When novel & dramatic events happen at roughly the same time, we humans tend to think they are causally related - that one event caused the other, or that they both had a shared cause.
For these reasons, we are often asked whether, and it is often suggested, that the PNW lowland sword fern die-off is caused by drought or climate change.
We suggest that the following multiple lines of evidence, in aggregate, conclusively establish that this is NOT the case.
The single strongest argument against the hypothesis that PNW drought causes sword fern mortality is this 2016 New Phytologist research paper by Jarmilla Pittermann and colleagues:
Not dead yet: the seasonal water relations of two perennial ferns during California's exceptional drought
Peer reviewed, published in a high impact journal, this paper presents lab and field evidence that sword ferns routinely survive extended severe drought. If these findings hold up, and if they are as applicable to PNW forests as to California redwood forests, then drought (alone) cannot explain the sword fern die-off.
The first reports in the region of dramatic die-off are from 2010. UW PhD atmospheric scientist Joe Zagrodnik looked for drought/die-off correlations and reported:
The period from 2007-2012 was generally characterized by near-normal temperatures and precipitation (2011 was a cool year). By all measures the current period anomalous weather started in 2013 and I can say confidently that anything happening prior to 2013 is not caused by unusually warm or dry weather patterns.
Reed College ecologist Dr. Aaron Ramirez and his undergraduate student Caleb Goldstein-Miller report
the ecophysiology analysis of ferns from healthy, intermediate and die-off areas in Seward park [show] that moisture stress is not a driver of physiological stress for Seward ferns.
More detail from Caleb's final report. The full text from 29 nov 2019 is here.
For the ecophysiology analysis portion, our hypothesis was that plants in non-die off areas would have higher values for stomatal conductance, Fv/Fm, and possess a Ψ closer to the optimum for the species in question (or further from the turgor loss point for the local population) in comparison to plants in active die off areas. Three distinct areas of decline were established at Seward Park, die off, intermediate, and healthy. These areas were characterized with the help of Paul Shannon (Seward Park Steward) and Olga Kildisheva (Verdant Counseling, LLC) and sampled on August 19th, at the peak of the dry season. We used the following metrics to compare physiological stress of ferns in the die off, intermediate, and healthy treatment areas: dark-adapted fluorescence (Fv/Fm) to determine the efficiency of photosystem II, stomatal conductance (Gs) to ascertain whether or not ferns are closing their stomata during photosynthetically active periods, and xylem water potential (Ψ) to determine water status (Toivonen and Vidaver, 1988; Schreiber and Bilger, 1987; Corcuera and Notivol, 2015; Angelopoulos et al., 1996; Jordan and Ritchie, 1971). Healthy tissue from ten ferns within each treatment group was analyzed for predawn and midday water potential and dark-adapted fluorescence. Stomatal conductance measurements were taken in between predawn and midday sampling. We found no significant differences between treatment groups in any of the metrics except midday water potential. The ferns from the die off treatment area had an average midday xylem water potential of -1.7574 MPa, versus -0.9074 MPa and -0.9474 MPa for the intermediate and healthy treatment groups respectively. This result is complicated by the fact that we ran out of compressed nitrogen during the midday water potential readings from the die off treatment area. Samples were immediately transported from Seward Park to Reed college in a cooler so that water potential measurements could still be taken. Due to the unstandardized approach in xylem water potential analysis between the die off and the other treatment groups, we cannot be confident of the accuracy of this result. However, a pressure-volume curve was created from samples taken across the three treatment groups, which gave us a turgor loss point of -2.3 MPa for P. munitum at Seward Park. Since the significant water potential result was well above this moisture stress-indicating threshold, we are confident in our conclusion that moisture stress is not driving the die off at Seward Park. This increases our confidence that the regional decline is caused by some still unknown biotic vector.
Dylan Mendenhall conducted transmission experiments at the UW greenhouse in the Fall of 2019, establishing that die-off like symptoms are reproducibly transmitted from affected fronds to unaffected fronds via shared sterile water. Full details here (todo).
We received in 2017 reports, photos and videos of die-off on Artillery Hill at Fort Warden, outside of Port Townsend, average rainfall about 25 inches. By 2019, all ferns had recovered. This, we suggest, is the normal response and recovery of sword ferns to drought stress. As an ancient and widespread PNW understory plant, it seems likely that it has well-established mechanisms with which to survive drought.
We are half-way through a five-year restoration planting experiment at Seward Park, described here. 36 ferns, in three groups, all ferns watered through drought months, show statistically significant survival - correlated with location - in an active die-off zone (low survival), and in area hypothesized to be free of any die-off activity (> 95% survival).
At Seward Park, and in the other regional die-off sites we monitor (ranging from near Quilcene on the Olympic Peninsula, to the Goldmeyer Hot Springs Road in the foothills of the Cascades) no other understory plants exhibit the die-off or decline symptoms we see in sword ferns. The die-off thus appears to be specific to P. munitum: no other fern species are affected, no other shrub or herbaceous species. It seems unlikely that the sword fern - famously robust and very long-lived ("1000 years is not out of the question" - David Barrington, University of Vermont, Polystichum genus expert) - that the sword fern alone would be affected by drought and temperature.
The die-off at Seward Park spreads in two ways: radially from heavily affected areas, and by leaps into distant previously unaffected areas. The radial spread includes movement uphill, jumping across trails after some delay, and downhill into presumably wetter soil. Large regions of low- and high-density fern populations are unaffected - within a short distance of affected areas. This distribution is hard to explain by drought effects. Pathogenicity, perhaps abetted by weather and climate, makes much more sense.
