28 years of vegetation standing crop data from the Mississippi river delta

Deltaic landscapes go through cycles of birth, growth, decline, to death governed by intertwined geological, biological, and ecological processes.  This study tracks deltaic lobes in the Balize Mississippi River Delta, Louisiana, USA over ~3 decades (study years 1984–2012). Hydrologic and geomorphic patterns and those which sustain patterns to wetland plant richness, diversity, and biomass are described.  Plant diversity and biomass were modeled by nMDS ordination.  Taxa (53) were harvested and dried (116,706g) from 965 (0.25 m2) plots and divided into three groups: I. 4 Foundation Species; 78.9% of total harvest; II. 9 Pioneer Species; 13.6% of total harvest; III. All Other taxa; 7.5% of total harvest (8 Miscellaneous Grasses, 8 Miscellaneous Sedges, 24 Miscellaneous Herbs).  Autogenic/allogenic processes (sedimentation, subsidence, plant colonization, succession events) affect composition and biomass.  Eleven important species were identified.  Taxa’s richness increased on mudflats during primary succession (15 to 25 taxa per site), then declined to fewer than 5 per site.  Niche-space theory explained patterns to community change.  There was similar total biomass/yr (~500 g/m2/yr) at study sites.  Quantile regression analyses showed water quality and quantity of the Mississippi River influenced biomass especially spring-time waters.  Stochastic events (storms, herbivory, salt-burn, flood-pulses) impacted biomass. Long-term studies like this are required in a future of climate unknowns.

Methods
In 1984 three sites were chosen for study; two sites on 2yr old (determined from NASA photographic records) rapidly elongating lobes of the BS; a north site (BSn) and a south site (BSs).  A third site was established ~150m inland from Octave Pass on the east side of the southeasterly dredged channel.  This constructed or created WSc site was originally a designed test location for wetland development to monitor with the two natural sites on BS.  Years later another lobe showed a future for growth on the same growing splay down flow from WSc but, not influenced by the dredged material from a decade before.  In 1994, this area was designated as the new WSn site. In 1984 at BSs, BSn, WSc, and at WSn in 1994, study site transect locations for plant collection on the newly emerged sites were marked by permanent stakes mid-summer nearshore at the most distal growing ends of each site’s lobe. Each year of substantial accretion (which forced elongation of the lobes) additional transects were added; two by 1986 at WSc and BSn, and two by 1989 at BSs.  All lobes had to have elongated ~50m or more for a new transect.  No further transects were deemed necessary after sufficient plots were established to describe the lobe plant community (usually three transects each with five plots). At the time of usual flowering (late-August early September), the peak plant standing crop along each transect was clipped in 5 quarter-meter2 (50cm x 50cm) plots separated by ~5 to 10m. All plant material, including the unidentifiable dead, was dried to constant weight and entered into Excel spreadsheets.  Voucher material was deposited in the herbarium of Tulane University, New Orleans, LA; since moved to the collections at Louisiana State University, Baton Rouge, LA.