Pilot Study: The lichens of Ahu Tahira

In July 2005 a volunteer research team consisting of Rapa Nui high school students and TAO instructors studied the lichens present at the archaeological site known as Vinapu. Work focused upon the back wall of a single ceremonial platform rock structure (Ahu Tahira). Constructed using large slabs of dressed basalt, the back wall of ahu Tahira comprises a high quantity of surface space that has been populated by various species of lichens. The main objectives of the project were to:
  1. map the wall at scale;
  2. photograph the wall at scale and with a high degree of detail;
  3. measure and record the distribution and nature of lichens upon an arbitrary square meter of the wall's surface

The site and ahu were chosen for a number of reasons. First, for lichenometric studies, historical data is essential. This was available for ahu Tahira at the local Sebastian Englert Museum in the form of written details and photographs from previous explorations of the site, principally those of Routledge (1919) and Heyerdahl (1955). Second, the site's relatively uniform and stable condition is historically documented, lending lichenometric research both scientific relevance as well as straightforward access for fieldwork. Before data collection began an introductory session to lichens and measuring techniques was held, during which methods were practiced and lichenometric principles discussed.

A 1 m x 1 m quadrat, divided into 10 cm squares was used and placed randomly on the planar surface of the west end wall of Ahu Tahira. Corner points were flagged for preceding sampling visits. Digital calipers were used to collect the width and length (mm) of all individual thalli within each square of the quadrant, and the data recorded was later collated into spreadsheets.

To document the site and to enable the future expansion of research, the back wall of ahu Tahira was mapped and photographed in resolution great enough for the study of discrete lichens. A composite and scaled photograph (1:50) was produced by maintaining a parallel relationship between the film plane of a digital single lens reflex camera (Canon 10D) and the back wall of Ahu Tahira, all at a consistent distance (4 meters) and focal length (80 millimeters). Approximately twenty-four images were stitched together to form the composite.

Mapping was conducted by establishing a datum and baseline, from which wall heights (base, basalt construction seams, top) and distances were taken. Plotted on graph paper using a coordinate system, the information was digitized in Arcview for integration with the photographic imagery.