Abstract:
The Strandveld mediterranean-ecosystem of the west coast of South Africa supports floristically
diverse vegetation growing on mostly nutrient-poor aeolian sands and extending from
the Atlantic Ocean tens of kilometers inland. The cold Benguela current upwelling interacts
with warm onshore southerly winds in summer causing coastal fogs in this region. We hypothesized
that fog and other forms of occult precipitation contribute moisture and nutrients
to the vegetation. We measured occult precipitation over one year along a transect running
inland in the direction of the prevailing wind and compared the nutrient concentrations with
those in rainwater. Occult deposition rates of P, N, K, Mg, Ca, Na, Al and Fe all decreased
with distance from the ocean. Furthermore, ratios of cations to Na were similar to those of
seawater, suggesting a marine origin for these. In contrast, N and P ratios in occult precipitation
were higher than in seawater. We speculate that this is due to marine foam contributing
to occult precipitation. Nutrient loss in leaf litter from dominant shrub species was
measured to indicate nutrient demand. We estimated that occult precipitation could meet
the demand of the dominant shrubby species for annual N, P, K and Ca. Of these species,
those with small leaves intercepted more moisture and nutrients than those with larger
leaves and could take up foliar deposits of glycine, NO3-, NH4
+ and Li (as tracer for K)
through leaf surfaces. We conclude that occult deposition together with rainfall deposition
are potentially important nutrient and moisture sources for the Strandveld vegetation that
contribute to this vegetation being floristically distinct from neighbouring nutrient-poor Fynbos
vegetation.
