Chapter 3 



Natural History of Terrestrial Vascular 

 Plants of Enewetak Atoll 



JANET O. LAMBERSON 



U. S. Enuironmental Protection Agenci; 



Hatfield Marine Science Center. Newport. Oregon 97365 



INTRODUCTION 



Enewetak Atoll has evolved over geological time from 

 a raised limestone island environment supporting mangrove 

 swamps and an upland mixed forest to a low coral sand 

 island habitat with typical strand vegetation. The 128 

 species of plants recorded from the atoll include many 

 introduced weeds and ornamental or food plants, although 

 43% of the flora is considered to be indigenous to 

 Micronesia. 



The dry, windy season from November through April 

 causes many of the plants to drop some or all of their 

 leaves or to die back completely. Summer — June through 

 September — is the season of maximum plant growth and 

 more flower and seed production. 



The effects of World War II and post-war military 

 activity, the nuclear test program from 1948 to 1958, and 

 the radiological cleanup and rehabilitation for the return of 

 the people of Enewetak have been significant with regard 

 to the vegetation. Many species were accidentally or inten- 

 tionally introduced to the atoll, and some species have 

 disappeared. The vegetation was greatly altered because of 

 destruction of habitat with removal of soil and nutrients 

 and exposure of the plants to salt spray and drying winds. 

 Several collections and studies of the flora have been 

 reported. 



This chapter describes stages in the succession of the 

 vegetation following disturbance. The vegetation of the 

 islands during the period 1975 to 1977 is also described 

 in some detail. 



ENVIRONMENT AND BIOGEOGRAPHY 



The islands and shallow reefs of Enewetak Atoll are 

 the top of a limestone cap approximately 1 km thick, rest- 

 ing on the basalt remains of an extinct volcano which was 

 active during the pre-Tertiary geological period. Core sam- 



ples from drilling on the island of Medren and on the 

 former island of Elugclab include basalt encountered at 

 depths of 1271 and 1405 m, respectively. The samples 

 also show that during the reef's geological history it was 

 exposed and submerged several times as a result of fluc- 

 tuations in sea level (Emery, Tracey, and Ladd, 1954; 

 Tracy and Ladd, 1974). 



Analysis of pollen grains in the cores (Leopold, 1969) 

 showed that during Miocene times Enewetak was a raised 

 limestone island supporting an intertidal mangrove swamp, 

 mangrove depressions on beach ridges, an upland forest of 

 salt-intolerant plants, and a typical strand vegetation (Fos- 

 berg, 1960) similar to that presently found at Enewetak. 

 Leopold (1969) listed 17 angiosperm genera among 54 

 species of plants found as fossil remains in the cores, 

 including Pandanus. Pisonia, Terminalia. Tournefortia, 

 Cordia, Morinda, and Guettarda. 



The islands of Enewetak Atoll have apparently reached 

 their present form within the Holocene. The reef around 

 the atoll grew to keep pace with the rising sea level, then 

 was eroded by wave action during a more recent drop in 

 sea level within the past 2000 to 4000 years (Buddemeier, 

 Smith, and Kinzie, 1975). The terrestrial plants that occur 

 now at Enewetak have been introduced by chance or inten- 

 tionally as seeds and cuttings brought to the atoll by ocean 

 currents, wind, birds, and people. 



In comparison with other atolls in the Marshalls, 

 Enewetak has fewer species of plants, and to those familiar 

 with the dense undergrowth and luxuriant ferns of the 

 southern atolls, Enewetak may seem somewhat barren. 

 Only 79 out of 155 species of plants, including those cul- 

 tivated in gardens, listed by Fosberg (1955) in the northern 

 Marshall Islands have been reported from Enewetak. 

 Species indigenous to Micronesia comprise 43% of the 

 total flora (55 species). (See Tables 1 through 4.) Sf)eci- 

 mens of all species in Table 1, except Cocos nucifera and 

 others noted in Table 4 were deposited in the reference 

 collection at the Mid-Pacific Laboratory at Enewetak. Addi- 

 tional specimens from Enewetak, including the new species 

 listed, are deposited at the Bishop Museum (BPBM) and at 

 the U. S. National Museum of Natural History (USNM). 

 The taxonomic nomenclature for the Dicotyledonae and 



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