The die-off at Ground Zero had reached steady state by 2014, leaving almost entirely bare ground. 10% of the original ferns, as we assessed in a June 2019 study. This quarter acre was subject to erosion and likely undergoing further ecological degradation: underground mycorrhizal networks need photosynthesizing plants, of which there were very few. No natural restoration had taken place until a few fringecups - most happily - appeared in 2019.
Suzanne and planted three lines of 12 ferns, two lines at Ground Zero, 1 line just north of what was then the boundary of the expanding die-off region. I hypothesized that the agent of the die-off, whatever it turns out to be, was no longer active at Ground Zero, but that it was likely to be active and virulent at the die-off's leading edge.
This hypothesis was a generalization from a single fern planted at Ground Zero in 2014. Inspecting the die-off with Seattle Parks plant ecologist Jillian Weed, I asked, "What should we do?". "Monitor closely", she said, "and why don't you plant a couple of nursery ferns?". We did, I watered the pair through a couple of summers, and one (dubbed "Jillian Weed #1") is thriving now six years later. A 50% survival rate for restoration planting is better than I usually achieve. So I surmised that the die-off agent/s were gone - at least temporarily.
I water all 36 plants weekly during summer drought, two liters per palnt. The Ground Zero ferns (with one exception) are thriving. The northern line ("ADZ" for active die-off zone) is about 50% dead or dying. I have made every attempt to treat all three lines identically.
In April, Bonnie Drew, Jeff Kelley and I twice independently surveyed all three lines. We counted fiddleheads, interpreting them as simple markers of overall plant health. Here are the results, summarized for each of the three 12-fern lines, in box plot form. The full dataset is available on request.
I provisionally conclude that there is currently no active agent affecting ferns at Ground Zero. The degraded site is now recovering, which is very fine to see. The agent may return.
(Seattle Parks contracted out some additional restoration planting at Ground Zero, installing more than 100 plants of mixed native species in the winter of 2018-2019. I water these plants in the summer as well, and part of the returning health of Ground Zero comes from this generous action by Seattle Parks.)
