Invasion by a N2-fixing Tree Alters Function and Structure in Wet Lowland Forests of Hawaii

Invasive species pose major threats to the integrity and functioning of ecosystems. When such species alter ecosystem processes, they have the potential to change the environmental context in which other species survive and reproduce and may also facilitate the invasion of additional species. We describe impacts of an invasive N2-fixing tree, Falcataria moluccana, on some of the last intact remnants of native wet lowland forest undergoing primary succession on 48-, 213-, and 300-yr-old lava flows of Kilauea Volcano on the island of Hawai‘i. We measured litterfall, soil nitrogen (N) and phosphorus (P) availability, light availability, species composition, and forest structure in native-dominated stands and in stands invaded by Falcataria. Litter inputs increased 1.3–8.6 times, N mass of litterfall increased 4–55 times, and P mass of litterfall increased 2–28 times in invaded stands relative to native stands. C:N and C:P ratios of litterfall were lower, and N:P ratios higher, in invaded stands relative to native stands. Resin-captured soil N and P values were 17–121 and 2–24 times greater, respectively, in invaded stands relative to native stands on each of the three lava flows. Native species accounted for nearly 100% of total basal area and stem density in native stands, while alien species accounted for 68– 99% of total basal area, and 82–91% of total stem density, in invaded stands. Compositional changes following Falcataria invasion were due both to increases in alien species, particularly Psidium cattleianum, and decreases in native species, particularly Metrosideros polymorpha. Results provide a clear example of how invasive tree species, by modifying the function and structure of the ecosystems that they invade, can facilitate invasion by additional nonnative species and eliminate dominant native species. Given the rarity and limited extent of remaining native-dominated wet lowland forests in Hawaii, and the degree to which Falcataria invasion alters them, we expect that the continued existence of these unique ecosystems will be determined, in large part, by the spread of this invasive species.