Vegetation dynamics at Raraku Lake catchment (Easter Island) during the past 34,000years

Abstract

Easter Island is a paradigmatic example of human impact on ecosystems. The role of climate changes in recent vegetation shifts has commonly been rejected without proper assessment. A palynological study of a long sediment core from Raraku Lake documents the vegetation dynamics for the last 34 ka and investigates their driving forces, particularly the effects of climate variability on vegetation changes. Significant relationships between pollen assemblage changes and sedimentary and geochemical proxies demonstrate the rapid response of vegetation to lake crater basin hydrology and climatic changes. The lake surroundings were occupied by an open mixed palm grove during the Last Glacial period. Poaceae and Sophora increased at the expense of palms and Triumfetta, and Coprosma practically disappeared, in response to slightly wetter and/or colder climate during the Last Glacial Maximum. Palms and Triumfetta thrived in a warmer and/or drier climate during the deglaciation. Minor vegetation changes (a slight increase in Sophora and a drop in Asteraceae and Poaceae) occurred between 13.2 and 11.8 cal. ka BP and can be related to rapid changes in the Younger Dryas chronozone. The increase in herbaceous taxa indicates a gradual shallowing of the lake and development of a mire during the Holocene, caused by sediment infilling and warmer and drier climate. Relatively rapid vegetation changes in the Holocene were caused by climate and by plant succession on the expanding mire. The rates of vegetation change observed in the mire were similar to those at the initial stages of human impact identified in a previous study. These results reveal significant vegetation changes prior to human presence, due to the interplay of climate variations (temperature and moisture), changes in lake basin form by infilling and intrinsic dynamics of plant succession. Hence, the potential contribution of these factors in vegetation shifts during the period of human presence should not be neglected.