COMPETITIVE BEHAVIOR OF UMBILICATE LICHENS - AN EXPERIMENTAL APPROACH

The lichens Lasallia pustulata and Umbilicaria spodochroa grow in dens e monospecific or mixed populations on the coastal cliffs of southern Scandinavia. Attached to the substrate by only a thin central holdfast , their shield-shaped thalli compete for light and space for growth by overlapping each other. Matched pair experiments in the laboratory an d held observations of interacting pairs show that different behaviour al responses to precipitation tend to result in the margins of U. spod ochroa overlapping those of L. pustulata within a few minutes. The beh aviour is apparently caused by different capacities for water absorpti on in the upper and lower cortices of the species. An initial period o f repeated encounter caused by thallus expansion and contraction durin g precipitation will be followed by a period in which U. spodochroa gr ows to overlap L. pustulata more and more. When the overlapping lichen s are wet, flexible and photosynthetically active, the thallus above r ests directly on the upper surface of the one below. Very little light is transmitted through thalli of U. spodochroa, and the shaded parts of L. pustulata are retarded in their growth and die off.