The biology of the crustose lichen Rhizocarpon geographicum

Abstract

The crustose lichen Rhizocarpon geographicum (L.) DC. comprises yellow-green lichenized areolae which develop and grow on the surface of a non-lichenized fungal hypothallus, the latter extending beyond the edge of the areolae to form a marginal ring. The hypothallus advances very slowly and the considerable longevity of R. geographicum, especially in Arctic and Alpine environments, has been exploited by geologists in dating the exposure age of rock surfaces (lichenometry). This review explores various aspects of the biology of R. geographicum including: (1) structure and symbionts, (2) lichenization, (3) development of areolae, (4) radial growth rates (RaGR), (5) growth physiology, (6) changes in RaGR with thallus size (growth ratesize curve), (7) maturity and senescence, and (8) aspects of ecology. Lichenization occurs when fungal hyphae become associated with a compatible species of the alga Trebouxia, commonly found free-living on the substratum. Similarly, 'primary' areolae develop from free-living algal cells trapped by the advancing hypothallus. The shape of the growth rate-size curve of R. geographicum is controversial but may exhibit a phase of decreasing growth in larger thalli. Low rates of translocation of carbohydrate to the hypothallus together with allocation for stress resistance results in very slow RaGR, a low demand for nutrients, hence, the ability of R. geographicum to colonize more extreme environments. Several aspects of the biology of R. geographicum have implications for lichenometry including early development, mortality rates, the shape of the growth-rate size curve, and competition.

Lichenometric dating (lichenometry) and the biology of the lichen genus rhizocarpon: challenges and future directions

“…Hence, the initial colonizers may have disintegrated and it is their surviving fragments that have grown to maturity and are present on rock surfaces (Armstrong ), a phenomenon frequently observed in foliose lichen species (Honnegger ; Honnegger et al . ), but also present in R. geographicum (Armstrong ) (Fig. ).…”

“…by direct measurement: a plot of radial growth rate (RaGR) versus thallus diameter on rock surfaces in north Wales. Three phases of growth were identified: (1) an early growth phase in which RaGR increased to a maximum, (2) a short phase in thalli 30-40 mm in diameter at which RaGR was maximal, and (3) a phase in which RaGR declined in thalli greater than approximately 50 mm in diameter (data from Armstrong 2005b, 2011). White 1972Luckman 1977), exponential-linear (Benedict 1967;Denton and Karlen 1973) or 'irregular' (Curry 1969).…”

“…; Smith et al. ), and especially the ‘yellow‐green’ species, that have been most frequently employed (Armstrong ). The most common subgenus, Rhizocarpon Ram.…”

“…Although many lichen species have been used in lichenometry, it is members of the crustose genus Rhizocarpon (Benedict 1988;Poelt 1988;Purvis et al 1992;Smith et al 2009), and especially the 'yellow-green' species, that have been most frequently employed (Armstrong 2011). The most common subgenus, Rhizocarpon Ram.…”

Within‐site variation in lichen growth rates and its implications for direct lichenometry

“…It is well established that RaGR is related to thallus size in both M. fuliginosa ssp. fuliginosa (Armstrong 1976) and R. geographicum (Trenbirth and Mathews 2010;Armstrong and Bradwell 2011;Armstrong 2011). In foliose lichens, RaGR increases in smaller diameter thalli becoming linear or asymptotic in individuals larger than about 1.5 cm (Armstrong 1974;Aplin and Hill 1979;Armstrong and Smith 1996).…”