Lichens and Lichenometry

Lichens are omnipresent in Rapa Nui's rugged semitropical environment. Close observation of nearly any barren surface reveals patchy coverings of these vegetative growths in a variety of colours: white, green, grey or orange. As primitive plants lichens are renowned for their ability to survive in environments that others cannot. A symbiotic creation resulting from the union of an algae and fungus, lichens take on many forms, functions and roles. Among the first colonizers of newly formed surfaces lichens play an important part in the generation of soil by initiating decomposition processes. They are able to thrive on nutrient poor substrates by taking sustenance mainly from the air and ambient water and as poor competitors lichens are often found in open habitats that lack vegetation (e.g. geologic substrate).

The majority of plants and animals found on Rapa Nui have been introduced over time by various agents of dispersal (Flenley, 1993). Lichens could have arrived on the island as vegetative spores carried by man or bird, wind or sea (McCarthy, 1999). Records from the expeditions of the biologists Zahlbruckner (1928) and Skottsberg (1956) document the presence of lichen species on Easter Island. Skottsberg includes five endemic species in a list of 23 species for both Easter and Juan Fernandez Island, noting that such a small number of species illustrates an 'insufficient knowledge' of the island's lichen flora (Skottsberg, 1956).

The most recent and up to date catalogue of Pacific island lichen species is available on the Internet (Feurer, 2006). The checklist includes all modern classifications of Rapa Nui's lichen species, which now stands at fifty in total. The publication regroups Physcia picta from Zahlbruckner and Skottberg's reports as Dirinaria picta (Feurer, 2006).

Historically, the presence of lichens is mentioned by numerous accounts. Islanders had distinguished the existence and import of lichens. In a creation chant recorded in 1886 and later published, lichens are recognized for their function in life: "Himahima-marao ki ai ki roto te Kihi-tupu-henua, ka pu te kihihi"; was translated into, "Himahima-marao by copulating with Lichen-growing-on-the-soil produced the lichen" (Metraux, 1940). Although Metraux (1940) regarded the translation as a 'free' and possible misinterpretation of the island's language at that time, the word 'kihihi' appears repeatedly for 'lichen' in further texts . In later verses the chant goes on to mention ferns, trees, grasses and plants, by an assortment of different names but suggests that a system of plant form differentiation enabled lichens to be easily recognized as a separate group by islanders.

Lichens gain mention in further accounts of the island's history. When Pierre Loti visited the island in 1872 he noticed how moai appeared to be 'gnawed by lichen…' and during an expedition to Rapa Nui Maziere (1968) described how islanders held certain moai in regard: 'The ones lichen does not grow on are still alive.' Moai, a fundamental element of Rapa Nui's widespread intrigue, play a further role in the lichen story. A 'briefer article' within the 1885 edition of the Botanical Gazette describes how lichens were discovered growing on stone statues returned to the US Smithsonian Institute by the steamer 'Mohican'. The lichens were identified by a Mr. Henry Willey and classified as: Usnea barbata; Physcia stellaris; and Parmelia laeviagata (Knowlton, 1885). Parmelia laevigata has since been reclassified and is now listed by the modern online checklist as Parmotrema laevigata.

Lichenometry (or lichenometric dating), the use of lichen size to determine the age of a substrate, was pioneered by geologist Roland Beschel during research in the Alps (Beschel, 1950). Since this original conception the method has been highly modified, however, the fundamental assumption remains the same: that a relationship exists between the diameter of the largest lichen thallus growing on a surface and the amount of time that the surface has been exposed to colonization. The ratio of time to thallus size makes possible the determination of a growth rate (Beschel, 1950; Webber and Andrews, 1973).

Lichenometry has been primarily used within the field of geomorphology to date glacial deposits and features in arctic and alpine environments and relies upon measurements of the species Rhizocarpon geographicum (Beschel, 1960; Innes, 1985; Henriquez & Caamano, 2002; McCarthy, 2003; Winchester, 2004). Lichens can provide a minimum value of how long a surface has remained immobile and undisturbed. In an archaeological context, lichenometry has been used for dating purposes (Follmann 1961). Ideally used in tandem with other dating techniques and/or historical data, it can provide valuable insights when calculating substrate age estimates.

The lichenologist Gerhard Follmann arrived on Rapa Nui in 1961. Follmann studied the island's lichens from a biological point of view, but he was also able to calculate dates for certain archaeological structures (i.e. to conduct lichenometry), and his work, following on the methods of Beschel, can be described as the first lichenometric work to be conducted on the island. The three lichen species Follmann used for his lichenometric studies included those previously described by Zahlbruckner and Skottsberg: Diploschistes anactinus, Lecidea paschalis and Physcia picta.