The prehistoric economy of Rapa Nui relied primarily on the production of the tuber crops of sweet potato (Ipomoea batatas), dryland taro (Colocasia esculenta), yam (Dioscorea spp.), and ti (Cordyline). All of these crops, with the possible exception of sweet potato, arrived with the first immigrant population to Rapa Nui around AD 800 and were the staple food for this society over the next 1000 years. It has been proposed that sweet potato was a secondary introduction to the island sometime around the thirteenth century (Wallin et al. 2005). In part this is based on 1 sigma calibrated date ranges of AD 988-1155 and AD 1409-1440 that bracket sweet potato fragments found in a cave deposit on Mangaia, central Polynesia (Hather & Kirch 1991; for a discussion see Green 2005, and Ladefoged et al. 2005). These starchy tubers were planted in small gardens, open fields and intensively cultivated plantations in both the coastal and elevated regions of Rapa Nui (Stevenson & Haoa 1998; Stevenson et al. 1999, 2002). In general, people were dispersed among the agricultural fields in single family or multiple family hamlets and produced food for household needs. In remote field systems located away from domestic sites, surplus foods were probably produced to support communal ceremonies and temple building projects. In this paper, we examine the character of agriculture on Rapa Nui, based on the age and internal structure of one garden through the analysis of a 100m long soil profile located at the base of Maunga Orito (Figure 1).
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Agriculture on Rapa Nui is dependent upon suitable soil conditions (see Ladefoged et al. 2005) and annual rainfall to supply moisture. Archaeological surveys have identified water diversion alignments but there is no evidence of irrigation (McCoy 1976; Stevenson 1997). In lowland coastal areas the average annual rainfall is c. 1100mm (Mataveri Station, Figure 1). However, deviations from the mean value can be significant. Within the 25-year recording period, dispersions from the mean value can be as much as 200mm and result in periods of high humidity or reduced moisture. In the latter case, this would have resulted in significant declines in crop production in open and unprotected settings. Hunt and Lipo (2001) note that moisture fluctuations do not appear to be predictable and therefore introduce a degree of uncertainty into agricultural production.
These difficulties are exacerbated by two additional factors. The first is the presence of a nearly continuous onshore wind that removes ground moisture by evapotranspiration. In very dry years, a high rate of evapotranspiration can even result in a moisture deficit for the year (Louwagie 2003). The second factor is the high soil moisture permeability in some areas (Louwagie 2003; Louwagie & Langohr 2002). These problems may not have existed during the early part of prehistory when the island was forested with palm and understory species (Flenley 1998; Flenley et al. 1991; Orliac 2000; Orliac & Orliac 1998). Swidden fields within a forest environment would have been well protected (Mieth & Bork 2003). However, with the extensive removal of trees and surface ground cover, higher evapotranspiration, reduced soil moisture retention and soil erosion became more significant impediments to a successful harvest.
Strategies for agricultural production
The prehistoric people of Rapa Nui would have quickly recognised the impacts of deforestation and rainfall variability on the productive potential of the agricultural system. These limiting and unpredictable factors would have introduced an uncertainty about the outcome of farming from year to year. As a result, the Rapa Nui implemented a set of agricultural innovations to counteract these negative effects. These developments included the use of a surface lithic mulch to facilitate water permeability, …