Gap-dynamics, recruitment and individual growth in populations of Lasallia pustulata

Abstract

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.