Lobe connections and lobe crowding are associated with growth rate in the lichen Xanthoparmelia conspersa

The association between lobe connections and the degree of lobe crowding and radial growth was studied in thalli of the foliose lichen Xanthoparmelia conspersa. In 35 thalli, 15% of the lobes were not physically connected to either of their neighbours before the lobes merged into the centre of the thallus. Twenty-five percent of the lobes were connected in pairs and 29% in groups of three. Approximately 5% of the lobes were interconnected in larger groups of six or more. The mean number of lobes per group in a thallus was positively correlated with thallus diameter and with the degree of lobe growth variation but was unrelated to annual radial growth rate (RGR). The degree of crowding of the lobes in a thallus was defined as a 'crowding index', viz., the product of lobe density and mean lobe width. Crowding index increased rapidly with size in smaller thalli but changed less with size in larger thalli. Crowding index was positively correlated with RGR but was unrelated to lobe growth variation. Lobes removed from large thalli and glued in various configurations to simulate different degrees of crowding did not demonstrate an association between lobe crowding and RGR over one year. These results suggest that the pattern of lobe connectivity of a thallus is associated with lobe growth variation in X. conspersa. The degree of lobe crowding is associated with the increase in RGR with thallus size in smaller thalli and by restricting lobe width, could also be a factor associated with the more constant growth of larger thalli.