Water and nitrogen use by invasive tree Prosopis pallida (kiawe) in Kīholo Bay, Hawaii: Implications for groundwater

Prior to European contact, coastal zones and near-shore upland zones in Hawaii were likely comprised of a diverse mix of native plants and plants introduced by the Polynesians. Today, dense stands of the non-native tree, kiawe (Prosopis pallida), dominate much of the coastal wetland and upland ecosystems of the Kona Coast. Kiawe (Prosopis pallida) is an N-fixing legume, introduced in the early 19th century, which now forms monospecific stands on much of lowland leeward Hawaii. Kiawe is also a tree that may use large amounts of water, essentially precluding its use by other plants. During primary succession on young Hawaiian lava flows, nitrogen is typically the nutrient most limiting to plant growth when water is in sufficient supply. Hence, processes that control N accumulation and loss in young soils are likely to affect terrestrial primary production in much of Hawaii. Currently we are following two areas at Kīholo Bay, a lowland site where the trees are close to sea level and are > 10 m tall and an upland site where the trees are 2-5 m tall, found at a much lower density and biomass, and where we have evidence of greater water and nutrient stress. In both the lowland and upland areas we have established five plots of 20 m radius (0.13 ha) in which there are dozens of trees. We aim to determine if the presence of this tree in our leeward Hawaii may have negative effects on water and nutrient availability in dryland and coastal ecosystems.

Our research focus at Kīholo addresses how water availability affects physiology and nitrogen fixation in the invasive legume Prosopis pallida, and feedbacks of this tree on soil chemistry. Physiological measurements, including measurements of leaf tissue chemistry, water potential, fluorescence, and sap oxygen isotopes of kiawe at upland and lowland sites are proceeding quarterly for a 2-year period. These data are supplemented by sapflux and climate observations including rainfall, temperature, and humidity at these sites collected constantly for the entire two-year period; and soil, rainfall and groundwater chemistry measurements collected quarterly. Continuous monitoring of weather data has been ongoing since December 2010, and six quarterly sampling events of all other measurements are complete. Results to date indicate both slower growth and less water availability at upland sites, where the trees may not be able to fix N as well, and are unable to access the deeper groundwater aquifer. Study begun in the last year examining N mineralization and release to groundwater from soils in Kiawe-dominated areas has shown large pools of biologically available N in soils at these sites, but that both mineralization and flow of this N to groundwater is strongly dependent on rainfall. This appears to have created a buildup of soil N at lowland sites, where loss of nutrients from soils is dependent on rainfall but addition of  N by Prosopis pallida  to soils (measured by litterfall collections) is not. A preliminary calculation of total groundwater use by this species at lowland sites has shown transpiration to be in excess of rainfall; further calculations will determine the proportion of total groundwater flow used by these stands. 

We are continuing sampling for a two year period because we are noticing strong seasonal differences in water and nutrient use. Work to quantify aboveground net primary productivity (litterfall mass and biomass) of the upland and lowland plots is now nearly complete. A nitrogen mineralization experiment is around half completed. ENDER work with the all other teams in a cross-cutting project has shown water availability to cause differences in metabolic compounds produced by Kiawe, in terms of their presence and abundance, and further work will continue in this area. In a collaborative effort with researchers from UC Berkeley, we are exploring the possibility of using stable isotope ratios of wood laid down in tree rings to determine whether Kiawe trees have altered the salinity of shallow groundwater over the 150 years they have been growing in this area.