Is Biological Nitrogen Fixation Strategy Tied to Invasive Success for Non-Native Woody Legumes in Hawai‘i?
Angalee Kirby
University of Hawaii at Hilo, Hilo, HI


III. Invasive Species & Biosecurity


Hawai‘i has been subject to biological invasion since the first settlers on the island chain. Invasive plant species have been shown to disrupt nutrient cycling and outcompete native flora. Many invasive trees have a nitrogen-fixing symbiosis, allowing them to contribute excess nitrogen to Hawaiian ecosystems that have evolved to survive in nitrogen limited conditions, facilitating further invasion by exotic plants that can utilize the nitrogen surplus. The spectrum of biological nitrogen fixation strategies includes over-regulation, obligate, facultative, and under-regulation. The present study investigates whether nitrogen fixation strategy is tied to the invasiveness of approximately twenty non-native woody legumes (Fabaceae) found in Hawai‘i, using the Hawai‘i/Pacific Weed Risk Assessment system to categorize trees as either high risk or low risk for invasion. In a greenhouse experiment trees are being treated with three levels of isotopically labelled nitrogen fertilizer, whose signature is distinct from atmospheric nitrogen, in order to calculate the percent of nitrogen derived from fixation. These data and relative growth rate data will explain which strategy each species uses in order to evaluate relationships between strategies and risk assessments. Results of this study will give insight into which plants should be monitored more carefully on the islands and which should have limited or restricted introduction to the islands, as well as contribute to the limited but growing knowledge of nitrogen fixation regulation in several species that have not yet been documented.