An Invader Alters Germination and Growth of Native Dominant Tree in Hawai'i

Myrica faya, an introduced nitrogen—fixing tree, is rapidly invading volcanically disturbed forests in Hawaii Volcanoes National Park on the island of Hawai'i. We investigated the direct effects of Myrica on the establishment and growth of the native tree, Metrosideros polymorpha, in a forest that is recovering from burial under a 10—200 cm deep layer of volcanic cinder. The number of Metrosideros seedlings found under Metrosideros trees, in the open, and under Myrica trees was 0.12/m 2 , and 0.01/m 2 , and zero, respectively. Myrica litter inhibited Metrosideros germination in field experiments; germination occurred only when Myrica litter was removed. In open sites, the addition of nitrogen—rich Myrica soils increased dry mass accumulation of transplanted Metrosideros seedlings. Shade from Myrica canopies (or shade cloth) increased survivorship and height growth but not dry mass accumulation of Metrosideros seedlings. Isolated Metrosideros trees increased diameter growth in response to nitrogen fertilizer but not in response to the presence of adjacent Myrica trees, despite previous findings that Myrica trees elevate available nitrogen in the soil. Myrica basal diameter and height growth were consistently greater than that of Metrosideros in all size classes. Although Myrica does not readily invade closed, late—successional Metrosideros forests, on young, volcanically disturbed soils it is rapidly establishing dense, monospecific stands under which Metrosideros does not regenerate.