Vegetation History of Laysan Island, Northwestern Hawaiian Islands

Paleoenvironmental investigations were undertaken on Laysan Island in the remote Northwestern Hawaiian Islands to investigate its flora before historical observations. Substantial impacts occurred to the island as a result of late nineteenthand early twentieth-century guano mining, commercial feather collecting, and denudation of vegetation by feral rabbits. An account of Laysan’s historically known vegetation is presented, followed by discussion of results from the investigation of a 16.41-m sediment core from Laysan’s central hypersaline lake. The 7,000-year pollen and seed record, besides indicating the former importance of Pritchardia palms on Laysan, showed the former presence of seven previously unknown taxa, only four of which could be identified. Diatom analysis indicated fresh to brackish lake water during the early Holocene, a finding supported by the mollusk assemblage. Diatom diversity gradually decreased over time until there is a near monoculture, with types indicating a gradual increase of salinity. Hypersaline conditions were first recognizable near the top of the sequence with the appearance of Artemia zooplankton. Generally wetter conditions seem to have characterized the island before about 5,150 yr B.P., with drier conditions thereafter. The pollen record also suggests two possibly very brief periods of much drier conditions, conceivably related to El Niño– Southern Oscillation episodes. A paleoenvironmental study, using a wetland/lake sediment coring methodology (e.g., Binford and Leyden 1987, Flenley et al. 1991, Colinvaux et al. 1998, Athens et al. 2002, Athens and Ward 2004), was conducted on Laysan Island in the far Northwestern Hawaiian Islands (NWHI) to investigate vegetation and other environmental attributes before human activities on the island (Figure 1). Particularly as a result of guano-mining operations starting in the early 1890s, and the subsequent introduction of rabbits in about 1903, many of the native plant taxa disappeared during the first two decades of the twentieth century (for the island’s history see Ely and Clapp 1973, Rauzon 2001, Athens et al. 2004). In 1923, the Tanager Expedition (Christophersen and Caum 1931) documented only four native plant species remaining on the almost completely denuded island (Portulaca lutea, Santalum ellipticum, Sesuvium portulacastrum, and Tribulus cistoides). Morin and Conant (1998:1) cautioned that the Laysan ecosystem may have been altered even before the start of guano mining because ‘‘there were numerous ship landings and at least one shipwreck dating before 1859.’’ Because of the rapid disappearance of lowland forests that occurred with Polynesian settlement of the main Hawaiian Islands (Athens 1997, Athens et al. 2002), it is reasonable to regard the same kind of disappearance as a possibility for Laysan if there was prehistoric contact or settlement. The apparent absence of traditional archaeological sites on Laysan suggests that there was no significant prehistoric occupation (though formal survey has not been undertaken), but prehistoric Hawaiians may have been aware of Laysan’s existence. They are known to have made regPacific Science (2007), vol. 61, no. 1:17–37 : 2007 by University of Hawai‘i Press All rights reserved 1 Manuscript accepted 2 March 2006. 2 International Archaeological Research Institute, Inc., 2081 Young Street, Honolulu, Hawai‘i 96826-2231 (e-mail: [email protected]). 3 The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates. 4 Department of Biological Sciences, Northern Arizona University, P.O. Box 5640, Flagstaff, Arizona