Contributions of exotic floating macrophytes to accretion in a rapidly subsiding coastal floodplain forest

Background/Question/Methods

Coastal floodplain forests in the Louisiana Deltaic Plain are undergoing rapid relative sea level rise with estimates as high as 11.9 mm yr^-1.  Where subsidence exceeds organic accretion, the hydroperiod is extended, regeneration of flood intolerant species is inhibited and coastal forests eventually retreat. These flooded forests often support abundant floating macrophytes.  Where the floating communities are dominated by highly productive invasive species, such as Salvinia minima and Eichhornia crassipes, accretion rates may at least partially offset subsidence.  The objective of this study was to compare organic accretion rates of these two invasive species to background control conditions. In June 2012, fifteen 1m² fine mesh enclosures were established in the swamp forest at Jean Lafitte National Historical Park and Preserve in coastal Louisiana.  In each enclosure, a feldspar marker horizon was applied to the mineral substrate. Five enclosures were randomly assigned to each of three cover treatments (no cover control, Salvinia, or Eichhornia).  In January 2013, we cored each enclosure using a thin-walled aluminum tube.  Cores were frozen and cut longitudinally into quarters.  Average depth of the accretion layer atop the feldspar was determined for each core; one-way ANOVA and post-hoc Tukey’s tests were used to compare accretion among treatments.   

Results/Conclusions

After six months, there were significant overall differences across the three cover types (F_2,12=48.29, p<0.0001).  The large and robust Eichhornia had significantly higher accretion (4.28 mm ± 0.28 SE) than the smaller Salvinia fern (1.33mm ± 0.04 SE) and control enclosures (0.55mm ± 0.165), which did not differ.  Although Salvinia accretion was not statistically different from the controls, the patterns suggest that differences between the two may increase with time.  Field observations of the control enclosures suggested that allochthonous litter and autochthonous algal inputs contribute to background accretion rates in this system.  These preliminary results suggest that exotic macrophytes are indeed contributing to accretion in the backswamp.  Additional cores will be collected seasonally over the next two years and tied in with a newly established Surface Elevation Table (SET) to determine the long-term impacts of invasive macrophytes on elevation dynamics in this rapidly subsiding coastal forest.