Comments, links and papers resulting from email sent to University of California fern biologists, and allies, on February 2nd 2017, who all generously responded.

My email:

Here in Seattle, and now apparently in a few locations in the Puget lowland, we are witnessing a novel die-off of sword ferns.

We have been tracking and monitoring the die-off for two years in league with plant pathologists from WSU and ecologists from the University of Washington. Phytophthora was suspected, but repeated PCR tests have so far been negative.  The cause is unknown. 

The full story is presented, along with narrative history, maps, videos, and lab reports here:

   http://sewardparkswordferndieoff.blogspot.com

Since we do not have marker or assay for a pathogen, our tracking of the apparent spread is imprecise, and thus subject to both under- and over-interpretation. 

One way to improve our tracking - and my motivation for writing - is for me to learn as much as possible about the habits of Polystichum munitum.   Could you direct me to resources for such an education?

To give an example of what I don't know:  nowhere I have been able to learn the typical life span of the sword fern, with what mean and what variability.  Perhaps the current age of a fern could be inferred by traces left by now absent fronds?

Alan Smith, research botanist at UC Berkeley Herbarium (retired)

My observation is that plants of this species in my yard live a long time, 25+ years.  Gradually, by either forking of the apex or by adventitious shoots (or both), new crowns are formed that radiate out from the original crown.  One plant becomes two, which becomes four and so on, in close proximity.  I doubt that in a mature redwood forest there is much sexual propagation, from spore dispersal -- that seems to happen in more open or exposed areas, maybe semi-disturbed, road banks, trailsides, etc. In a mature redwood forest, the plants on the forest floor then must be essentially a clone, or many clones from some original establishment(s)... So the populations could be hundreds, if not thousands of years old, depending on disturbance and openings in the canopy.  But I have never seen good documentation of this, either long-term ecological studies or molecular studies. 

David Wagner, Director and Curator of the University of Oregon Herbarium 1976 to 1993, currently operates the Northwest Botanical Institute

There is no way to accurately [date] old sword ferns although it is clear some are quite ancient. Many years ago I had a group of students in a summer Ferntastic Ferns course study a population of relatively young ferns. Each student dug up a single fern that had only one apex or crown. Most old ferns have multiple crowns because the stems branch regularly and often extensively. In these single crown ferns we counted the number of fronds produced in that year and then counted stipe bases as far back as possible. Most of these still possessed stipe bases all the way to the tapered base because they are so resistant to decay. The average number of stipe bases divided by the average per year gave us an average age of something like 50 years (can’t lay my hands on class notes immediately). This had to be an underestimate because in the early years of life they do not produce as many fronds per year. I’ve never published this because it is so generalized but it tells you why I believe sword ferns live for centuries. They often survive stand destroying forest fires, so it is not unreasonable to presume many ferns are older than the trees in the canopy above them.

The other thing known is that reproduction is only sporadic and dependent on disturbance. I never found juvenile plants in a mature forest floor. Gametophytes grow well only on mineral soil such as root balls, landslides or artificial sites as road and trail cuts.

I have some population data in my monograph of Polystichum (1979.  Systematics of Polystichum in western North America north of Mexico.  Pteridologia 1: 1-64). I will attach below a pdf of a paper that includes Polystichum.  (see "Belowground traits of herbaceous species in young coniferous forests of the Olympic Peninsula, Washington", below)


Consulting with east coast fern biologists on longevity:

Dr. David Barrington, University of Vermont, Professor of Plant Biology, Polystichum genus expert

I very much like the idea that they are old individuals, established at a remote time when there was disturbance in the forest.  Fire would create such a disturbance.  As to age, 1000 years is not out of the question, possibly much longer but there are no data at all for deciding. There are very few sites for the establishment of new plants via gametophyte in the stable mature forests of the Northwest, though a single treefall would create such an opportunity.  Polystichum munitum is essentially unbranched, which lends support to the idea that the populations are made up of independently originated individuals, not fragments of some giant clone.

 Dr. Robbin Moran, Curator at the Institute of Systematic Botany, New York Botanical Garden:

If I had to guess, I would say that the individuals are probably very old, perhaps dating back to the time of original disturbance in the area. This seems likely given that there seems to be little or no sexual reproduction going on, and the mature sporophytes can persists by their perennial and bifurcating rhizomes.

I wonder whether age of an individual close could be extrapolated by trying to determine the size (diameter) of an individual clone and taking into account its annual growth rate. Something like this has been done for Osmunda regalis and Pteridium aquilinum, but I cannot remember the references off-hand. I think one study on Pteridum aquilinum was done by Park and Werth in 1993 near the Mt. Lake Biological Station in Virginia.  

Emily Burns, PhD (UC Santa Cruz, on the physiological influence of climate on coast redwood forest plants_, currently Director of Science at the Save the Redwoods League, and Research Associate in the Pittermann lab at UCSC (see next blog post).

First of all, I’m really sorry to hear about the Polystichum munitum die-off in your area. The drought of 2012-2015 has certainly taken its toll on sword fern in California and the forests around Mt. Tamalpais in the Bay Area experienced a similar crown die-back trend last year. It seems that in our area, the stress brought on by the drought caused the sword fern to be more susceptible to thrips, a small blackish pest which discolors the leaves and can lead to foliage decline. While many of the ferns appeared quite dead last year with all of their leaves affected, with more rain they did send up new fronds and appear to be recovering. I hope that a similar recovery is possible for your local ferns. Starting in March (at least in CA), ferns that are alive will begin to send up new leaves as fiddleheads so keep a watch out for those.

We don’t know how old ferns can get, however their leaves are considered evergreen and live for approximately 2.5 years with a single cohort of leaves being produced each spring (meaning during the summer, there are three ages of leaves on the crown and the oldest leaves, usually on the outer perimeter of the crown, look like they are beginning to decline and will die in the fall. In the case of a forest fire or other disturbance that kills all the fronds, a new cohort of leaves can flush and replace the crown.

Belowground traits of herbaceous species in young coniferous forests of the Olympic Peninsula, Washington

pdf

Of relevance: