Primary succession | Secondary succession

Succession is initiated when vegetation is destroyed or a new substrate is exposed. Ecologists often distinguish between primary succession (vegetation development has to start anew) and secondary succession (vegetation development commences following (local) destruction - in this case at least the soil remains more or less intact and the remaining plant propagules contribute to the revegetation process).

In the relatively cold climate of the alpine environment succession is a rather slow process, and with increasing altitude the rate of succession and the number of species involved usually decrease. Regardless of the often declining species richness with increasing altitude, differences in species composition can be found between vegetation patches at the upper limit of the alpine zone that reflect the age or successional stage of the vegetation.

Primary succession

In a primary succession, the establishment of new plant communities depends on the immigration of seeds or vegetative parts and on the ability of species to establish on new substrates. The best known primary successions in alpine environments are in areas exposed by the retreat of glaciers. These areas are especially useful for ecological research because it is often possible to determine the exact age of the various successional stages. This research is also important due to the accelerated retreat of glaciers in the wake of global climate change.

So, where do the seeds come from that initiate primary succession in glacier forefields and similar areas?
Most of the seeds or plant fragments probably reach the area by downslope processes (such as streams or avalanches) from the vegetation that is situated above the newly exposed ground. However, wind or animals may also contribute some seed input from other sources.

In contrast to many lowland environments, where new substrate may be colonized rapidly by ephemeral plants, the first communities in alpine areas mostly consist of single plants of a few pioneer species which are able to develop and establish under severe environmental conditions. Seedlings often establish in the vicinity of large stones. Without the presence of such microtopographical niches, colonization may be considerably delayed. The combination of the local dispersal of seeds and sharply bounded microsites often produce a strikingly clustered and patchy distribution of the first pioneer plants in primary succession.

After the first species have got a foothold in the new area, further vegetation development may be promoted by clonal growth of some of the established species and by increased colonization by plant species in established vegetation nearby. These species may take advantage of improvements in soil conditions (for example increased organic matter and soil nitrogen) brought about by the pioneer vegetation. Further succession can produce a variety of different plant communities depending on the habitat conditions. The pioneer species will then more or less remain restricted to unstable sites such as riverbanks, solifluction slopes, small screes, etc. Under the severe environmental conditions above treeline, it may often take about 100 years for the establishment of climax species.

Secondary succession - cyclic processes

Whereas primary succession normally occurs over large spatial ranges and is strongly directional (that is, it leads to a more or less stable end state), secondary succession takes place on a smaller scale (e.g. after the death of single plants) and is often part of a cyclic process.

Alpine vegetation is often a mosaic of patches at different successional stages that is maintained by frequent, small-scale disturbances. For example, if Loiseleuria procumbens, characteristic of wind-exposed sites in the Alps, is damaged by frost, age, or animals, subsequent wind erosion leads to a gap in the Loiseleuria carpet. The bare substrate offers a suitable habitat for various lichens, and species like Minuartia recurva or Primula minima can grow more abundantly here than in the surrounding vegetation. This process initiates the regeneration of the Loiseleuria carpet.

1 - Gap within a Loiseleuria procumbens carpet, 2100 m, Oberalppass, Switzerland (127K)
2 - Gap within a Loiseleuria procumbens dominated community, with lichens growing within the gap (126K)

Strong interactions between abiotic factors and vegetation processes may produce some interesting cyclic processes, a kind of successional 'perpetuum mobile' in which a stable state is never reached. For example, in the alpine tundra of Wyoming, large peat hummocks may be abraded by strong winter winds carrying ice crystals (1). Soil frost action and wind may then further degrade the hummock and sort stones to form polygons (2). At the margins of these polygons mosses and vascular plants establish (3), initiating the formation of small hummocks (4) which may grow until they are abraded again (5-7) (after Billings and Mooney, 1959).

29 August 2011
© ALPECOLe 2002-2007