Ecosystem impacts of invasion by an N-fixing tree in Hawaii: Sure, ecosystem nitrogen stocks increase, but what about stocks of phosphorus?

Prior studies demonstrate coincidental increases in aboveground productivity and N availability when N-fixing species invade ecosystems. The impact of such invasions on aboveground stocks and availability of phosphorus – a nutrient often limiting growth of N-fixers – is less clear, particularly regarding acquisition and use of P in ecosystems. Specifically, do N-fixing species increase bio-available stocks of P through increased mineral soil exploitation, or are they capable of maintaining higher rates of production with similar amounts of available P through higher P-use efficiency? To address this, we estimated aboveground bio-, C, N, and P mass, as well as ANPP and N and P mass contributions to ANPP, in native-dominated forest stands and adjacent forest stands invaded by the N-fixing tree, Falcataria moluccana, in lowland wet forests of Hawai’i. As expected, ANPP was 1 to 6 times greater, and N mass of ANPP 3 to 50 times greater, in Falcataria-invaded stands compared to native-dominated stands. Regarding P, aboveground stocks were 2 to 42 times greater, and P-ANPP 2 to 28 times greater, in Falcataria-invaded stands compared to native stands. In contrast, P-use efficiency (defined here as biomass ANPP divided by P-mass ANPP) was 1 to 3 times greater in native stands compared to Falcataria stands. Results indicate that, rather than increasing efficiency in the use of P, invasion by this particular N-fixing tree species dramatically increases the overall ecosystem stocks and cycling of bio-available P, both for itself and for all other organisms in the system.