Proceedings of the Third International Conference on Invasive ...

Chapter 2: Spartina Distribution and Spread<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive Spartinato NAVD88 (Morris et al. 2005). At North Inlet, <strong>the</strong> meanhigh water level was 0.618 m (2001 through May, 2003)with a mean tidal range <strong>of</strong> 1.39 m. Details about <strong>the</strong> NorthInlet marsh can be found in a variety <strong>of</strong> sources (Eiser &Kjerfve 1986; Dame et al. 2000; Morris 2000; Morris et al.2005). Below, <strong>the</strong> model is generalized for two or morespecies and its predictions for <strong>the</strong> case <strong>of</strong> two hypo<strong>the</strong>ticalspecies are discussed.The Elevation <strong>of</strong> <strong>the</strong> Marsh SurfaceThe elevation <strong>of</strong> <strong>the</strong> marsh platform equilibrates at aposition where erosion and deposition are equal. It is clearthat sediment deposition must approach zero at elevationsnear that <strong>of</strong> <strong>the</strong> highest high tide and should increase as<strong>the</strong> concentration <strong>of</strong> suspended sediment and hydroperiodincrease. This logic is based upon a consideration<strong>of</strong> abiotic processes alone and predicts that intertidalmarshes approach an equilibrium elevation that approximatesMHW (Krone 1985). A rise in relative sea levelwill increase flooding <strong>of</strong> <strong>the</strong> marsh and inundation time<strong>the</strong>reby increasing <strong>the</strong> opportunity for sediment depositionand re-establishing <strong>the</strong> elevation <strong>of</strong> <strong>the</strong> marsh relativeto <strong>the</strong> new MSL (Pethick 1981, Krone 1985, French 1993).Sedimentation rate is <strong>the</strong> product <strong>of</strong> settling velocity andtime <strong>of</strong> inundation. Since depth below mean higher highwater (MHHW) and time <strong>of</strong> inundation are roughly proportional,<strong>the</strong> net rate <strong>of</strong> change in <strong>the</strong> elevation <strong>of</strong> <strong>the</strong>marsh surface (dY/dt), is proportional to <strong>the</strong> depth (D) <strong>of</strong><strong>the</strong> marsh surface: dY/dt D. Strictly speaking, sedimentationrate should be proportional to depth, whereaso<strong>the</strong>r processes that affect elevation, like compactionand decomposition, may or may not be depth-dependent.However, compaction can be taken to be a constant that isaccounted for in <strong>the</strong> local rate <strong>of</strong> sea-level rise.Feedbacks between <strong>the</strong> marsh vegetation and <strong>the</strong> sedimentsare also important. The rate <strong>of</strong> change <strong>of</strong> elevation <strong>of</strong><strong>the</strong> marsh platform is a positive function <strong>of</strong> <strong>the</strong> standingdensity <strong>of</strong> plant biomass (B). For simplicity it is assumed thatthis relationship is linear: dY/dt q + kB, where parametersq and k are proportional to <strong>the</strong> settling velocity and <strong>the</strong> efficiency<strong>of</strong> <strong>the</strong> vegetation as a sediment trap, respectively. Theproduct <strong>of</strong> kB represents <strong>the</strong> positive effect that abovegroundbiomass density has on <strong>the</strong> trapping <strong>of</strong> suspended sediments(e.g. Leonard & Lu<strong>the</strong>r, 1995; Christiansen et al 2000). Thevalues <strong>of</strong> q and k are likely to vary locally and regionallyas a function <strong>of</strong> sediment availability and tidal range (e.g.Stevenson et al. 1986). Note that k also may vary by species.This implies that <strong>the</strong> efficiency <strong>of</strong> sediment trapping may varyamong species (e.g. Rooth and Stevenson 2000). Combining<strong>the</strong>se concepts, <strong>the</strong> absolute change in elevation <strong>of</strong> <strong>the</strong> marshsurface, for depths (D) within <strong>the</strong> intertidal zone, can beapproximated by:dY/dt = (q + kB)D, for D>0 (1)The kB term implicitly accounts for <strong>the</strong> contribution <strong>of</strong>organic matter accretion when <strong>the</strong> model is calibrated to <strong>the</strong>total accretion rate. It assumes that organic matter accretionis proportional to <strong>the</strong> standing biomass density <strong>of</strong> <strong>the</strong> vegetation.We have recent unpublished results that show that thisassumption overly simplistic, but <strong>the</strong> qualitative behavior<strong>of</strong> <strong>the</strong> model with respect to total sediment accretion andsea-level rise is not changed by breaking out organic matteraccretion as a separate term.The Vertical-Distribution <strong>of</strong> Standing BiomassBiomass density, B (g/m 2 ), is variable and changes witha number <strong>of</strong> environmental conditions including <strong>the</strong> relativeelevation <strong>of</strong> <strong>the</strong> marsh platform. For any intertidal species,<strong>the</strong>re exist upper and lower limits <strong>of</strong> relative elevation. For<strong>the</strong> hybrid in San Francisco Bay as for S. alterniflora on <strong>the</strong>Atlantic coast, <strong>the</strong> lower limit is probably set by <strong>the</strong> hypoxiaresulting from tidal flooding, while <strong>the</strong> upper elevation isdetermined by salt stress, desiccation, and competitivepressure from o<strong>the</strong>r species. The dominant representativesfrom different plant communities along a topographicgradient will have different biomass distributions (a, band c values in Eq. 2) that may or may not overlap, andinterspecific competition or facilitative interactions maymodify <strong>the</strong> shapes <strong>of</strong> <strong>the</strong> curves where overlap occurs (eg.Bertness 1991; Emery et al. 2001; Bertness & Ewanchuk2002; Pennings et al. 2005). These distributions can bedescribed by a family <strong>of</strong> curves:B i= a iD + b iD 2 + c i(2)where a, b, and c are coefficients that determine <strong>the</strong> upperand lower depth limits, and magnitude <strong>of</strong> B and where <strong>the</strong>subscript i refers to a specific dominant species or communitytype. Depth, D (cm), is positive for depths lessthan MHHW. These curves can be viewed as dimensions<strong>of</strong> a species’ fundamental (in <strong>the</strong> absence <strong>of</strong> competitors)or realized (in <strong>the</strong> presence <strong>of</strong> competitors) niche, sensuHutchinson (1957). The values <strong>of</strong> <strong>the</strong> coefficients a, b, and cwill also differ regionally as a function <strong>of</strong> tide range, salinity,or climate. In examples discussed below, <strong>the</strong> values <strong>of</strong>a, b, and c where chosen to represent hypo<strong>the</strong>tical specieswith biomass distributions that span different ranges alongan intertidal gradient.MHHW is used as <strong>the</strong> zero datum (D 0) for conveniencesince it is approximates <strong>the</strong> elevation where biomassdensity and sedimentation rate approach zero. However,because MHHW is based upon an arithmetic average <strong>of</strong><strong>the</strong> higher high water height <strong>of</strong> each tidal day observedover <strong>the</strong> National Tidal Datum Epoch, D 0may differ fromMHHW regionally, depending on factors such as windtides. The departure <strong>of</strong> D 0from MHHW is probably greatestin microtidal estuaries where wind tides <strong>of</strong>ten dominateastronomical tides. Moreover, MHHW will vary due tolow frequency variability in mean sea level and with <strong>the</strong>18.6-yr lunar nodal cycle (Stumpf and Haines, 1998). Thelunar nodal cycle changes <strong>the</strong> tidal amplitude by about 5 cmand is simulated below by allowing D 0to vary by 5 cm at afrequency <strong>of</strong> 1/18.6 yr.- 110 -