Growth Curve of the Crustose Lichen Ochrolechia parella Obtained from Marginal Growth Rings

Thalli of the crustose lichen Ochrolechia parella have marginal ‘growth rings’ enabling rapid estimation of radial growth rates (RaGR) over successive years. I obtained a growth curve, viz. changes in RaGR with thallus size, from rings of two populations of O. parella and in one population, and compared the growth curves obtained from rings and by measuring growth directly. Both methods revealed that RaGR in O. parella increased with thallus size in smaller thalli becoming more constant in larger thalli, a growth curve similar to that of foliose and placodioid species rather than crustose lichens such as Rhizocarpon geographicum in which growth declines in larger thalli. In O. parella the marginal zone comprises a layer of confluent areolae which may be better able to support and maintain a more constant growth rate compared with R. geographicum in which the marginal areolae are often isolated and scattered.