Radial growth and carbohydrate levels in the lichen Parmelia conspersa on north and south facing rock surfaces

Parmelia conspersa (Ehrh. Ex Ach.)Ach. is a foliose lichen found more frequently on south facing compared with north facing rock surfaces in South Gwynedd, Wales, UK. The radial growth of thalli of P. conspersa from a north and a south facing rock surface was measured in situ at intervals of two months for 1 yr during 1990/1991. Mean annual radial growth rates were greater on the south compared with the north facing rock surface. In addition, the pattern of radial growth varied during the year with maximum growth recorded in the Feb/Mar. period especially on the south facing rock surface. The levels of ribitol, arabitol and mannitol were measured in individual lobes of P. conspersa collected from the same rock surfaces on 4 days (2 Jun; 7 July and 30 Nov. 1990 and 29 Mar. 1991) during 1990/1991. The total of the three carbohydrates varied between days; the levels of arbitol and ribitol being significantly lower in the 7 July sample on both north and south facing rock surfaces. In addition, the levels ribitol, arabitol and mannitol were higher on the south facing rock surface especially in the summer samples. The ratio of arabitol plus mannitol to ribitol and the mannitol/arabitol ratio varied more between days sampled than between north and south facing rock surfaces. The level of ribitol in individual thalli was positively correlated with arabitol on the north facing and with mannitol on the south facing slope. These results suggest that differences in the radial growth of P. conspersa thalli with aspect are more likely to reflect higher rates of photosynthesis on the south facing rock surface rather than large difference in the way carbohydrates were partitioned on the different surfaces. Lower radial growth rates may place P. conspersa at a competitive disadvantage on north facing rock surfaces.

Competitive interactions between four foliose lichens on north and south facing rock surfaces

Competition between four foliose lichen species which have distinct aspect distributions on slate rock in South Gwynedd, Wales, U.K. was studied in the field using a factorial experimental design. The lichens were grown as fragments glued to pieces of slate in monoculture and in two-, three- and four-species mixtures. The pieces of slate were placed to face a northerly or southerly direction. Growth in area (mm2) was used as a measure of performance in the experiment. The growth in area of Parmelia conspersa in south facing plots was not reduced in the presence of any of its competitors but its growth was reduced in the presence of Parmelia saxatilis in north facing plots. The growth of Parmelia glabratula ssp. fuliginosa was reduced in the presence of P. conspersa and P. saxatilis in south and north facing plots. Physcia orbicularis was reduced by P. conspersa in south facing plots and by both P. glabratula ssp. fuliginosa and P. saxatilis in north facing plots. The growth of P. saxatilis was increased by P. glabratula ssp. fuliginosa in south facing plots but was not reduced by any of its competitors in north facing plots. Significant two and three factor interactions suggested that the results from the three- and four-species mixtures were not always predictable from the results of the two-species mixtures. The results of the experiment may help to explain the existing aspect distribution of the four species on slate rock in South Gwynedd.

The reponse of lichen growth to transplantation to rock surfaces of different aspect

Thalli of four saxicolous lichens on slate rock fragments were transplanted from rock surafces to horizontal boards and then to south-east-facing and north-west-facing rock surfaces. The radial growth rate of Physcia orbicularis and Parmelia conspersa thalli declined after transplatation to north-west-facing rock surfaces and was unchanged after transplantation to south-east-facing rock surfaces cmpared with growth rates on the boards. The radial growth rate of P. glabratula ssp. fuliginosa thalli declined after transplantation to south-east-facing rock surfaces and was unchanged after transplantation to north-west-facing rock surfaces compared with grwoth rate on the boards. The radial growth rate of P. saxatilis thalli was similar on the horizontal boards, south-east-facing and north-west-facing rock surfaces. These results are dsicussed in relation to the aspect distribution of the four lichens in South Gwynedd, Wales.

The effect of rock surface aspect on growth, size structure and competition in the lichen Rhizocarpon geographicum

Rhizocarpon geographicum is a crustose lichen found frequently on rock surfaces of southern aspect and less frequently on rock surfaces of northern aspect in Gwynedd, North Wales. This study tested the hypothesis that the radial growth of R. geographicum thalli predicts aspect distribution. Thalli of all sizes, however, exhibited significantly greater radial growth over 18 months on northwest compared with southeast facing surfaces. The hypothesis that a more intense competitive environment on northwest facing surfaces may explain the aspect distribution of R. geographicum was then tested. The size frequency distributions of thalli revealed a higher proportion of thalli in the smallest size class and a more restricted thallus size range on the northwest facing surfaces. In addition, thallus mortality appeared to be greater on northwest facing surfaces. Significantly more associated lichen species were present on rock surfaces of northern aspect at sites where R. geographicum was present. The mean frequency of the associated lichen species, however, was significantly lower on surfaces of northern aspect where R. geographicum was present. In addition, two common foliose species at these sites were demonstrated experimentally to overgrow thalli of R. geographicum. It is concluded that the growth of R. geographicum over the study period did not predict aspect distribution and that differences in the competitive environments on northwest and southeast surfaces may be an important factor determining aspect distribution. © 2002 Elsevier Science B.V. All rights reserved.

Competitive interactions between four foliose lichen species with and without nutrient enrichment

Competition between four foliose lichen species, common on slate rock surfaces in South Gwynedd, Wales, UK, was studied in experimental plots with and without nutrient enrichment by bird droppings. Fragments of the four lichens were glued to pieces of slate on horizontal boards in monoculture and in two-, three- and four-species mixtures in a factorial experimental design. In monoculture, nutrient enrichment increased thallus area of Parmelia conspersa (Ehrh. ex. Ach.) Ach., decreased thallus areas of Parmelia saxatilis (L.) Ach. and Parmelia glabratula ssp. fuliginosa (Fr. ex. Duby) Laundon, and did not affect thallus area of Phaeophyscia orbicularis (Necker) Moberg compared with untreated thalli. In the mixtures, P. conspersa and Ph. orbicularis were equally effective competitors in plots with and without nutrient enrichment. Addition of bird droppings, however, altered the ability of P. saxatilis and P. glabratula ssp. fuliginosa, to compete with the other species, the competitive ability of both species being reduced in some mixtures but increased in others. The results suggest that nutrient enrichment may alter the competitive balance between the four lichen species and this may be a factor determining their relative abundance on rock surfaces in South Gwynedd.

Competition between three lichen species using a factorial experimental design

Competition between three foliose lichens common on slate rock in south Gwynedd, Wales was studied in the field using a factorial experimental design. The lichens were grown as fragments glued to pieces of slate in two- and three-species mixtures. In the two-species mixtures, Parmelia conspersa (Ehrh. Ex Ach.) Ach. outcompeted Parmelia glabratula ssp. fuliginosa (Fr. ex Duby) Laund. strongly and Physcia orbicularis (Neck.) Poetsch less strongly, while P. orbicularis outcompeted P. glabratula weakly. Significant two-factor interactions indicated that the results from the three-species mixture could not be predicted from the two-species mixtures. Parmelia glabratula and P. orbicularis grew better in the presence of two competitors than one. This result suggests that the three species may co-occur on well-lit rock surfaces at the site.

The biology of the crustose lichen Rhizocarpon geographicum

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. © The Author(s) 2012.

Factors associated with degeneration of the thallus centre in foliose lichens

Degeneration of the older parts of foliose lichen thalli often lead to the formation of a space or 'window' in the centre of the colonies. The percentage of thalli of different size which exhibited 'windows' was studied in twenty saxicolous lichen populations in south Gwynedd, Wales. The proportion of thalli with 'windows' increased with thallus size. The size class at which 50% and 100% of thalli exhibited 'windows' varied between populations. Differences between populations were not correlated with distance from the sea, aspect, slope or porosity of the substrate or the total number of lichen species present. However, a higher percentage of smaller thalli had 'windows' on rock surfaces with a greater lichen cover. There were no significant differences in the levels of Ca, Mg, Cu or Zn in large (>4 cm) and small (<2 cm) Parmelia conspersa (Ehrh. ex Ach.) Ach. thalli or in the centres and marginal lobes of these thalli. The concentration of ribitol, arabitol and mannitol was significantly reduced in the centre of large thalli compared with the margin of large thalli and the centre of small thalli. However, carbohydrate levels were similar in the centre of large thalli and the margin of small thalli. The data suggest that loss of the thallus centre is a degenerative process related to thallus size. In the field, the formation of 'windows' may be related to the intensity of competition on a substrate. Central degeneration was not associated with a deficiency or an accumulation of Ca, Mg, Cu and Zn in the thallus centre. However, degeneration may be associated with a reduction in carbohydrates in the centre compared with the marginal lobes.

The levels of ribitol, arabitol and mannitol in individual lobes of the lichen Parmelia conspersa (Ehrh. ex Ach.) ACH.

The levels of the soluble carbohydrates ribitol, arabitol and mannitol were measured in individual lobes of the lichen Parmelia conspersa (Ehrh. ex Ach.) Ach. Lobes were collected from a north and a south facing slate rock surface in South Gwynedd, Wales, U.K. on 4 days during 1990-1991. On each day sampled, the most significant variation in soluble carbohydrate levels was between the individual lobes of a thallus. In addition, carbohydrate levels were significantly greater on the south facing rock surface on 2 of the 4 days sampled. Factorial analyses of variance suggested that the levels of individual carbohydrates varied significantly between days but not between north and south facing rock surfaces. Mannitol levels varied less between days than arabitol levels. Levels of ribitol, arabitol and mannitol were positively correlated in individual lobes. A stepwise multiple regression suggested that on the north facing rock surface, arabitol and mannitol levels could be explained by variations in the level of ribitol. By contrast, on the south facing rock surface, the levels of fungal carbohydrates were less dependent on the level of ribitol and there was evidence of a relationship between arabitol and mannitol. Variations in carbohydrate production, allocation and metabolism could help to explain lobe growth variation in foliose lichens and the radial growth of lobes over a longer period of time. Greater carbohydrate production rather than differences in allocation and metabolism may explain the increased growth and frequency of P. conspersa on south facing rock surfaces in South Gwynedd. © 1994.

The influence of the frequency of wetting and drying on the radial growth of three saxicolous lichens in the field

Three lichen species were wetted with distilled water at different frequencies during August 1973 to July 1974. The radial growth rates of Parmelia glabratula ssp. fuliginosa and Physcia orbicularis thalli declined with increased wetting while the radial growth rate of Parmelia conspersa thalli increased with wetting frequency until ten experimental wettings per month but at fifteen wettings per month fell to a value near to the control. In the summer months, wetting resulted in a decline in the radial growth of P. glabratula ssp fuliginosa compared with the control but had little influence on the growth of P. conspersa and Physcia orbicularis. In the winter months, wetting had no significant influence on the radial growth of Parmelia glabratula ssp. fuliginosa, while the radial growth of P. conspersa increased and Physcia orbicularis declined compared with controls. These results are interpreted physiologically and in relation to the aspect distribution of the three lichens on rock surfaces.

The influence of aspect on the pattern of saesonal growth in the lichen Parmelia glabratula ssp. fuliginosa (Fr. ex Duby) Laund

The radial growth of samples of thalli of Parmelia glabratula ssp. fuliginosa were measured in situ on a south-facing and a northwest-facing rock surface each month from August 1973 to July 1974. In the periods August to October 1973 and March to July 1974 the radial growth of thalli in the northwest population was greater than in the south population. In the period November 1973 to February 1974 the radial growth of thalli in the south population was greater than in the northwest population. A physiological basis for the differences in seasonal growth in the two populations was suggested. The mean annual radial growth rate (in units of mm/year) was not significantly different in the two populations. However, the variability in radial growth rate between thalli was signifiacntly larger in the northwest than in the south population. These results may be explained by genetic difference between the populations and environmental differences between the rock surfaces.

The biology of the lichen Rhizocarpon geographicum: symbionts, growth, ecology, and lichenometry

Rhizocarpon geographicum (L.) DC. is one of the most widely distributed species of crustose lichens. This unusual organism comprises yellow-green ‘areolae’ that contain the algal symbiont which develop and grow on the surface of a non-lichenized, fungal ‘hypothallus’ that extends beyond the margin of the areolae to form a marginal ring. This species grows exceptionally slowly with annual radial growth rates (RGR) as low as 0.07 mm yr-1 and its considerable longevity has been exploited by geologists in the development of methods of dating the age of exposure of rock surfaces and glacial moraines (‘lichenometry’). Recent research has established some aspects of the basic biology of this important and interesting organism. This chapter describes the general structure of R. geographicum, how the areolae and hypothallus develop, why the lichen grows so slowly, the growth rate-size curve, and some aspects of the ecology of R. geographicum including whether the lichen can inhibit the growth of its neighbours by chemical means (‘allelopathy’). Finally, the importance of R. geographicum in direct and indirect lichenometry is reviewed.

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

Variation in lichen growth rates poses a significant challenge for the application of direct lichenometry, i.e. the construction of lichen dating curves from direct measurement of growth rates. To examine the magnitude and possible causes of within-site growth variation, radial growth rates (RaGRs) of thalli of the fast-growing foliose lichen Melanelia fuliginosa ssp. fuliginosa (Fr. ex Duby) Essl. and the slow-growing crustose lichen Rhizocarpon geographicum (L.) DC. were studied on two S-facing slate rock surfaces in north Wales, UK using digital photography and an image analysis system (Image-J). RaGRs of M. fuliginosa ssp. fuliginosa varied from 0.44 to 2.63 mmyr-1 and R. geographicum from 0.10 to 1.50 mmyr-1.5. Analysis of variance suggested no significant variation in RaGRs with vertical or horizontal location on the rock, thallus diameter, aspect, slope, light intensity, rock porosity, rock surface texture, distance to nearest lichen neighbour or distance to vegetation on the rock surface. The frequency distribution of RaGR did not deviate from a normal distribution. It was concluded that despite considerable growth rate variation in both species studied, growth curves could be constructed with sufficient precision to be useful for direct lichenometry. © 2014 Swedish Society for Anthropology and Geography.

Lichen growth and lichenometry

Lichenometry is one of the most widely used methods available for dating the surface age of various substrata including rock surfaces, boulders, walls, and archaeological remains. It depends on the assumption that if the lag time before colonisation of a substratum by a lichen is known and lichen age can be estimated, then a minimum date can be obtained by measuring the diameter (or another property related to size) of the largest lichen at the site. Lichen age can be determined by variety of methods including calibrating lichen size against surfaces of known age (‘indirect lichenometry’), by constructing a growth rate-size curve from direct measurement of lichen growth (‘direct lichenometry’), using radio-carbon (RC) dating, and from lichen ‘growth rings’. This chapter describes: (1) lichen growth rates and longevity, (2) methods of estimating lichen age, (3) the methodology of lichenometry and (4) applications of lichenometry. Despite its limitations, lichenometry is likely to continue to play an important role in dating a variety of surfaces and also in providing data that contribute to the debate regarding global warming and climate change.

The use of the lichen genus Rhizocarpon in lichenometric dating with special reference to Holocene glacial events

Lichenometry is one of the most widely used methods of dating the surface age of substrata including rock surfaces, boulders, walls, and archaeological remains and has been particularly important in dating late Holocene glacial events. Yellow-green species of the crustose genus Rhizocarpon have been the most useful lichens in lichenometry because of their low growth rates and longevity. This review describes: (1) the biology of the genus Rhizocarpon, (2) growth rates and longevity, (3) environmental growth effects, (4) methods of estimating lichen age, (5) the methodology of lichenometry, (6) applications to dating glacial events, and (7) future research. Lichenometry depends on many assumptions, most critically that if the lag time before colonisation of a substratum is known and lichen age can be estimated, then a minimum surface age date can be obtained by measuring the size of the largest Rhizocarpon thallus. Lichen age can be estimated by calibrating thallus size against surfaces of known age (‘indirect lichenometry’), by constructing a growth rate-size curve from direct measurement of growth (‘direct lichenometry’), using radio-carbon (RC) dating, or from lichen ‘growth rings’. Future research should include a more rigorous investigation of the assumptions of lichenometry, especially whether the largest thallus present at a site is a good indicator of substratum age, and further studies on the establishment, development, growth, senescence, and mortality of Rhizocarpon lichens.