being dissolved by substances released by the organisms (Friedmann, 1982).

However, Williams & Rudolph (1974) reported that free-living fungi showed greater ability than either fungi or algae isolated from lichens to chelate ferric iron in culture, and Brodo (1973) considered rock weathering, humus formation and entrapment of wind-blown particles by lichens to be extremely slow. Indeed, a well-developed cover of lichens, or of bryophytes, may under certain circumstances retard weathering by protecting rock surfaces from erosion by wind-blown particles, by providing insulation against freeze-thaw cycles, or by absorbing precipitation and thus further reducing the incidence of frost shattering (Holdgate, Allen & Chambers, 1967; Lindsay, 1978). Mosses appear to be more effective than lichens in paving the way for colonisation of rock surfaces by vascular plants. Crustose lichens are commonly the first colonisers, but their presence is not always necessary to permit establishment by mosses, often in cracks or depressions. Moss rhizoids and associated fungi penetrate at least 5 mm into some rocks, permitting water entry and accelerating physical weathering (Hughes, 1982). Mosses have higher growth rates than lichens, and their colonies have a greater capacity to trap wind-blown material and contribute organic matter to developing soil (A. J. E. Smith, 1982). Oosting & Anderson (1939) considered these processes of greater significance in temperate regions than the influence of mosses on rock weathering.

Polunin (1936) defined stages of a xerosere in northern Lapland as dominated by: (1) cyanobacteria, (2) appressed lichens, (3) foliose lichens, (4) mosses and fruticose lichens, (5) herbs, (6) xeric dwarf shrubs, (7) taller shrubs and (8) birch climax. He regarded stages 1-3 as proseral, in that the mosses of stage 4 were in places able to colonise bare rock directly. The larger mosses, and species of Cladonia and Stereocaulon that grow on them, were considered primarily responsible for creating conditions favouring higher plants by trapping dust and accumulating as humus. Comparable processes have almost certainly occurred widely in mild- and cool-polar regions during the development of angiosperm-dominated tundra.

The traditional view that mosses and lichens precede flowering plants during colonisation is less generally valid for deposits of sand, gravel and till than for rock. Plant succession in frigid-Antarctic soils begins with the appearance of algae, followed by bacteria and other microorganisms, and finally by mosses and lichens (Llano, 1965). Recently deglaciated rocks in the cold-Antarctic commonly show a lichen trimline at distances up to 2.5 m from the retreating ice. Intervening rocks lack colonising lichens, whereas beyond this boundary there is a progressive increase in species diversity, and in colony size in the first established species. However, Drepanocladus uncinatus and other mosses become established on local deposits of moist sand, even within the lichen trimline. Similarly on moraines, the first macrophytic colonisers appear to be lichens on rock surfaces, and mosses or the grass Deschampsia antarctica in pockets of soil (Corner & Smith, 1973; Lindsay, 1971; R. I. Lewis Smith, 1982a). On South Georgia, some moraines subject to cryoturbatic disturbance are colonised by grasses, with mosses becoming abundant later in association with older grass plants. Crustose lichens may appear first on more stable soils, giving way to bryophyte colonies in which flowering plants later become established (Heilbronn & Walton, 1984; Smith, 1984a).

A range of plant types can thus act as pioneer macrophytic colonisers in the Antarctic depending on the nature of the substratum and other factors. Worsley & Ward (1974) likewise concluded that mosses and grasses become established more or less simultaneously on newly formed moraine ridges near a retreating glacier in northern Norway. The different patterns of colonisation on rock and finer substrata were evident on Surt-sey, an island formed in 1963 by volcanic activity off southern Iceland. Colonisation of lava flows was effected primarily by mosses, whereas sandy beaches were initially invaded by both mosses and flowering plants with only minor association between the two (Fridriksson, 1975). Kuc (1970) considered that pioneer bryophytes on finely divided substrata are commonly dwarf acrocarpous mosses forming short-lived colonies of high fertility (Chapter 8).

Although cryptogams do not invariably precede the pioneer flowering plants, these plants undoubtedly influence pedogenesis and therefore the course of succession on immature, mineral soils by contributing to the development of organic matter, and through their influence on mineral contents (Chapter 7). Dawson, Hrutfiord & Ugolini (1984) have demonstrated that usnic acid and other compounds liberated by Cladonia mitis in Alaska are sparingly water-soluble and mobile within the soil profile, and they suggested that such compounds may contribute significantly to podsolisation and other aspects of profile development.

Mosses also play a part in hydroseres. Sphagnum spp form floating turfs which extend from the shore towards the centre of pools. As the turfs increase in thickness other plants characteristic of mires, and eventually of mesic communities, can become established. In a rather different manner, mosses such as Drepanocladus and Scorpidium spp, growing submerged or partly so, influence early stages of other hydroseres by accelerating the build-up of organic matter and sediments on the bottoms of ponds and lakes (Polunin, 1935).

Directional succession

An analysis of lichen succession on two types of ultra-basic rocks in montane tundra in the northern Urals (Magomedova, 1979) illustrates several of the preceding points. Plant cover was recorded in more than 950 quadrats, each assigned to a stage of succession according to degree of rock weathering, silt accumulation and other features. Table 3.1 summarises data for pyroxenites on which lichens colonise microindentations between rock particles and a high cover is eventually attained. Dunites (Table 3.2) differ both physically and chemically: colonisation occurs principally in crevices, the surface is unstable, and percentage lichen cover

Table 3.1 Succession in lichen communities on pyroxenites in the northern Urals

Stage of succession

Number of species §

Number of species of different growth form types

Stage of succession

Number of species §

Number of species of different growth form types

0 0

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