A proposal for experimental restoration of long-term bare ground die-off sites at Seward Park

Submitted to Seattle Parks & Verdant LLC on May 31st 2018

Here for your consideration and critique is my strawman proposal for experimental restoration in two small die-off regions at Seward Park. 

- Paul

Premises:

1) We do not know the cause/s of the die-off, nor its expected duration
   and eventual extent.  Crucially, we do not know if this is a transient, cyclic,
   or endemic phenomenon.

2) We DO know that in Seward Park, die-off of formerly sword fern dominated
   areas are not, after four years, regenerating.   This is in marked
   contrast to post-disturbance regimes of natural restoration described
   by Swanson, Franklin et al (2002), "The Forgotten State of Forest: Early
   Successional Ecosystems on Forest Sites".  The cumulative effects, upon
   general forest health, of unregenerated bare ground, may be significiant.

3) The selection of restoration strategies may benefit from an evaluation of
   the differennce between S&F's intense disturbances(fire, volcano,
   clearcut, insects, wind) and the context in which their regeneration
   occurs.  Specifically, our sites have an intact overstory and
   a possibly reduced local seed sources.

4) Ground Zero is in GSP zone "MF4", with target forest type PSME-ARME/HODI/LOHI
   (doug fir, madrone, ocean spray, hairy honeysuckle).  "Extended
   ground zero" - across the Hatchery Trail to the north, MF7, has
   target forest type PSME-TSHE/MANE-POMU.  A good case can be made
   that ground zero is PSME-TSHE/MANE-POMU as well.

5) My immediate concern is with the previously sword fern dominated bare
   ground sites.  That fern dominance may not be a necessary feature of
   those bare ground sites:  this may have been a contingent assemblage
   produced by quirks in the plant geography and historical dynamics of the forest.
   Given that the (still unknown) cause of the die-off may be still be present,
   or may return, it is unwise to restore these areas to anything like the
   sword fern (near) monoculture which existed there before.


Proposal:

  1) To choose two small currently mostly bare areas (70' x 70'?), one north and
     one south of the Hatchery Trail for restoration planting and seeding, using
     the characteristic species of the PSME-TSHE/MANE-POMU forest type, as
     described in Christopher Chappell's DNR report, "Upland Plant
     Associations of the Puget Trough Ecoregion", 2004, page 109, as listed below.

   2) Perhaps conventional 1-gallon pot seedling planting can be accompanied by
      judicious experiments with direct seeding.

   3) There may be some urgency to returning these moribund areas to
      good health - made up from a heterogeneous community with complex
      food webs, nutrient flows and physical structure inspired by some of the
      intact heterogeneous late succession plant communities found elsewhere
      in the forest at Seward.  We may therefore wish to perform careful
      monitoring of survival rates for both seeds and plants, and to
      inistitute manual watering protocols through the first two summers,
      to maximize the chances of successful restoration, and to refine
      practices for reuse elsewhere.

Candidate Plant species

Douglas-fir                                                       Pseudotsuga menziesii var. menziesii 100 48
western hemlock                                                   Tsuga heterophylla 87 38
western redcedar                                                  Thuja plicata 82 35
bigleaf maple                                                     Acer macrophyllum 60 19
grand fir                                                         Abies grandis 22 7

Shrubs and Dwarf-shrubs
dwarf Oregongrape                                                 Mahonia nervosa 100 18
red huckleberry                                                   Vaccinium parvifolium 80 3
trailing blackberry                                               Rubus ursinus var. macropetalus 78 1
salal                                                             Gaultheria shallon 73 3
vine maple                                                        Acer circinatum 49 15
beaked hazelnut                                                   Corylus cornuta var. californica 42 5
baldhip rose                                                      Rosa gymnocarpa 40 1

Graminoids
Coast Range fescue                                                Festuca subuliflora 33 1

Forbs and Ferns
sword fern                                                        Polystichum munitum 100 23
sweet-scented bedstraw                                            Galium triflorum 62 2
western starflower                                                Trientalis borealis ssp. latifolia 62 2
bracken fern                                                      Pteridium aquilinum var. pubescens 49 1
western trillium                                                  Trillium ovatum ssp. ovatum 47 1
spreading woodfern                                                Dryopteris expansa 42 1
vanillaleaf                                                       Achlys triphylla 36 3
twinflower                                                        Linnaea borealis ssp. longiflora 31 3
inside-out flower                                                 Vancouveria hexandra 29 5
threeleaf foamflower                                              Tiarella trifoliata var. trifoliata 29 1

Fort Worden Recovery

Darrell Howe reports on a recent visit to Fort Worden that the ferns we thought were dying in 2017 are in fact recovering, sprouting new fiddleheads.   See his iNaturalist report and photos.   Last year's blog post of what we then interpreted as die-off is here.

This suggests a few different possibilities:

1. The putative pathogen which we think causes the die-off is not always fatal: it can also cause a die-back from which ferns can recover.  We may see this phenomenon at Ground Zero in Seward Park, where two ferns (out of ~100),  which I previously judged to be dead, are now recovering.
2. Some other phenomenon lies behind last year's apparently dead ferns at Ford Worden.  

Whatever the explanation - and ongoing observation may clarify - this recovery is very good news.

This recovery at least partially fits the pattern reported by the Pitterman Lab describing sword fern die-back due to drought in the redwood forests of California.   See the blog post here.  In brief:  fern response to drought includes stomatal closure followed by xylem embolisms followed by die back.  The ferns return in successive years, but continued cycles of drought + embolism lead to die-off.

Note that the California drought was more severe and lasted longer than what we have seen in the Pacific Northwest.  See precipitation records and California comparion here.

What's Next? A proposal to plant Oregon Grape from native seed

My initial shock has diminished - at the broad loss of sword ferns to the north of Ground Zero, between the Hatchery enclosure and the sqebeqsed trail.  Some of that loss can be seen in this photo, looking northeast from near the sqebeqsed/Hatchery trail junction.  The original Ground Zero is out of frame to the right (south and a bit east).

Tim and Kramer have put in some observations plots, and Suzanne put in a 12-fern experimental planting,  north of (and to the left in) this photo.   Matt & co. planted perhaps 50 ferns just down-slope from the scene pictured here as part of (I hope I have this right) an eco-physiological study.

About 30 yards to the north of this site the forest understory gets healthy again.   Last week I went looking in that area for healthy ferns to monitor.  We do not yet have a detailed record of how fern fronds are affected, from health to first symptom to full death.   I figure that a photographic record of a few individual fronds on different plants may be useful.

While looking around for plants to monitor, I came across a few large areas dominated by healthy Oregon Grape, mahonia nervosa. At Seward, healthy ferns often appear in mixed communities with this species.  Salal can be  intermixed as well.  All three species appear in various ratios, with complete dominance by a single species sometimes occurring,  but often in mixtures on a sliding scale, of the three species.

I now feel that complete sword fern recovery at Seward Park is a distant prospect - at best.  There is little to no evidence of natural regeneration  - of ferns, or any other species.  The only exception to this which I have seen thus far at Seward is a large bloom of the invasive weed Herb-Robert - in an area (not quite visible here) at  the top left of the above photo.

This all leads me to suggest that

1. The next urgent task at Seward is restoration of native understory where the sword ferns have died off.
2.  Oregon Grape and Salal are the best candidates for planting
3.  Growing these from native seed, collected from this forest,  means that restoration will be inexpensive and low risk.

I bet there is a lot of practical knowledge and advice around on how to grow mahonia from seed.  Some casual web searching suggests that seeds are best collected in early September and then planted right away so that 'stratification' happens naturally over the winter.  Growth is slow in early years.  Maybe extra watering will not be needed.   Perhaps salal is similar?

I am eager to hear advice, suggestions, and comments on this plan.

Invasive Earthworms?

Astute reader Betsy Dowling, a Seattle native now studying urban horticulture at Mesa Community College in Arizona, noticed the similarity between the Seward Park 2011-2017 before and after photos which introduce this blog, and this pair, from the 5th edition of Soil Science and Management, Edward Plaster, 2008 - copied here (alas) without permission:

On a quick trip to Seward Park last night I performed a ridiculously ad hoc assay, digging and examining a cubic foot of soil from three spots:  ground zero (GZ: 1 worm),  active die-off zone (ADZ: 4 worms) and a healthy dense sword fern community off the Erratic Trail (ET: 4 worms).   This establishes nothing other than that earthworms (no species ID) are present.  

Here is a hodgepodge of academic papers.   2002 research by Michael Gundale apparently introduced the topic, describing on the effects of invasive earthworms on a rare, mostly underground fern:

Influence of Exotic Earthworms on the Soil Organic Horizon and the Rare Fern Botrychium mormo

The research is summarized in the  Conservation Magazine.  The original paper is listed here, with this abstract:

Forests north of the last glacial extent have no native earthworms. Exotic earthworms are now colonizing forests that are naturally free of earthworms. It is currently unknown how these exotic earthworms might affect rare plants. To determine whether there is an association between the presence of an exotic earthworm species and extirpation of the rare fern Botrychium mormo, I surveyed 28 populations documented and counted previously. I estimated current population sizes of B. mormo, soil horizon thickness, earthworm species present, and carbon content, nitrogen content, and pH of the A soil horizon. Two earthworm species were abundant, Lumbricus rubellus and Dendrobaena octaedra. Dendrobaena octaedra had no significant association with any soil variable or with B. mormo extirpation. Lumbricus rubellus was significantly associated with B. mormo extirpation and a mull humus type. Where L. rubellus was present, O1 and O2 horizons were significantly thinner. I conducted a laboratory microcosm experiment to determine whether L. rubellus could create the conditions it was associated with in the field. Microcosms with L. rubellus resulted in a significant reduction in the thickness of the O1 and O2 horizon and a significant increase in the thickness of the A horizon. This experiment suggests that L. rubellus created the conditions with which it was associated in the field. The intrinsic rate of increase (r) of B. mormo was best explained by the O2 depth, which implies that this soil layer supplies a critical resource. My results support the idea that exotic earthworms alter the forest floor, leading to negative changes in native vegetation.

 2014:  Impact of Exotic Earthworms on Organic Carbon Sorption on Mineral Surfaces and Organic Carbon Inventories in a Northern Hardwood Forest  

A 2016 review from the German Centre for Integrative Biodiversity Research:  

European earthworms decrease species diversity in North America

2009:  Distribution and impacts of invasive earthworms in Canadian forest ecosystems.  A short section titled Coast and Columbian forests of British Columbia will be of some interest.

A Lazarus Fern?

When I returned to Ground Zero a week after the two lines of twelve ferns were planted, in order to take the first set of time-lapse photos, I was shocked and delighted to discover a healthy fern which had not been planted by us.    Since sword ferns, we are told, will not reproduce under a closed forest canopy, I assume that this fern has come back from the dead:  enough life lingered in the rhizome for it to regrow after three years of inactivity.   Hence: the Lazarus Fern.   Definitive evidence for this claim - of regrowth rather than new growth - could come from removing soil and locating (or not locating) a large rhizome; young ferns do not acquire a rhizome for at least five years.    I do not want to disturb this rare, resuscitated, old survivor - or is it a new young plant? - not yet.

Based upon the size of the seven fronds, and the time of year, these fronds must have been 2017 growth.   I did not notice this frond last year.  This is puzzling, but not completely unlikely:  inasmuch as  I did not expect to see any fern regrowth,  I may well have overlooked this one instance.

There are a few other rather straggly ferns surviving at ground zero.  They are all - except for this fern, and one other I will describe in a moment - found very close to the trail or to large trees.   All of these survivors are failing, and fewer of them are found with each passing year.   Perhaps some sort of micro-site effects, or interspecies cooperativity, is responsible for their imperfect survival. 

The other healthy fern found at ground zero is one of the two nursery ferns we planted in November 2014, at the suggestion of Jillian Weed, parks plant ecologist.   I watered both ferns through the first two summers; one has survived:

This survivor, and the surprising regrowth of the Lazarus Fern suggests a possibility: that the putative pathogen killing the ferns,  which spreads at the rate of about 30 meters a year, may now be absent from ground zero.    Perhaps something in the genetic makeup of the Lazarus Fern, or its micro-site, of some unknown factor - some apparently unique factor/s allowed it to regrow.   The survival of one of the two 2014 plantings is consistent with this hypothesis:  whatever kills the ferns now appears to be gone.    This may be a cause for optimism - or it may be that the killing agent will return and once again attack ferns at ground zero if the ferns are re-established.  

For now, I will take comfort in the possibility that the scourge has moved on, and that at least one native plant is resistant.   We will collect spores from the Lazarus Fern in July and raise up a bunch of baby ferns,  which will also possibly be resistant, for further experimental planting.   If this is a genetic trait, and if it has high penetrance, we may have stumbled upon a partial remedy for the sword fern die-off.

Experimental planting at Seward Park

In February we planted three lines of young sword ferns, twelve ferns in each line.  These ferns were part of our annual generous allotment of three hundred restoration plants from Seattle Parks and the Green Seattle Partnership.  They come from regional native plant nurseries; the plants are about two years old. 

Two of these lines are in the original ground zero at Seward Park, an area that has been barren now for three years, with no natural regeneration of any species.    The third line is about fifty yards north and just outside of the current boundaries of active die-off.   Every fern was "watered-in".  Half of them - every other fern in each line - were infused with mycorrhizal spores.

I hypothesize (based on evidence presented in the next blog post "A Lazarus Fern?") that the likely pathogen responsible for die-off has swept through ground zero and is no longer present.  Thus I predict that the two lines of twelve ferns in ground zero, if we water them through the first summer or two, will mostly survive.  As the die-off zone spreads it will soon encompass the northern line of twelve ferns.  I predict that most of these twelve ferns will die.    More specifically, and as a nod to statistical rigor, I predict a p-value < 0.05 when, after five years, we compare the survival rates at the two contrasting sizes, ground zero (24 ferns) and the northern line (12 ferns, aka AD for "active die-off).  Here this is mocked up in a little R code:

set.seed(17)

GZ <- as.integer(runif(24) + 0.7)       #   75% survival: 0 1 1 1 1 1 0 0 1 0 1 0 1 1  
                                                           #                           1 1 1 0 1 1 1 1 1 1 

AD <- as.integer(runif(12) + 0.25)    #   25% survival: 1 0 0 0 1 0 0 0 1 0 0 0

t.test(GZ, AD)$p.value                      #   0.0047

I photograph all thirty-six ferns every week and will create a time-lapse video for each fern as these weekly photos accumulate.  The two GZ lines are shown in the picture above.

Report of Die-off in Snohomish County: Lord Hill Regional Park

Amy Lucas, Senior Park Planner with Snohomish County Parks, reports:

We recently received a citizen report of a similar die-off at our Lord Hill Regional Park. I went in the field yesterday with one of our Senior Rangers and found 3 suspect sites. Two sites were small, and surrounding salal had spots, much like black spot on a rose plant.

We hiked to a much larger site that showed signs of complete sword fern mortality that looked like it was spreading radially. The ground at that site was barren, and the brown fronds were spreading to surrounding plants. 

These photos seem convincing to me, though I have not yet visited the site for a close inspection, and we as yet do not have measurements of extent or rate of spread.  Leaves in the bottom-most photo perhaps obscure dead fern crowns?  It usually takes two years to progress from first symptoms to crown stubble.  

This report, if confirmed, indicates a new direction of spread, to the north and east of Seattle, about 30 miles from Seward Park, and 30 miles west of Baring, a small and apparently isolated residential die-off   - Paul