Seward's Forest "Split Site": a natural experiment
We have been tracking, and trying to understand dramatic and
novel mortality of sword ferns in our old-growth forest's
understory. Only sword ferns (Polystichum munitum) are affected.
The full twelve year project is documented
here.
Two Univeristy of Washington students spent the summer surveying
throughout the Puget Lowlands of Washington state, establishing
that the sword fern blight is intermittently present throughout
that region. This project is reported
here.
At Seward Park, we have observed the "split site" (see map
tab) informally for two years. This summer (2025) we
approached it systematically, by
- Mapping and classifying each of the 39 ferns.
- Analyzing the soil in each half
The site provides
a
natural experiment in that the
two halves have
- apparently identical abiotic conditions,
- are equal in sword fern density and count
- are separated only by one meter horizontally
- have the same slope and aspect
- have no obvious canopy differences (though
month-by-month insolation data have not been collected)
- are separated only by a fallen Douglas-fir log which, in the site, has a
ground clearance averaging three inches (need to verify this!)
allowing easy passage air, insects, small mammals and rainwater.
Nonetheless, and for several years, sword mortality is
dramatically different between the two halves. The remnant dead
fern crowns (dark grey markers on the map) indicate that
mortality struck here three-to-five years ago. This summer (I
write in September 2025) a few of the "healthy side" ferns show
their first signs of blight.
On the affected half-site, the survival of 3 of 21 ferns (14%)
fits our informal sense that even in heavily affected pure
stands, and perhaps due to genetic variability, 10% of the
stand survives the blight.
A simple chi-square calculation shows that the significance
(p-value) of the separation is 4.887e-07. In lay terms:
there is 1 chance in two million that this contrast happened by
chance.
Thus this site, judiciously studied, and with many conditions
and features identical, and dramatically different outcome, may help us to
understand the nature of the sword fern blight.
Results of Soil Tests Conducted Separately on the Two SubSites
Four soil samples were collected on each subsite, to a depth
of three inches. These were pooled to create two aggregate
samples, one for each side, and sent to
EarthFort of Corvallis Oregon
for analysis.
We suggest that the contrasting values across the two sites
are likely to be the result of east subsite fern mortality -
not its cause.