The tiny chance of a seed to become an adult

Most seeds are lost during dispersal. Either, they land on an unsuitable place on rocks, on stones, in water, on top of a closed canopy or in other situations unsuited for the germination of seeds. It is really not surprising that most plants produce a lavish number of seeds. During their live-time perennials may produce millions of potential offspring.

Germination and establishment

Once seeds have reached the ground, germination and establishment of a new plant may occur. This is one of the most elementary stages in the life cycle of a plant and at the same time the most risky one. If seedlings do not manage to rapidly anchor deep into the ground, they will die. Particularly, in alpine habitats environmental conditions like topsoil desiccation, heat on bare soils, nighttime needle ice formation, frost heaving and during winter cryogenic processes in the soil permit only few seedlings to survive. In the Rocky Mountains, Jolls and Bocks (1983) found that the number of seedlings of Sedum lanceolatum declined from lower montane (2300 m) to alpine grassland (3700 m) from 25 per m2 to almost zero.

The concept of "safe sites" was introduced by Harper et al. (1965) to describe the specific requirements that allow the seeds of a particular species to emerge successfully from the soil. "Safe sites" in alpine habitats can be found on bare soil close to big stones, in small depressions on dry substrate where moisture from rainwater is available, in shaded places near established plants, or in vegetation gaps created from different sort of disturbances. Unlike at low elevation, protection and facilitation by established plants plays a crucial role in alpine habitats.

safe sites
1 - Surviving seedlings of Trifolium pallescens (Fabaceae, left) and Achillea moschata (Asteraceae, right) in a sandy, dry site (Epilobietum fleischeri rhacomitrietosum).
depression zone shading

Seedling mortality

Belowground investments are the key to survival of seedlings. In Oxyria (Polygonaceae) for instance, during the first period after germination, hardly any shoot growth occurs and establishment and growth of the root system prevails (Billings and Mooney 1968).

Usually, the mortality risk for newly established plants is highest during the first year either during the dry summer period or in the first winter. The study of Stöcklin and Bäumler (1996) on glacier foreland revealed small summer and high winter mortality in moist sites near the retreating glacier and similar mortalities in summer and during winter at older, better drained sites (see table below).

A characteristic of the conditions at high altitude is the considerable year to year variation in seedling abundance. Establishment from seeds at high altitude is risky and years with a high number of successful seedlings are probably more the exception than the rule. The low seedling recruitment is considered as the most important reason for the various and high frequency of clonal growth strategies of plants in cold environments.

Germination and seedling mortality on a glacier foreland in the Swiss Alps
Wet sites Dry sites
Seedlings early summer Mortality during summer Mortality after one year Seedlings early summer Mortality during summer
Linaria alpina 170 12 % 73 % - -
Oxyria digyna 190 3 % 9 % 10 100 %
Cerastium pedunculatum 411 27 % 38 % - -
Saxifraga aizoides 1500 15 % 80 % - -
Epilobium fleischeri 420 32 % 99 % - -
Rumex scutatus 422 23 % 76 % 17 71 %
Achillea moschata 536 1 % 5 % 8 100 %
Trifolium pallescens 404 17 % 65 % - -
Hieracium staticifolium 275 15 % 90 % 2 50 %

Approximately 2000 seeds were sown in experimental plots, each was 0.3 x 0.6 m.