Population structure and size-dependent reproductive effort in Umbilicariaspodochroa

The size structure and the relationship between individual thallus size and apothecium production was investigated in crowded populations of the saxic olous, umbilicate lichen Umbilicaria spodochroa in coastal cliff communitie s in southern Norway. There was a strong positive correlation between apoth ecium production and the mass and diameter of individual thalli. The study also indicated the existence of developmental maturation thresholds, a mini mum diameter and mass that has to be reached before apothecium production c an commence. Linearity of the relationship between thallus mass, thallus up per surface area and minimum number of apothecia produced after maturation suggests that in these size stages there is a constant proportion of resour ce allocation to reproduction. The populations exhibited pronounced size hi erarchies, commonly with many small individuals and fewer big ones. Thus at any time the few large thalli contribute more to the propagation of the po pulation than many of the more numerous small individuals. The Gini-coeffic ient of inequality of mass between individuals ranged from 0.48 to 0.70 in the sampled populations and was correlated with population density.