Pioneers in Primary Succession

Primary succession occurs when extreme disturbances, such as landslides and volcanic eruptions, create new habitats by removing or covering existing vegetation and soil. Pioneers that initiate primary succession must be able to establish and grow on substrates that are nutrient poor and that often have unfavorable moisture conditions. The most extreme sites are exposed unweath-ered rock surfaces. Here, colonization may be limited to cyanobacteria ('blue-green algae'), lichens, and bryo-phytes, with no further vegetation development. Somewhat more nutrient-rich conditions associated with weathered or fragmented bedrock surfaces, such as the scree slopes of landslides, are often dominated by tree species. Sites still richer in mineral nutrients, which may contain some residual organic soil, such as the deposi-tional zones of glacial moraines, in turn are often colonized by herbaceous species and grasses with faster growth rates (Figure 1).

For pioneers in primary successions, nitrogen is often the most limiting resource. Unlike other mineral nutrients that can be released through weathering of underlying rock, nitrogen must either be transported to primary successions through leaching and deposition, or fixed in situ. Some of the most inconspicuous pioneers on exposed rock faces are nitrogen-fixing cyanobacteria. Rates of nitrogen fixation by cyanobacterial 'biofilms' on rock surfaces may be considerable; thus, nitrogen-rich leachate from these surfaces may affect community development at down-slope sites. Cyanobacteria may also form symbiotic associations with lichens (e.g., Stereocaulon spp.).

Lichens, e.g., Stereocaulon

Mosses, e.g., Polytrichum

Grasses

Figure 1 Pioneer species typical of substrate types exposed following deglaciation. More fertile (nitrogen-rich) substrates often support herbaceous species and grasses, whereas rock surfaces and glacial till support lichens, bryophytes, and woody species. Modified from Grubb PJ (1986) The ecology of establishment. In: Bradshaw AD, Goode DA, and Thorp E (eds.) Symposium of the British Ecological Society, Vol. 24: Ecology and Design in Landscape, pp. 83-97. Oxford: Blackwell.

Lichens, e.g., Stereocaulon

Mosses, e.g., Polytrichum

Trees, e.g., Salix, Betula Herbs, e.g., Saxifraga, Lupinus

Grasses

Figure 1 Pioneer species typical of substrate types exposed following deglaciation. More fertile (nitrogen-rich) substrates often support herbaceous species and grasses, whereas rock surfaces and glacial till support lichens, bryophytes, and woody species. Modified from Grubb PJ (1986) The ecology of establishment. In: Bradshaw AD, Goode DA, and Thorp E (eds.) Symposium of the British Ecological Society, Vol. 24: Ecology and Design in Landscape, pp. 83-97. Oxford: Blackwell.

These lichens are among the first colonists of landslides and lava flows (Figure 2). Nitrogen fixation by lichens on these sites (0.2-0.4 kg N ha-1 yr-1) may be important in facilitating the later colonization of these sites by vascular plants.

Relatively few vascular plant pioneers are nitrogen fixing. An exception is the perrenial lupine (Lupinus lepidus). Lupine is a conspicuous pioneer of the extensive ash and pumice fields that were created by the eruption of Mt. St. Helens (Washington State, USA) in 1980. In the first decade after the eruption lupine patches spread rapidly, increasing available nitrogen in the soil tenfold, potentially facilitating the growth of other colonizing plant species. More recently, however, the spread of lupine patches has slowed as specialist insect herbivores have colonized the plants. The patchiness and unpredictability of vegetation colonization on Mt. St. Helens also highlights the importance of constraints other than nutrient availability that limit recruitment success. Dispersal limitation, described as the failure of seeds to arrive at suitable establishment sites, may limit the rate at which pioneers colonize available substrate, and may be important in shaping the trajectory of succes-sional change. Similarly, requirements for safe sites that provide favorable conditions for seedling establishment may account for the nonrandom distribution of pioneers on substrates such as glacial till. Small seed size and wind dispersal are particularly common traits among vascular plant pioneers. Wind dispersal is favored when animal dispersal vectors are rare. Small seed size may increase the probability of seeds becoming trapped in cracks and small depressions where germination and seedling survival are likely to be enhanced. Conversely, it has been suggested that small seed size limits the initial nutrient resource supply available to the plant and may prevent seedlings from developing mutualisms with nitrogen-fixing bacteria.

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