Understory Succession Following a Dieback of Myrica faya in Hawai'i Volcanoes National Park

Abstract

Studies of invasion by the introduced nitrogen-fixing tree Myrica faya Aiton in Hawai'i Volcanoes National Park have led to predictions that the nitrogen-rich soil M faya creates will promote invasion by nonindigenous plant species. An insect-caused dieback of M. faya that began in the late 1980s provides an opportunity to test this hypothesis. We compared· percentage cover and density of all plant species under live and dead M. faya, as well as total nitrogen in soil and plant tissue. Mean percentage cover of four common species increased significantly, and no species decreased in cover after dieback. Cover of native shrubs and herbs increased from 4.8 to 15.2%, largely due to the spread of Carex wahuensis C.A. Mey, and introduced grasses increased from 2.3 to 14.1%. Density of native shrubs did not differ beneath live and dead M. faya, but immature introduced grass individuals were significantly more numerous beneath dead M. faya. We found no differences in total nitrogen in soil or plant tissue collected beneath live versus dead M. faya. Beneath dead M. faya, cover of C. wahuensis increased with total soil N, and introduced grass cover decreased. This surprising result may be the legacy of shading effects from the live M. faya canopies, for which total soil N may be an indicator. Success of grass seedlings compared with failure of native shrubs to recruit from seed suggests that dieback promotes nonnative grass species. Replacement of M. faya with introduced grasses may greatly increase fire risk.