Seed Entrapment in Alpine Ecosystems: Effects of Soil Particle Size and Diaspore Morphology

Abstract

The spatial distribution of diaspores in seed banks can be significantly affected by physical processes that act on diaspores after they reach the soil surface. We examined how diaspore morphology and soil particle size affect diaspore incorporation into soil in a disturbed alpine ecosystem on the Beartooth Plateau, Montana, USA. Diaspores of alpine species with varying morphology were sown over soils of five different particle sizes and later collected from three depths. Regardless of diaspore morphology, the total number of diaspores trapped increased with increasing particle size until a threshold soil particle size was reached above which no more diaspores were trapped. At small particle sizes (0.5—1.0 and 1.0—2.0 mm) small diaspores and diaspores with adhesive seed coats were trapped, but most large diaspores moved horizontally across the surface and were not trapped. The majority of those diaspores trapped were at the 0—1 cm depth at small particle sizes. At large particles sizes (2.0—4.0, 4.0—8.0, and 8.0—16.0 mm) high numbers of large diaspores were trapped, and more diaspores moved vertically through the soil column. In small particle size soils small diaspores reached greater depths than large diaspores. Diaspores with adhesive seed coats responded more like large diaspores in terms of vertical movement. Diaspore length and eccentricity (length/width) were highly correlated with entrapment at small particle sizes and appeared to have the greatest effect on horizontal movement. Mass and width were significantly correlated with numbers of diaspores trapped in large particle sizes and were influencing vertical movement. Models based on the Weibull probability distribution were used to describe diaspore "survival" on the soil surface and to describe vertical movement in the soil column. This study indicates that on exposed soils in windy environments diaspore morphology and soil particle size greatly affect the spatial distribution of diaspores in seed banks. For diaspores of a given species, the optimal soil type traps a high number of diaspores but precludes significant downward movement.