Effectiveness of cattail (Typha spp.) management techniques (mowing, burning, and herbiciding) depends on exogenous nitrogen inputs

Background/Question/Methods

In the Great Lakes Region, invasive wetland plants are often managed using fire, herbicide, mowing, or a combination of these, but long-term effectiveness across a range of exogenous N inputs is not well known.  Additionally, the impact of these management techniques to carbon storage and nutrient retention is poorly understood. MONDRIAN is an individual based model, simulating growth and competition for nutrients and light among individual ramets; N cycling both drives, and is driven by, plant growth, litter production, and biogeochemistry of litter and sediment organic matter. MONDRIAN was modified to model burning, mowing, and herbiciding as management techniques.  We modeled the effectiveness of these management techniques on controlling Typha xglauca ten years after invasion of a 3-species native community across a N loading gradient (4 to 30 g N m^-2 yr^-1).  Simulations ran for 15 years post invasion and included all combinations of burning, mowing, and herbiciding.  Mowing and herbiciding occurred at peak biomass, while burning occurred in the spring or fall.  Management lasted 1, 3, or 6 continuous years.  Invasion success as both invasive species NPP and NPP proportion were measured on a yearly basis, as well as N retention and C storage

Results/Conclusions

Before management techniques were implemented, Typha was unsuccessful at low N loading, but formed monotypic stands at high N.  For all treatments, the effectiveness of management depended on N loading. Herbiciding alone reduced invader biomass more than burning or mowing alone in eutrophic wetlands.  When treatments were combined, herbicide+burning resulted in the greatest decrease in invader success across the N loading gradient (up to 80% decrease in invader NPP at high N loading).  Overall, three years of management was often better than one year, though six years was seldom more effective than three years.  At low N loading, some management techniques (e.g., herbiciding) actually benefited invasives likely due to N pulses associated with litter decomposition from herbicided tissue.  Total ecosystem C storage was generally resilient to management, though there was a 5-10% decrease during the implementation of the herbicide/burning treatment.  However, wetland N retention declined dramatically (up to 50%) during the initial stages of all management treatments.  Invader biomass, N retention, and C storage recovered relatively quickly (2-4 years) after management ended.  These simulations strongly suggest that to control Typha invasion a combination of both herbiciding and burning is required, especially at high N loading where Typha is most successful.