Tests of the model against data suggest it provides an accurate, robust, and universal framework for understanding the growth dynamics of both large and small lichens in nature. Regarding the wide range of lichen size at oldest ages, difference may be mostly due to the scarcity of available i large lichens on rock surfaces, ii old reference surfaces for long term control of lichen growth, and iii to the mortality of a lichen population that is subject to rapid turnover, impeding the initial cohort to survive while the rock-surface gets older Loso and Doak, 2006. However, the second of these basic statements is an assumption unsupported by ecological properties and population dynamics of lichens Joachimsen, 1973; McCarthy, 1999. Mean annual diametral growth rate ranged from 0. Questions such as influence of moisture on growth rate Beschel, 1961 , linear or exponential growth curve e. In this approach, all thalli of a certain species typically R.
Icelandic glaciers are sensitive to climate variability on short-term timescales owing to their North Atlantic maritime setting, and have been undergoing ice-marginal retreat since the mid-1990s. Cet article propose un examen des utilisations de la lichénométrie en Islande depuis 1970, différents auteurs ayant mis en place plusieurs techniques pour résoudre des questions de chronologie des processus géomorphologiques. They constructed a growth curve from repeated measurements of 'largest lichens' diameters at three sites: Mt. The reliability of a lichenometric date will depend on the quality of the calibration curve, the size of the sample, the nature and postoccupational history of the substratum, and the ability of the archaeologist to recognize potential disturbance factors. In Iceland, most lichenometric studies have been performed in proglacial environments to date moraine sequences e. Gravestones are ideal for checking accuracy. The bases for these two simplest approaches are grounded, according to Beschel 1950 , on: i the lichen cannot be older that the surface it grows on, ii the radial development of a single thallus is uniform when averaged over the long-term, iii the largest thallus grows under optimal conditions and therefore indicates the minimum age of a surface that is closest to the actual age.
It was found in the area that had been exposed for a maximum of 240 years. If this individual thallus became established at about the time the habitat was exposed, its average annual diameter increase has been 0·4 mm per year. Légende Numbers refer to table 1 and figure 1. During flow, the fastest moving material was propelled in front of the leading edge of the moving mass and was deposited as a relatively fine-grained debris apron. This is the main reason it took off so quickly amongst the geomorphological community, along with the fact that R. Joachimsen M 1973 — Does the size of lichen thalli really constitute a valid measure for dating glacial deposits? Matthews, 1974;Erikstad and Sollid, 1986;Bradwell, 2004b. Working with two lichen taxa, we used multinomial-based likelihood functions to compare model predictions with measured lichen populations, using only the thalli in the largest 25% of the size distribution.
In particular, we perform a detailed comparison among methods based on the statistical analysis of either the largest lichen diameters recorded on geomorphic features or the frequency of all lichens. To my view, this debate is pointless as looking for more or less complex statistics is not improving the quality of the raw data, i. Erikstad and Sollid, 1986; Innes, 1985; Matthews, 1974 Matthews, , 1975 Matthews, , 1977 Matthews, , 2005 Winkler, 2001. Contemporary variants of the lichenometric dating technique depend upon statistical correlations between surface age and maximum lichen sizes, rather than an understanding of lichen biology. Optimal sampling area has been the subject of much research, bearing in mind that relative slope location does affect growth rates: values are 10-15 mm lower on moraine crests than near slope bases. Largest lichen measurements on rockfall blocks that accumulate incrementally as hillslope talus in earthquake-prone California plot as distinct peaks in frequency distributions.
Earth Surface Processes and Landforms, 2, 737-748. Rhizocarpon geographicum and Sporastatia testudinea lichens generally are very rare and even absent within the limits of the 1850 advance of the Grubengletscher. Nine lichenometry curves of exponential form are constructed from largest lichens on four past glacier margins of known age, and the age of four margins of unknown age predicted. However, lichenometric results are often simplistically interpreted in Iceland: there, contrasting environments are found over very short distances, and a wide range of geomorphic processes impede rock surface stability; in addition, and as seen previously, the validity of many measurements can be questioned as lichenometric ages do not always accord with tephrochronologic ages. To reduce this sampling effect, several authors have chosen to increase the number of measured lichens per geomorphic feature and consequently, to analyse a set of large diameters instead of a single one Farrar, 1974; Matthews, 1974Matthews, , 1975Matthews, , 1977Innes, 1983Innes, , 1985McCarroll, 1993McCarroll, , 1994Pech et al.
La lichénométrie est une méthode facilement applicable sur le terrain, ne requérant aucun équipement lourd ou coûteux, et offrant des résultats qui sont affichés comme simples à interpréter. Cosmogenic surface-exposure dating of Holocene moraines has the potential to test some of the assumptions of the lichenometric technique and to facilitate the development of a new set of improved lichen dating curves for Alaska. Predicted ages are geologically plausible, and reveal glacier terminus retreat after a Little Ice Age maximum advance around ad 1816, with accelerated retreat starting in the early to mid twentieth century. The stonewalls which surrounded the area also had many lichens on its surface. Each species of lichens has distinguishing characteristics which separates one from another.
While the age of two contiguous moraines is well constrained between 1721 and 1755 through the identification of an aeolian layer that i shows the absence of tephra K 1721 deposited after the Katla volcano eruption between May and August 1721 and ii shows the presence of tephra layer K 1755 idem, eruption active between October 1755 and February 1756 ; the lichenometric analysis of 550 thalli on one moraine and 717 thallus on the other indicates a date of 1854 in one case and 1831 in the other, using several methods largest lichen, five largest lichens, size-frequency. Arctic and Alpine Research, 19, 296-304. Three cobbles from each dated land surface were sampled, except for the 1750 surface, which had five cobbles. Botanists make yearly measurements of lichen sizes that describe highly variable radial expansion of young, and old, Rhizocarpon subgenus Rhizocarpon that is a function of thallus size and age. The method involves no interference at all with the rock art itself, and if samples were taken of dead lichen matter their removal would not have any adverse effect, in contrast to sample removal for other dating methods.
Emphasis is laid on various factors that need to be taken into consideration in order to avoid errors. After a personal experiment, several lichens were measured on gravestones and a stone wall in the same area. Kirkbride and Dugmore, 2001, 2008. The calculated ground temperature lapse rates compare favorably with those already reported in the literature for the region. The method has a limitation though, especially if a new event of a bigger magnitude totally covers the previous deposits.
This may be due to the impact of fires, the volcanic activity or the natural mortality in a pio- neering community Loso and Doak, 2006. I investigated its use in the age estimation of relatively recent Austrian Alpine petroglyphs in 1965, but later neglected to develop my experience further. These misguided activities have resulted not only in the denuding of thousands of petroglyph sites, but also in the destruction or chemical contamination of accretionary deposits Childers 1994 , removal of mineral mass, and acceleration of deterioration — while the claimed degradation by lichens remains unproven. Among the shrubs a number of demographic and behavioural patterns are identifiable as successional features; other patterns are explicable in relation to changing histories of disturbance and site moisture relationships. On a site north of Mýrdalsjökull, joint observations of lichens and tephras enabled testing the validity of absolute lichenometric dating for the moraine formation.