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 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)

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)