The colonization of a slate rock surface by a lichen

Thalli of Parmelia glabratula ssp. fuliginosa on a vertical slate rock surafce in South Gwynedd were signifiacntly larger at the top of the face than at the bottom. The radial growth rates of the thalli measured over 1 year were not correlated with height on the face or with variation in thallus diameter down the face. These results suggest that the lichen thalli may be older at the top than at the bottom are are consistent with the hypothesis that the lichen colonized the top of the face first and then gradually spread downwards.