Discussion: the potential of lichenometry on Rapa Nui

The application of lichenometry is a potentially valuable asset to aid the archaeological dating of select structures on Rapa Nui. Such a scenario is most promising "where a consistent methodology can be applied involving one species on one type of substrate within a restricted geographical area" (Aptroot and James, 2002). This pilot study establishes such a dataset that can be built upon through future research. There are however many factors that limit the potential of lichenometry as a dating method. As many researchers have previously discussed, a multitude of environmental and biological factors affect lichen growth and lichenometry experimental design (Jochimsen, 1973; Webber & Andrews, 1973; Innes, 1985; Winchester, 2000). Furthermore, the design and execution of lichenometry experiments must be carefully scrutinized.

Environmental factors
The environmental characteristics of Vinapu and ahu Tahira include the site's extreme climatic variation and exposure to sun, wind, rain and sea spray. These will undoubtedly overtime affect the microclimate of the basaltic rock surfaces and what ever is able to grow there. Katherine Routledge wrote of the workmanship and effort that went into producing Ahu Tahira's 'beautifully' finished, and almost smooth main wall basalt surface (Routledge, 1919).

The site is also interestingly in close proximity to the island's main airport and runway, but the chance of aviation fuel vapor polluting local lichen populations is highly unlikely given the current flight frequency and the island's location. Sources of air pollution in the future could come from increased vehicular traffic as the island's population increases.

Biological factors
Lichen growth must be considered in terms of physiological factors (Jochimsen, 1973). Factors that need to be taken into consideration include: symbiotic nature of lichen; individuality of lichen thalli; means of propagation and dispersal; growth rate and senescence; competition between species and individuals; and the survival or dispersal of mature lichen. The growth of lichen may be interrupted or terminated by sudden earth movements, the encroachment of vegetation, or the trampling of animals. Follmann noted that lichens on the island had a slow growth rate for sub-tropical species. He accredited this to high rock porosity, low nutrient availability and strong oceanic winds (Follmann, 1961).

Experimental factors A main advantage of conducting lichenometry is the ease with which measurements can be made in the field. Difficulties begin when incorrect species are identified - there are failures and errors in the recording of thalli - and the incorrect interpretation of lichen data. A further difficulty of conducting lichenometry on Rapa Nui is that few, if any dated reference lichen substrates exist. Therefore estimating a date for lichen colonization becomes problematical. Direct measurements of lichen growth over a defined time period can help to remedy this situation. In Wales data from churchyards was used to provide extra evidence of lichen growth rates when lichenometric methods were used to date landslide activity (Winchester & Chaujar, 2002). The authors did note that problems were inherent when comparing between the churchyard and landslide sites, yet the baseline data collected was imperative to the study.

An additional pilot study examined lichen growth in Hanga Roa's churchyard, the results of which can be seen in the appendix to this report. The mean growth rate calculated for the unidentified graveyard species was calculated as 1.97 mm yr-1. Complications remain unanswered for the churchyard pilot study such as whether or not residents clean lichens away from their relatives' headstones, and whether or not headstone material is quarried or collected. The growth rate calculated at the churchyard could not be applied to Ahu Tahira, due to the major environmental differences between each site, and therefore also potentially lichen species composition.

Further considerations for lichenometry on Rapa Nui Follmann as a biologist collected primary data on lichens that led him to be able to conduct lichenometric studies. In his publication are some extremely valuable and interesting results, yet Follmann concluded: 'It is none of our (author's) business to discuss the archaeological or ethnological foundations of these observations' (Follmann, 1961). However, many people have cited his work and Follmann is often quoted as having, 'dated the stone statues of Easter Island' (Innes, 1985; Armstrong, 2004). The dates that Follmann did calculate, for the age of certain moai and ahu are very close to those estimated by Father Sebastian Englert when he established, using genealogical records. a time line of the island's history. Statue and ahu ages were placed between 380-480 years. The age Follmann calculated for Vinapu (exact ahu not stated) was 460 yrs +/- 7% (Diploschistes anactinus) and 380 yrs +/- 8% (Physcia picta). As Follmann acknowledged, human alterations and renovations to many sites and statues alter rock surfaces and interfere with lichen growth, which can render results unreliable (Follmann, 1961.). This very fact causes some to question Follmann's attempt at lichenometry and his rationalization of the results (Aptroot & James, 2002). Ideally a wider investigation would have allowed clearer conclusions to be drawn but it seems that at the time, resources were extremely limited and lichenometry was still very much a developing field.

In the case of Rapa Nui it is interesting to note that since the German-Franco expedition, 'Projekt Osterinsel' of 1988, when a silicone mould was made of the north end of Ahu Tahira, a freshly exposed rock surface was created. Although the project in reality stripped the area of the ahu of a significant lichen population, it could now be monitored for lichen recolonization, establishment and growth. Collection of this type of data is exactly what is needed as a basis for future lichenometric studies, and what a number of researchers have already called for (Van Tilburg, 1990a; Aptroot & James, 2002). Any possible chemical imbalances caused by residues of the mould taking process would need to be considered and possibly tested for during such a monitoring project (Smith, 2005. Pers. comm), a process that at Ahu Tahira used a methyl cellulose skin and silicone mould (Kaufmann, 1990; Van Tilburg, 1990b).

The use of photography to assess lichen populations is an interesting prospect. Simple photogrammetric methods have been employed in the past to measure lichen growth over certain time periods (Brink, 1973). Digital photography has also been used to assess lichen growth, in terms of environmental change (Purvis et al., 2002). The study, using 15 permanent quadrats and annual photographs over a 6-yr time period, was able to process images and distinguish growth rate differences between different species. By using a single color for each species, and then correlating the area of each lichen with a range of environmental characteristics (e.g. temperature, rainfall, pH and pollution data such as SO2 levels), the researchers were able to detect changes due to changing conditions (Purvis et al., 2002). If, on Rapa Nui, one or more inconspicuous sites were established where permanent quadrats could be placed and photographed on an annual basis, evidence could be collected to provide a direct indication of growth rates over time. From the photographs taken during Heyerdahl's 1950 expedition, lichen growth at Ahu Tahira is clearly visible. The popular and photogenic location appears in many publications. Therefore the possibility exists of measuring lichen growth in-between photographs, once scale is accounted for and calibrated, to determine growth patterns.

A long-term, lichen-monitoring project on the island has many potential beneficiaries. Not only would the data add to debates regarding weathering and erosion of the monumental archaeological remains and rock art (Van Tilburg, 1990a & 1990b; Tratebas & Chapman, 1996; Milstein, 2005), but also global climate change and, from a purely biological point of view, Pacific island biodiversity. Lichens, as previously discussed, are extremely sensitive to environmental changes and could act as indicators of divergence in the farthest reaches of the Pacific.

This study highlights the potential that a site such as Ahu Tahira has for a long-term lichen-monitoring project, and how in turn, this could impact future lichenometric and archaeological studies on Rapa Nui. The use of digital photography as a method of collecting much needed baseline data is suggested as part of joint initiative between upcoming TAO high school groups, the museum and Chilean organizations with a presence on the island (e.g. CONAF).