Diversity and distribution of indigenous bradyrhizobia populations associated with Acacia koa in Hawai'i

Acacia koa is a tree legume species endemic to Hawaii that nodulates with bacteria of the genus Bradyrhizobium . Koa is one of eleven Hawaiian endemic legumes. As a dominant canopy species for a majority of Hawaiian forest ecosystems, koa's symbiotic interactions with the indigenous bradyrhizobia may be an important regulator of the nitrogen cycles retained within the ecosystem. Furthermore, koa is an important species in restoration, and effective symbiotic nitrogen fixation of young seedlings may enhance these efforts in reestablishing native forests. This dissertation characterizes bradyrhizobia associated with koa at different scales in order to describe the biological diversity of the symbiotic interactions of koa. The Hawaiian Archipelago is known for its wide range of environments where koa is established. I have identified adaptive traits from bradyrhizobia residing in these environments. On an island-scale, bradyrhizobia isolated from the older islands of Kauai and Oahu displayed higher aluminum and salt tolerances than isolates from the younger islands of Maui and Hawaii. Symbiotic interactions with Bradyrhizobium are often characterized with a broad host range of legumes. Hawaii has a wide range of endemic and exotic legumes. On Mauna Kea, koa and the exotic legume gorse (Ulex europaeus) reside within the same landscape and were found to interact with the same indigenous population of bradyrhizobia. On a landscape scale, symbiosis with promiscuous bradyrhizobia has contributed to the successful invasion of a noxious weed legume in this koa habitat. In wet mesic forests, I have discovered that many mature koa can nodulate on adventitious roots established within canopy pockets that contain trapped organic soils derived from decomposing heartwood and phyllode litter of the host tree. These canopy pockets were found to contain higher nutrient levels than the adjacent terrestrial soils. Within a single tree, canopy bradyrhizobia isolates made up a homogenous population that was distinct from the proximal terrestrial population. On a tree-scale, canopy nodulation may function in maintaining symbiosis under stresses occurring within the rhizosphere of the terrestrial environment. This research may provide future applications directed towards enhancing koa forest restoration throughout Hawaii.