The vanished palm trees of Easter Island : new radiocarbon and phytolith data
Résumé
Easter Island was formerly covered with palm trees that constituted one of the most distinctive attributes of the landscape. Twelve 14C dates were obtained from fragments of wood and of nuts discovered in archaeological sites and in cracks in cliffs. These dates, which cover a period between AD 1210 and 1440, are mainly centred around the 14th century which suggests that the population of palm trees was still important at this time. The lack of dates after AD 1450 suggests that the palm forest greatly decreased during the 15th century, without disappearing completely, as is proved by the European accounts. The existence of palm trees on Rapa Nui is confirmed by the presence of numerous palm phytoliths in archaeological sediments. Phytoliths are biogenic opal particles produced by plants. Because of their chemical composition, they are usually well preserved even in sediments unfavourable to the preservation of organic remains. Palm (Arecaceae) produce great quantities of phytoliths, including a very characteristic “spherical echinate” morphotype. The morphometric analysis of that kind of phytolith made it possible to improve our knowledge of Paschalococos disperta, the Rapanui extinct palm. The statistical comparison of fossil Easter Island palm phytoliths with phytoliths extracted from various palm species (Jubaea chilensis, Juania australis, Cocos nucifera, various species of Pritchardia) showed that phytolith assemblages produced by Jubaea chilensis are close to those from Easter Island sediments. Nevertheless, because of the differences between the two pools of data, we put forward the hypothesis that more than one species grew on the Island. Moreover, phytolith morphotypes sometimes vary from one part of the plant to another. We investigate the differences between trunk and leaf phytolith spectra, in order to determine which parts of plants are involved in archaeological deposits. In addition to paleoethnobotanical implications, the characterisation of leaves versus stem (which is currently impossible by wood anatomy criteria) will improve the interpretation of radiocarbon data. Indeed, if the trunk is a long-lasting organ that can sometimes be several centuries old, leaves represent a shorter period, and thus are more accurate for radiocarbon dating.