Long-term impacts of grass invasion and fire, Hawaii National Park

1. Despite decades of research documenting invasive plant impacts, very little research has followed invaded sites over decades and even fewer studies have contrasted how long- term compositional impacts from invasion compare to ecosystem-process impacts over time.
2. Using direct measurements of composition and ecosystems processes, we evaluate how ecosystem structure, net primary production (ANPP), and aboveground and soil nutrient pools change over 25 years since fire and plant invasions disrupted seasonally dry Hawaiian woodlands. We compare structure and function between intact primary woodland that has not burned and is largely native-species dominated, with sites that had been the same woodland type but burned in alien grass fueled fires in the 1970s and 80s. The sites have not experienced any further fires since 1987. They were studied by two of the authors intensively in the 1990s which forms a basis for evaluation of the longterm impacts.
3. We found that community composition and structure continue to be dramatically changed by the initial invasions and fires that occurred 25 years ago. ANPP and N storage, which showed dramatic decreases in the initial years after invasion and fire, have since increased and are now indistinguishable from values measured in intact woodlands. While soil carbon pools were resilient to both invasion and fire over time, plant carbon storage has not recovered to the pre-fire state.
4. ANPP has recovered because of secondary invasion of degraded sites by an N-fixing tree rather than because of recovery of native species. This invasive N-fixing tree is unlikely to return C storage of the invaded sites to those of uninvaded woodland because of its tissue characteristics.
5. We conclude that fire and secondary plant invasions have moved the ecosystem into a new persistent state for structure and plant C storage, despite dramatic recovery of ANPP and N pools. The fact that N pools and ANPP recover because of secondary invasion highlights the unpredictability of ecosystem trajectories in the face of dramatically altered regional species pools.