Studies on the Vegetation of Shetland: II. Reasons for the Restriction of the Exclusive Pioneers to Serpentine Debris

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Abstract
The exclusive pioneers are a group of boreal species almost confined in Unst to poorly colonized serpentine debris and apparently occupying open habitats throughout their distribution range. From study of a percentage cover analysis, seedling distribution and edaphic evidence in Unst, it appears that the restriction of many of the exclusive pioneers occurs at establishment and that they may be eliminated, as total cover increases, not by competition for space from other plants but by factors associated with changes in the soil. The frequent association of rare species with open habitats may not arise from their inability to compete for space in closed habitats.