Composed cushions and coexistence with neighbouring species promoting the persistence of Eritrichium nanum in high alpine vegetation

Abstract.

Zoller H. and Lenzin H. 2006. Composed cushions and coexistence with neighbouring species promoting the persistence of Eritrichium nanum in high alpine vegetation. Bot. Helv. 116: 31–40.

Cushions of high alpine plants may consist of several individuals, either of the same or of different species (composed cushions). To clarify the role of this coexistence for the life history of alpine plants, cushions of Eritrichium nanum (Boraginaceae) were investigated at 17 sites (2170–3320 m a.s.l.) distributed over the European Alps. We determined the age structure of large cushions using herbchronology, described their root systems, and recorded how frequently E. nanum directly coexists with other plant species. The maximum age of individual plants was 35–40 years, but cushions contained up to 25 individuals, including seedlings and juveniles, which suggests that composed cushions can become much older than individual plants. Unlike other cushion plants, E. nanum does not develop adventitious roots to take up nutrients trapped in the interior of cushions. Its root system consists of a primary axis bearing plenty of fine rootlets, two to four tap roots, and long plagiotropic side roots, which explore a large soil area and allow the effective exploitation of nutrients from humus-rich patches. About half of the surveyed plants grew individually on open soil, while the rest lived in close contact to other plants of the same or different species.When it coexisted with other species, E. nanum either acted as an epistratum, overgrowing dense cushion plants (63.5% of the contacts), or as a hypostratum, being overgrown by graminoids such as Festuca halleri. We conclude that at altitudes above 2500 m, E. nanum mainly profits from its coexistence with other species, which together with the joining of individuals into composed cushion seems to improve recruitment success and the persistence of stable populations.