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 marsheshas been recognizedsince 1968

Coastal Studies Bulletin No. 5 / Special Sea Grant Issue /February 1970

SPARTINA “DIE-BACK” IN LOUISIANA MARSHLANDS

W. G. SmithCoastal 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)

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

- 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.

- These areas cannot maintain their elevation relative to sea-level rise

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 Phragmitesaustralis (“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 pptLethal 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.

Avicenniagerminans

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)