Topographic Variation in the Abundance of Hieracium Floribundum: Relative Importance of Differential Seed Dispersal, Seedling Establishment, Plant Survival and Reproduction

Abstract

(1) The perennial herb Hieracium floribundum was found less often at the bottoms of small depressions (1-4 m diameter and 0-5-1 m deep) than at the tops in an abandoned pasture in Ontario, Canada. This study was to determine whether this variation was due to differential seed dispersal, seedling establishment, plant survival or reproduction. (2) More wind-dispersed seeds landed at the top of depressions (9300 m-2) than at the bottom (4100 m-2). (3) Topographic position had little effect on seedling establishment within a particular habitat type (i.e. vegetated or canopy removed or bare soil). However, the best habitat type for seedling establishment (bare soil) was more abundant at the top of depressions (6.7-26.4%) than at the bottom (0.4%). Consequently, seedling establishment would be more likely to occur at the top of depressions. (4) Seedlings planted at the top and bottom of depressions died at about the same rate during a 4-year period. The percentage of the transplants reproducing during this period was greater at the bottom of depressions. (5) Differential seed dispersal and seedling establishment were more likely responsible for the greater frequency of occurrence of H. floribundum at the tops of depressions than either differential survival or differential reproduction. This supports the view that critical factors determining plant distribution and abundance are those operating early in the life cycle.