Regeneration by seeds and vegetation structure in alpine plant communities, subarctic Finland

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>The aims were to examine the importance of regeneration by seeds, the influence of plant traits and disturbances, and the role of seed-seedling conflicts in regeneration and in the determination of vegetation structure. The study was carried out at in a subarctic alpine area (Kilpisjärvi 69°01′N 20°50E′, Finland).</p><p>Seed bank and seedling densities were high in many plant communities (ranges 99–1109 viable seeds/m² and 0.2–227 seedlings/m², respectively). Effective seedling recruitment is reflected in vegetation as a high proportion of plants with poor or no vegetative reproduction ability. This development may take place in meadows and snowbeds where herbs (e.g. <em>Gnaphalium supinum</em>, <em>Sibbaldia procumbens</em>, <em>Veronica alpina</em> and <em>Viola biflora</em>) are abundant. On the other hand, the low proportion of these plants in heath vegetation reflects ineffective seedling recruitment.</p><p>Floristic similarities between the consecutive phases in the regeneration pathway may be low despite effective seedling recruitment. Clonality, large and small seed sizes and appendaged diaspores limit the movement of species from phase to phase.</p><p>Generally, disturbances facilitate effective regeneration by seeds. Grazing promotes species with large seed banks and is therefore one reason for high seed bank densities. Freezing and melting processes negate a negative influence of altitude on seed bank densities in the phase of seedlings. However, if disturbances are severe and continuous and the soil is compact, unstable or dry, disturbances are not beneficial. The same is true if there is a shift in the species composition of seedlings from gaps to closed vegetation. This phenomenon occurred in a rich meadow.</p><p>Seed-seedling conflicts limit regeneration by seeds in low-herb snowbeds and <em>Ranunculus glacialis-Gymnomitrion</em> snowbeds. Vegetative reproduction and infrequent pulses of seedling recruitment negate an influence of short-term seedling recruitment on the spatial structure of vegetation. Extreme conditions, such as low temperatures, instability of the soil and late snowmelt modify the influence of factors that are important in more moderate conditions.</p><p>To conclude, all transitions limit regeneration by seeds. However, favourable conditions (e.g. moist conditions in a meadow) partly eliminate the obstacles against seedling emergence. Regeneration by seeds therefore has a major impact on the dynamics and structure of vegetation. In heath vegetation, where bare soils are dry and the moss cover is thick, large seed banks and seed rains do not guarantee effective seedling recruitment. The regeneration process is reduced in the early phases, and plants that reproduce primarily by seeds have a minor role in vegetation. The accumulation of seed banks is effective in these circumstances.</p></div>