Effects of density and water level on recruitment, mortality and shoot size in Sphagnum populations

Abstract

Peatmosses (Sphagnum) form a stem of indeterminate growth and branches of determinate growth from a single apical meristem. Vegetative regeneration occurs e.g. as a newly formed branch is converted into a second apical meristem. A shoot can die by being overgrown by its neighbours. In field and growth chamber experiments I tested the effect of differing shoot density on mortality and recruitment in Sphagnum shoot populations. A negative but under-compensating density-dependence was observed and density increased also at high initial density. The moss samples expanded or decreased in area as a result of a plastic response in shoot size (which was most important in the experiments where changes in area covered were small and sometimes negative) or by a numeric response in shoot number. Shoot size in natural populations generally decreased with increasing height above the water table. The species differ in their ability to show numeric or plastic response to density and environmental conditions.