The Central and Site Hypotheses of the VCR LTER


Outline


The Central Hypothesis

The central hypothesis of the VCR LTER is that ecosystem, landscape and successional patterns are controlled by the relative vertical positions of the land, sea, and fresh-water table. Large transient and small progressive changes in the position of these free surfaces result in disturbance of ecosystem processes and in physiological stresses that may lead to system state change. Variations in the elevations of these critical surfaces result from local weather or climate change, such as short-term storm-generated fluctuation in sea/land levels or long-term sea level rise. Ecological processes, including species extinctions and invasions that alter rates of erosion and deposition, also affect the positions of these free surfaces relative to one another. In many instances, the joint effects of contemporaneous disturbances at different temporal and spatial scales result in state changes.


The Central Hypothesis as it is applied to the Four Major Research Sites

1. The Megasite (MS)

Long-term progressive and short-term disturbance-driven changes in the intersections of land, lagoon and water-table free surfaces of the VCR cause changes in the spatial (10s m to 10s km) organization of VCR ecosystems and shifts in ecotones.

2. The North Hog Island Chronosequence (HI)

Terrestrial vegetation on coastal barrier islands is limited primarily by the relationship of the non-parallel land and water table free surfaces. The relative elevation of the fresh-water table and the magnitude of the reserve of fresh water available structures the vegetation of the barrier islands.

3. The Hog Island Bay Lagoon and Marshes Site (LM)

The type of community and level of productivity of emergent marsh vegetation on lagoonal marshes is controlled by the relative elevations of the parallel marsh land free surface and the parallel, oscillatory lagoon water free surface.

4. The Mainland Marsh Transition Site (MM)

The vertical position of the mainland-marsh ecotone is a function of the interaction of the non-intersecting, sloping free surfaces of the saline lagoon water, fresh ground water table, and land surface. Within the ecotone structured by these sloping free surfaces, multiple states are ordered along a gradient maintained by storm-generated disturbances against a background of chronic sea level rise. Changes in state along the gradient are caused by mechanisms which are characteristic of each change of state (Brinson et al. 1994).


Examples of working sub-hypotheses at each of the Four Major Research Sites

1. Sub-hypotheses at the Megasite (MS)

  1. The sedimentary layer produced by the 1933 hurricane that buried then-existing marshes is present throughout Hog Island Bay and is thickest where sea grass meadows and were extensive before 1933.(Zieman and Hayden)
  2. The shorezone of the barrier islands switches from accretional to erosional with a return period of approximately 80 years. We further hypothesize that the transition from accretion to erosion patterns is an Atlantic Coast-wide phenomenon that is caused by changes in winter storm(s) climate and long-term changes in the subtropical anticyclone of the North Atlantic. (Hayden and Dolan).
  3. Seaside vegetation composition on the barrier islands is a response to winter storm overwash disturbances (changes in land free surface elevation) and marine flooding; the effects vary depending on whether sections of the island are erosional or accretional. (Hayden and Shugart).
  4. Elevation of the land free surface above sea and fresh water surfaces are major determinants of the capacity of an island to support viable populations of vertebrates. Extinctions of vertebrate populations may result from changes in one or more of these surfaces. (Dueser, Porter and Moncrief).
  5. Reworking of regressive strandplain land free surfaces during the late Holocene transgression produced a characteristic stratigraphy due to the release of "fines" into the water column which increased lagoonal turbidity. The "fines" are preferentially deposited in wave-protected areas, and the marshes of small watersheds on the lagoon margins. (Oertel)
  6. Lagoonal waters are presently light-limited and turbid as the result of both inorganic and organic (high C/N values) particulate suspension. Increased primary production by phytoplankton and an increase in macroalgae will reduce organic turbidity in lagoonal waters. It is further hypothesized that this increase in clarity will be a key factor in the return of seagrasses to the VCR. ( Zieman and Hayden).

2. Sub-hypotheses at the North Hog Island Chronosequence (HI)

  1. The depth of the fresh water table below the surface of the island depends on the elevation of the land above mean sea level. (Kochel, Hayden, Furman, and Porter)
  2. In stable or accreting regions of the islands, species composition of the vegetation is determined by island elevation, depth to the freshwater table, and variations in water table elevation due to rainwater input and evapotranspiration losses. (Brinson, Porter and Hayden)
  3. In unstable regions of the island, plant species composition and successional processes are determined by depth to the freshwater table, climate variations and by the frequency and magnitude of disturbance from coastal storms. (Young and Hayden)
  4. Accreting regions of the islands have landscapes ordered in age from the sea landward. Along this chronosequence the relative positions of the land and water-table free surfaces have been in place for periods ranging from less than one year to about 120 years. Within the chronosequence, autogenic succession accounts for biogeochemical variations. (Day, Young, Porter, and Shugart)
  5. Above and below ground production are limited by proximity to the fresh water and available nitrogen. (Young and Day)
  6. Decomposition rate is a function of soil moisture and thus proximity to the water table. (Day)
  7. Nutrients (primarily N) are derived from either N-fixation by Myrica or atmospheric deposition and spatially are a function of exposure to oceanic influences and soil moisture and thus proximity to the water table. (Young, Macko and Galloway)
  8. Utilization of the island landscape by small mammals varies with age and complexity of the landscape and vegetation type. (Dueser and Porter)
  9. Mammalian consumers affect the composition and structure of vegetation which, in turn alters the rates of change of free surfaces through alteration of aeolian deposition and evapotranspiration (Dueser, Young and Porter)
  10. A change in the mean level of the water table free surface will result in a change in vegetation composition and productivity. (Hayden, Porter, Furman, and Young)

3. Sub-hypotheses at the Hog Island Bay Lagoon and Marshes Site (LM)

  1. Spartina alterniflora growth form and productivity vary as a function of the difference between marsh surface elevation and mean sea level. Lowering the marsh land surface will increase productivity and promote dominance by the tall form of S. alterniflora. (Zieman and Hayden)
  2. VCR Marshes that were buried during the 1933 hurricane are elevated and relatively flat, and are therefore dominated by the short form of S. alterniflora. (Zieman and Hayden)
  3. Hypersaline marshes are restricted to regions where the marshland-surface is level at elevations between mean high tide and spring tide. (Zieman)
  4. Storm-derived marine sediments deposited below mean high tide level develop a chemistry and texture compatible with marsh colonization and succession. (Furman and Zieman)
  5. Reduction of pore water salinity by infiltration of precipitation and the activitiy of invertebrates enhances Spartina altinaflora productivity. (Zieman and D. Smith)
  6. Exchange of water between ocean and lagoon is accomplished by a standing tidal wave and tidal currents moving through well-developed antecedent valleys. The exchange is very efficient, and the resulting lagoonal water body maintains a relatively constant salinity at 30-32 ppt. Exchange is determined by water-mass trajectory path, not diffusion of stratified flows (Oertel).
  7. Water mass boundaries in lagoons are determined by differences in current "drag" created by bottom friction and fluctuating water levels. (Oertel, Porter, and Hayden)

4. Sub-hypotheses at the Mainland Marsh Transition Site (MM)

  1. The forest to high marsh transition is induced by storm-generated influxes of saline water that stresses and kills trees, tree mortality is primarily a result of salt induced water-stress and not water-logging. (Brinson and Christian)
  2. Tree mortality allows light penetration to the forest floor and is followed by replacement of forest by marsh. (Brinson and Christian)
  3. The transition from organic high marsh to mineral low marsh is facilitated by erosion of tidal creeks headward and/or along creekbank margins resulting in subsidence of the surface of the organic high marsh. (Christian, Brinson, Blum, and Wiberg)
  4. Increased oxidation of sediment organic matter contributes to subsidence of the organic high marsh surface. Increased oxidation is a result of better drainage created by the steeper hydraulic gradient between the marsh water table and the creek at low tide. (Christian and Anderson)
  5. The organic high marsh surface subsides in part because of decreasing accumulation of below ground organic matter and in part as a result of decreased allocation of resources for root and rhizome growth. (Blum and Christian)
  6. Inundation and salinity by themselves are insufficient to cause species replacements in organic high marsh communities. Disturbance due principally to wrack deposition must be involved to maintain within state heterogeneity (although fire, trampling, and ice scouring may be additional disturbances). (Christian and Brinson)
  7. Sediment deposition on mineral low marsh surfaces is facilitated by the frictional resistance (i.e., baffling effects) of marsh plants and /or infiltration of water through the marsh surface on falling tides. Erosion of marsh sediment is inhibited by the presence of vegetation and sediment particle aggregation by invertebrate fauna (Wiberg and D. Smith).
  8. Nutrient transfers from the mainland into the mainland-marsh tidal creeks depends on the distance of transport and the number and type of marsh zones through which the flow must pass. (Macko, Anderson and Blum)