The mystery of the trees of Rapa Nui (Easter Island) is a complex problem within a dynamic ecosystem. With new physical cores from the volcanic crater lake Rano Kao, this dissertation uncovers a detailed 15,000 year ecological history of Easter Island and its climatic variability. New radiocarbon dating methods establish a more precise chronology which shows that the island before human habitation was very different than what we know today. It had a simple but prolific ecology that transitioned into a barren grassland. What factors caused the transition are unclear but are likely to be human related. As the forests slowly disappeared, it could have triggered changes in regional and local climate, particularly rainfall, which generally leads to a rapid loss of ecosystems. With virtually stable climate conditions over the last 2,000 years, Easter Island has lost 33 species of plants including the giant palms, and still has not recovered today. This research challenges the previously accepted theory that humans deforested the island for the sole purpose of moving the Moai around, and focuses rather on uncovering the role of climate change that may have altered the ecosystem. New cores were obtained in 2005 from Rano Kao that were radiocarbon dated using scirpus seeds. The nine meters of core were sampled for oxygen isotope analysis of the lake water changes, a new science to the island. Detailed palynological studies of pollen and non-pollen palynomorphs uncovered changing vegetation patterns that aligned with a 700 year drought cycle. Conclusions support a cool-dry event that occurred 545 years ago, at which time the giant palms disappeared. While it was not the focus of this study, evidence confirms human occupation for more than 500 years before the disappearance of the trees.