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
Historical Perspective
Die-back of
“Spartina alterniflora marshlands in Louisiana frequently have
SPARTINA
LOUISIANA
MARSHLANDS
large
areas of“DIE-BACK”
standing deadIN
stubble.
These killed
areas were
Spartina
marshes
first noted by the author in the Grand Isle area on
W. G.
Smith
hasNovember
been
recognized
10,
1968……
Coastal Studies Institute
Louisiana
State
University
since
1968
As of December
31,
1969,
no
recovery
was evident………”
Coastal Studies Bulletin No. 5 / Special Sea Grant Issue /February 1970
(Smith 1970)
Salt Marsh Die-back in
Louisiana
Since the 1970’s more research concerning
salt marsh die-back has been conducted in
Louisiana than probably anywhere else in
the world
“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.
Current Marsh Dieback
Potential Causes of Die-back
• Background Statement
• Evidence
– Data collected
– Literature
Biotic Factors
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.
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.
Herbivory
• Evidence against Herbivory: -Not all die-back areas have
high densities of snails or evidence of snail feeding on live
tissue.
Herbivory
• Evidence against Insects:
– No evidence of insect outbreaks or insect
damage out of the ordinary.
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.
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.
Abiotic Factors
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.
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.
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.
Buildup of Sulfide / Fermentative
Products
• Evidence against Phytotoxins: Unaffected
species are not more tolerant of sulfide, e.g.,
Avicennia germinans.
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.
High Salinity
• Evidence for Salinity: More salt-tolerant
species (Avicennia and Batis) have survived
alongside the dead Spartina.
High Salinity
• Evidence against Salinity:
– Measured salinities (<40 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)
High Salinity
• Evidence against Salinity:
– Less salt tolerant species such as Juncus
roemerianus have survived in die-back areas.
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.
High Water Levels
• Evidence for High Water Levels:
– Pattern of die-back shows that lower elevation
areas (interior marsh) are experiencing
extensive mortality.
High Water Levels
• Evidence against High Water Levels:
– Other species that are equally or less flood
tolerant than Spartina have survived.
Avicennia
germinans
Distichlis spicata
Low Water Levels
• Evidence for low water levels: Records indicate
low water levels at some locations during early
part of year.
Low Water Levels
• Evidence against low water levels: Low elevation
sites appear to be more affected than high
elevation sites.
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).
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)