Marsh Dieback: History, Potential Causes,

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Marsh Die-back History, Potential Causes,
Current Evidence

• Karen L. McKee
• U. S. Geological Survey
• Irving A. Mendelssohn
• Louisiana State University
• Michael D. Materne
• Natural Resources Conservation Service

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Historical Perspective
Die-back of Spartina marshes has been
recognized since 1968
Coastal Studies Bulletin No. 5 / Special Sea
Grant Issue /February 1970 SPARTINA DIE-BACK
IN LOUISIANA MARSHLANDS W. G. Smith Coastal
Studies Institute Louisiana State University
Spartina alterniflora marshlands in Louisiana
frequently have large areas of standing dead
stubble. These killed areas were first noted by
the author in the Grand Isle area on November
10, 1968 As of December 31, 1969, no recovery
was evident (Smith 1970)
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Salt Marsh Die-back in Louisiana
Since the 1970s more research concerning salt
marsh die-back has been conducted in Louisiana
than probably anywhere else in the world
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Historical Spartina alterniflora Die-back
- Typical die-back of Spartina occurs in the
marsh interior
- These areas cannot maintain their elevation
relative to sea-level rise
- Our research has shown that this historical
die-back has occurred because excessive
submergence leads to sulfide accumulation, which
in turn causes reduced growth and eventually
plant death.
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Current Marsh Dieback
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Potential Causes of Die-back

• Background Statement
• Evidence
• Data collected
• Literature

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Biotic Factors
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Pathogens

• Background Bacterial, fungal, or viral
  pathogens or insect outbreaks can cause
  widespread mortality of plants.
• Evidence for Pathogens
• In Texas Florida, a fungal involvement has been
  identified in connection with Spartina die-back,
  but infection usually opportunistic on stressed
  vegetation.
• Evidence against Pathogens
• Examination of Spartina culms from LA by
  pathologists has so far revealed no obvious
  pathogens.

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Herbivory

• Background Herbivores (nutria, waterfowl) can
  cause large eat-outs in marshes.
• Evidence for Herbivory Some, but not all,
  die-back areas have large concentrations of
  snails that are eating the dead vegetation.

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Herbivory

• Evidence against Herbivory -Not all die-back
  areas have high densities of snails or evidence
  of snail feeding on live tissue.

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Herbivory

• Evidence against Insects
• No evidence of insect outbreaks or insect damage
  out of the ordinary.

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Low Genetic Diversity

• Background Generally, the higher the genetic
  diversity the broader the ecological amplitude of
  a species. Changes in the environment that lead
  to a substructuring of populations may render a
  marsh more vulnerable to sudden extremes that
  exceed the genetic capacity of the population to
  adjust.

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Low Genetic Diversity
Evidence European research shows two genetic
groups of Phragmites australis (deep water reed
and land reed), each of which may be
completely eliminated from an area by
manipulations favoring the other genotype and
preventing the establishment of new populations
after old ones have been destroyed. The
surviving population is more likely to
experience die-back.
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Abiotic Factors
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Chemical Spill (e.g., petroleum)

• Background Chemical spills can kill large areas
  of marsh in a short period of time.
• Evidence for Spills
• There were no signs of a chemical spill in the
  die-back marshes.
• Evidence against Spills
• Pattern and extent of die-back is inconsistent
  with this hypothesis.

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Buildup of Sulfide / Fermentative Products

• Background Organic matter is generally
  accumulated in wetlands, and the anaerobic carbon
  decomposition in wetland sediments produces a
  range of fermentative products that are toxic to
  plants (organic acids and sulfide) at high enough
  concentrations. This process can also create a
  high soil oxygen demand that can stress plants by
  competing for their internal oxygen.

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Buildup of Sulfide /Fermentative Products

• Evidence for Phytotoxins
• Historical die-back of Spartina alterniflora has
  been linked to sulfide accumulation in Louisiana
  marshes.
• Current die-back areas exhibit elevated
  concentrations of sulfide. However, death of
  plants will generate these compounds a cause and
  effect relationship cannot be assumed.

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Buildup of Sulfide / Fermentative Products

• Evidence against Phytotoxins Unaffected species
  are not more tolerant of sulfide, e.g., Avicennia
  germinans.

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High Salinity

• Background The recent severe drought, combined
  with low river flow, may have increased salinity
  in die-back marshes.
• Evidence for Salinity
• Surface salinities did increase in the past year,
  according to records examined thus far.
• Porewater salinity is slightly elevated in some
  of the dead marshes.

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High Salinity

• Evidence for Salinity More salt-tolerant
  species (Avicennia and Batis) have survived
  alongside the dead Spartina.

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High Salinity

• Evidence against Salinity
• Measured salinities (lt40 ppt) do not exceed
  tolerance limits of S. alterniflora.

Upper Limit of Ecological Range 45-50
ppt Lethal Salinity 83-100 ppt From Hester,
Mendelssohn, McKee (1996)
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High Salinity

• Evidence against Salinity
• Less salt tolerant species such as Juncus
  roemerianus have survived in die-back areas.

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High Water Levels

• Background Spartina alterniflora is very flood
  tolerant, but there are limits to its tolerance.
  When oxygen is cut off from the plant roots for
  24 hours, the meristems (growing tips) will begin
  to die and the entire plant can succumb within a
  few days.

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High Water Levels

• Evidence for High Water Levels
• Pattern of die-back shows that lower elevation
  areas (interior marsh) are experiencing extensive
  mortality.

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High Water Levels

• Evidence against High Water Levels
• Other species that are equally or less flood
  tolerant than Spartina have survived.

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Low Water Levels

• Evidence for low water levels Records indicate
  low water levels at some locations during early
  part of year.

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Low Water Levels

• Evidence against low water levels Low elevation
  sites appear to be more affected than high
  elevation sites.

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Interacting Environmental Biotic Factors

• Background Several factors may be interacting
  to cause die-back. A pre-existing stress
  condition may have made the plants more
  vulnerable to a second stress factor or a
  pathogen that alone would not have caused
  mortality.
• Evidence
• In Louisiana, natural subsidence, sea-level rise,
  and sediment deficiency have been implicated in
  historical die-back of interior marshes.
• In Europe, Phragmites die-back has been linked to
  a combination of eutrophication, artificially
  stable water-levels (stagnation), and disturbance
  (harvesting, fire).

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Summary

• Factors least likely to be involved a chemical
  spill, herbivory
• Factors possibly involved, but with little or no
  data from a broad survey of current die-back
  sites pathogens, low genetic diversity
• Factors possibly involved, with some evidence
  from several sites water level extremes,
  salinity, natural toxins (e.g., sulfide)