Biological Invasion by Myrica Faya in Hawai'i: Plant Demography, Nitrogen Fixation, Ecosystem Effects

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Abstract
Myrica faya, an introduced actinorhizal nitrogen fixer, is invading young volcanic sites in Hawaii Volcanoes National Park. We examined the population biology of the invader and ecosystem—level consequences of its invasion in open—canopied forests resulting from volcanic cinder—fall. Although Myrica faya is nominally dioecious, both males and females produce large amounts of fruit that are utilized by a number of exotic and native birds, particularly the exotic Zosterops japonica. In areas of active colonization, Myrica seed rain under perch trees of the dominant native Metrosideros polymorpha ranged from 6 to 60 seeds · m—2 · yr—1; no seeds were captured in the open. Planted seeds of Myrica also germinated and established better under isolated individuals of Metrosideros than in the open. Diameter growth of Myrica is >15—fold greater than that of Metrosideros, and the Myrica population is increasing rapidly. Rates of nitrogen fixation were measured using the acetylene reduction assay calibrated with 15N. Myrica nodules reduced acetylene at between 5 and 20 µmol · g—1 · h—1, a rate that extrapolated to nitrogen fixation of 18 kg · ha—1 · yr—1 in a densely colonized site. By comparison, all native sources of nitrogen fixation summed to 0.2 kg · ha—1 · yr—1, and precipitation added —1 · yr—1. Measurements of litter decomposition and nitrogen release, soil nitrogen mineralization, and plant growth in bioassays all demonstrated that nitrogen fixed by Myrica becomes available to other organisms as well. We concluded that biological invasion by Myrica faya alters ecosystem—level properties in this young volcanic area; at least in this case, the demography and physiology of one species controls characteristics of a whole ecosystem.