DISTURBANCE REGIMES AS DETERMINANTS OF SEED BANKS IN COASTAL DUNE VEGETATION OF THE SOUTHEASTERN CAPE

Soil-stored seed banks of grassland, fynbos and thicket, all growing on calcareous dunes and each subject to different disturbance regimes, were examined. Seed banks were determined from counts of germinants from 50 soil cores from each type. Aboveground estimates of plant species cover in 10 1-m2 plots were used in determining vegetation/seed bank similarities. There was no evidence for seed bank densities to be markedly higher in the most frequently disturbed community (grassland - 4273 seeds/m2) than the least disturbed community (thicket - 3417 seeds/m2). Highest similarity between seed bank and above-ground vegetation composition in terms of species and growth form/life-span classes was recorded for grassland (CC = 50%). Lowest similarity (CC = 13%) was found in the less frequently disturbed thicket where no seeds of climax trees were recorded in the seed bank. A fynbos community on a north-facing (warm, dry) slope had intermediate-sized seed banks (1683 seeds/m2) with intermediate vegetation/seed bank similarity (CC = 46%). However, on the south-facing slope, which has a large post-fire ephemeral herb component, seed banks were larger (4518 seeds/m2) but less similar to above-ground vegetation (CC = 39%). Ordination (DCA) of vegetation data from the four communities was different from an ordination of their seed bank data. Fynbos shrub species were absent from seed banks of both grassland and thicket, even though secondary succession proceeds from grassland, through fynbos to thicket. Their seed banks appear less persistent than those of European heath or Californian chaparral shrubs.