Other hypotheses

Drought

Although drought conditions have preceded dieback events in southern states, it does not fit with what's been going on in Cape Cod. Perhaps the best evidence is that vegetation losses on Cape Cod are ongoing and continue to occur in the absence of drought. For example, the photos below show the recent disappearance of S. alterniflora between 2004 and 2008 during which there were no significant droughts.

In addition, it is the creekbanks that have lost the most vegetation over the years. However, these areas are flooded daily with seawater, which negates any possible long-term effects of drought on soil chemistry (such as decreased pH and mobilization of metals).

Finally, if severe drought were the primary cause of vegetation decline, the expectation would be for large-scale, simultaneous losses in multiple marshes during a single year. This has certainly not been the case. On Cape Cod, vegetation losses began at different times in different marshes and have been progressive over a period of many years.

 

Recent loss of S. alterniflora in a permanent monitoring plot at CACO.  This occurred in the absence of drought and no interim phase of dying foliage ever occurred (photos by Smith, NPS).

 

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Pathogens

Fungal infections of the Fusarium spp. variety have been noted during the decline of S. alterniflora in southern states diebacks. On Cape Cod, however, vegetation losses are ongoing and have been closely monitored since 2004. During this time, no symptoms of disease in plants have been observed (note: plants normally have fungal spots on foliage during autumn senescence). Thus, while it is possible that pathenogenic fungi may play some secondary role in vegetation losses on Cape Cod (e.g., they might affect the ability of plants to regrow after herbivory), there is currently no evidence that they are a primary driver. In fact, plants immediately surrounding denuded areas are typically very healthy, reaching heights of up to 6 ft or more in many places.

Dr. Wade Elmer, Dr. James LaMondia (Connecticut Agricultural Experiment Station), and Dr. Frank Caruso (UMASS Cranberry Research Station) are investigating the ways in which pathogens may influence the fitness of salt marsh plants.

 

Recent losses of S. alterniflora in a Wellfleet Bay marsh in the absence of drought and without symptoms of diseased foliage (photos by Smith, NPS).

 

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Observations that can lead to invalid conclusions

We now know that some of our earlier reports of dieback were clearly wrong. In certain years, S. alterniflora may enter the senescence phase earlier than usual. In 2006 this happened in Middle Meadow (a CACO marsh) and a mid-August browing-up of S. alterniflora was thought to be "dieback". The next spring, however, all the same plants resprouted and came back green and healthy that year. A similar process occurred in the Herring River (Harwich) during 2002-2003.

Lesson: it is imperative to track the growth of plants at least through the next growing season, since the death of aboveground foliage outside the period of typical senescence does not necessarily mean plant mortality.

 

Early senescence in response to a record heat wave in August 2006, followed by complete recovery in 2007 (Middle Meadow) (photos by Smith, NPS).

 

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New shoots emerging in the spring of 2007 from a marked plant that had brown, dead foliage in mid-August of 2006.  All marked plants recovered fully from early senescence the previous year (photo by Smith, NPS).

 

Herring River (Harwich): the upper right photo (NEERS SWD website) shows foliage that is dead by late September 2002, which may be slightly early for this marsh.  However, the next year's photo (2003) shows that this was not a mortality event as green, healthy vegetation grew back in all the same places.  The bare patches are previous losses of vegetation.

 

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