Searching for the Rarest Plants ever found on Avery Island

June 4th, 2019

For decades I have known about the rarest of all native plants ever discovered on Avery Island, and for decades I have searched for and yet still not relocated three of these rarest species. Each was collected only once back in the 1930s and then never seen again. The plants have always intrigued me, and I have spent countless hours exploring deep ravines on the Island in hopes of locating them again. My searches have not been in vain, for I have made numerous discoveries of other rare plants, several of which had never been reported for Avery Island. But my hopes of relocating these three original discoveries have not yet materialized. This month, however, I made a special effort to track down the location of the actual herbarium specimens of these rare plants.
My first search led me to the Philadelphia Academy of Natural Sciences (PANS) where famous botanist Edgar T. Wherry (1885–1982) deposited specimens he collected from Avery Island in 1936. Wherry found two species of North American ferns new to Avery Island, the Adiantum pedatum (“northern maidenhair”) and the Diplazium pycnocarpon (“narrow-leaved glade fern”).

Both species occur elsewhere in eastern North America, including Louisiana, but are considered extremely rare throughout the southern part of their range. Their discovery on Avery Island represents the southernmost location for both species in the United States and was a range extension of about 150 miles from the nearest known locality in the Tunica Hills north of St. Francisville, Louisiana. The topography of Tunica is remarkably similar to that of the largest salt domes along the Gulf Coast: Avery Island, Weeks Island, and CÔte Blanche Island. All of these areas are riddled with deep ravines that offer a unique habitat for some of North America’s rarest native flora.
After contacting the PANS herbarium curator, I was able to acquire high-resolution images of the very specimens that Wherry collected on Avery Island. I was eager to see what notes about their discovery might be included on the label data.

Much to my surprise there are two specimens of the Adiantum. Both are mature fronds that were very fertile. This indicates that the species was apparently thriving in this location at the time it was collected. Unfortunately, the collection data is insufficient, and as such the original area where these specimens were found remains a mystery. The single specimen of Diplazium pycnocarpon is equally interesting: it is a rather large plant, though sterile. Wherry clearly indicates on the collection label that he only observed a single plant of this species. Again, the locality data on the label is quite vague: “north facing gully cooled by moving air.” Too vague to determine where he found them. Since all three specimens have the same collection date of September 9, 1936, and similarly vague habitat descriptions, I think it is likely all three came from the same location. It is equally interesting that there are no subsequent records of either species ever being collected again on Avery Island, despite the fact that other botanists are known to have collected on Avery Island in 1938, only two years after Wherry. They did not report either of these plants in their collections.

One of those other botanist who visited Avery Island in 1938 was Donovan S. Correll (1908-1983). He collected a specimen of Diplazium loncophyllum (“lanced leaved glade fern”). At that time it was the first and only record known from North America. It is a tropical species occurring in the Caribbean, Mexico, and Central America. Its occurrence on Avery Island is likely due to Gulf winds or storms that transported the spores. Correll made a single collection of this species that is on deposit in the herbarium at the New York Botanical Garden. Once again, the collection data is quite vague: “deep heavily wooded ravine, south side of Avery Island.” He does indicate that the plant is “rather abundant.” This is the only collection of this species known from Avery Island.

I located a small but thriving population of Diplazium loncophyllum on Weeks Island in the 1970s. Subsequent trips were made to this location many years later, and the plants were still thriving. In the late 1980s I finally had the opportunity to explore CÔte Blanche Island. Much to my delight on that trip I located a single small plant of D. loncophyllum. The specimen resides at the UL Herbarium (LAF).

I have searched Avery Island ravines and gullies for 45 years and have failed to relocate these three species. Nevertheless, I still have hopes of rediscovering these elusive ferns, because there are potentially many of these ravines on the Island yet to explore.

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