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