Nutrient and Organic Matter Inputs to Hawaiian Anchialine Ponds: Influences of N-Fixing and Non-N-Fixing Trees1

Invasive nitrogen-fixing plants often increase energy and nutrient inputs to both terrestrial and aquatic ecosystems via litterfall, and these effects may be more pronounced in areas lacking native N2-fixers. We examined organic matter and nutrient inputs to and around anchialine ponds on Hawai‘i Island’s leeward coast from an invasive, N2-fixing tree, Prosopis pallida, and a non-N2-fixing tree, Thespesia populnea. On a monthly basis we quantified: (1) tree basal area and stem density surrounding the ponds, and (2) nutrient content and quantity of P. pallida and T. populnea litter inputs. Tree density and basal area significantly predicted litterfall mass inputs for both species, but there was no difference in mean total annual litterfall mass, foliar N concentration, or litterfall N between the pond types. Time did not significantly influence litterfall mass, C, or P but did affect litterfall N, likely due to the production of P. pallida pods. In contrast, P concentrations were three times greater in T. populnea litter than in P. pallida litter, resulting in significantly higher P deposition where T. populnea was the prevalent tree species. Because this deposition of labile P by the non-N2-fixing T. populnea has the potential to strongly influence water quality and soil chemistry, its replacement by P. pallida in tree communities of lowland Hawai‘i may alter the functioning of anchialine pond ecosystems.