In the coastal cliff landscape of southern Scandinavia dense
populations of the saxicolous, lichen-forming fungus Lasallia pustulata
are subject to disturbances creating small or larger gaps; one or
several thalli are removed by strong winds, icing, rock slides, log-falls,
branch sweeping or trampling. New patches for colonization are also
created when mats of bryophytes, fruticose lichens or pine
needles are removed by the same forces. The disturbance gaps are
major sites of recruitment in the populations and the age
structure of a population is spatially structured by the pattern
of gap formation. The revegetation of experimentally created gaps
were monitored over a four year period, at the end of which
the gaps were filled with new, 1–9 mm sized thalli, unevenly scattered,
on average 6500 individuals m−2,
covering less than 1% of the substrate. In addition there were
clusters of small thalli sprouting
from the former attachment site of some of the large thalli that
had been removed. The growth of individual thalli in natural gaps
and mature populations was also monitored for four years. The
largest annual increment in thallus diameter was 15 mm, the average
5·5 mm. Some thalli did not increase in size at all, some
became slightly smaller, and some were completely eliminated, apparently
by self-thinning. There was no correlation between initial thallus
size and annual growth; and age and size are not correlated. The
individual variations in growth rate seem to reflect the degree of
initial crowding, subsequent competitive success, and local supply
of nutrients. The average growth and initial size of individuals plus
their density and dispersion are used to model the speed of gap-closure,
estimated to be 10–15 years.