Seasonal growth of the crustose lichen Rhizocarpon geographicum (L.) DC. in South Gwynedd, Wales

Seasonal growth was studied in the slow-growing crustose lichen Rhizocarpon geographicum (L.) DC. in an area of South Gwynedd, Wales. Radial growth rate (RGR) of a sample of 20 thalli was measured in situ at three-month intervals over 51 months on a southeast-facing rock surface. There were five periods of significant growth: July-September of 1993, 1994 and 1995, in January-March of 1996, and in April-June of 1997. In four of these periods, growth coincided with a mean temperature maximum (Tmax) over a three-month period exceeding 15°C and three of the maxima with greater than 450 sunshine hours. Two of the growth maxima coincided with periods of total rainfall exceeding 300 mm and one with greater than 50 rain days in a three-month period. There were no significant linear correlations between RGR and the climatic variables measured. However, there were significant non-linear relationships between RGR and Tmax, the mean temperature minimum (Tmin), the total number of air and ground frosts and the number of rain days in a growth period, the relationship with Tmax being the most significant. Hence, in south Gwynedd, maximum growth of R. geographicum occurs in any season although the period July-September appears to be the most favourable. Relationships between growth and climatic variables were non-linear, temperature having the most significant influence on seasonal growth. ©2006 Balaban.