Nitrogen (N) fixing trees are frequently used to restore soil organic matter and nutrient cycling functions to degraded ecosystems. Yet, because elevated soil N availability can increase the presence of non-native species in many systems, N-fixers could enhance invasive species abundance during restoration. Although N-fixers are lumped into a single functional group, the quality and quantity of the plant material they produce and the rate at which they add N to the cycling pool likely varies which could affect the composition and structure of the associated community. This talk will address the following questions: (1) How does N-cycling differ among three common N-fixing tree species? And (2) Does the understory composition and structure reflect differences in N cycling rates among the fixer species? To address these questions, we investigated planted stands of two Hawaiian native N-fixing trees (Acacia koa and Sophora chrysophylla) and ‘natural’ stands of an invasive N-fixing tree (Morella faya) in burned seasonal submontane woodlands in Hawaii Volcanoes National Park. We measured the relative availability of nitrogen in the soil pool and understory plant assemblage. We also characterized the rate and amount of N-cycling in these stands in the field and using long-term laboratory soil incubations.
We found that N cycles very differently under the three N-fixing tree species and that this corresponds with differences in the understories that assemble beneath them. High exotic cover common to S. chrysophylla stands is associated with faster N-cycling and greater N availability compared to all other site types. Incubated S. chrysophylla soils mineralized almost twice as much N as any other soil type over the course of the experiment and in the field, these stands supported more cover and biomass of non-native plant species than either of the other two N-fixing tree species. The comparatively high-N environment under S. chrysophylla suggests that litter quality (low C:N) in combination with light availability may be more important than litter quantity (which is generally lower for S. chrysophylla than the other species) in determining the success of weedy N-loving exotic species in the understory.