Research Proposal: Assessing Western Hemlock Health Status in Two Contrasting Sites in the Old-Growth Forest at Seward Park

Since 2015 there have been intermittent reports of Western Hemlock (Tsuga heterophylla) decline and mortality in the Puget Lowlands. A variety of causes have been suggested, but none have been verified.

To the best of our knowledge, the Pacific Northwest forestry community has done no systematic study of this regional phenomenon. We can find no peer-reviewed publications. Opinions are offered, for instance, that climate change or normal hemlock senescence explain the decline. Or perhaps that an endemic fungus such as Annosus, Laminated Root Rot, or Rhizoctonia butinii is responsible. Phytophthora pluvialis has been suggested. The decline may be due to a combination of such factors, perhaps different combinations in different sites.

In order to move from conjecture to reproducible research, we propose a study site for PNW hemlock decline. The 120 acre urban old-growth forest in Seattle’s Seward Park has two contrasting hemlock-dense regions. A two hectare site with heavy mortality lies in the northwest section of the peninsula’s forest. Less than a kilometer away, towards the southeast, there is a one hectare site is dense with mostly healthy hemlocks.

We propose that these two sites at Seward offer an unusual opportunity to study hemlock decline. But first we must establish their statistical difference with respect to mortality, and their similarity with respect to density and age distribution. Tw0 UW Capstone Research students from UW’s Program on the Environment will join forest steward Paul Shannon to assess all trees in both sites. If our preliminary observations are borne out, we hope to attract the interest of experienced forest ecologists and pathologists to explore for the causes and mechanisms causing hemlock decline.

We have several audiences in mind for this proposal, and endeavor to address them all:

• Seattle Parks Plant Ecologists, who must approve this research proposal before we begin.
• UW Student participants, and other volunteers, so that they can understand the scope, purposes, background and field methods.
• Seward’s Forest Health Scientific Advisory Board, so that we may refine the proposal in response to their suggestions.
• Regional forest ecology and pathology researchers who, if we make a convincing case for study in this pair of contrasting sites, may be interested in grant-funded research into mechanisms.

Our initial map. These two “walkabout” polygons enclose hemlock dense regions, north and south. The actual survey will extend beyond these preliminary borders.

For seven weeks in the heat of July and August 2021, three fourteen year-old interns from CHOOSE180, along with forest steward Paul Shannon, surveyed hemlocks in three regions of the forest. We did an exhaustive survey in the middle section, then a partial survey of all hemlocks near the trail in north and south regions. It was here that we developed our version of a protocol originally suggested by WSU’s Marianne Elliott, and it was here that we learned of the uneven distribution of hemlock mortality. We were also advised by UW’s Tim Billo and professor emeritus Bob Edmonds. The final report is here.

The final report includes our tentative claim that possibly relevant symptoms of disease are visible on needles during the hot days of summer. That is, the phenotype of the hypothesized disease, and indeed the hypothesized disease itself, may be correlated to temperature and time of year. The report provides photos consistent with this hypothesis, and links to n=19 healthy/affected microscopy videos. This hypothesis is unproven, speculative, and possibly useful.

The mostly gray and black cluster to the north in this map are in the Hemlock Graveyard.

Using hand-held GPS devices, smart phones and diameter tapes, we will locate every hemlock within and nearby the two regions and record its diameter at breast height (DBH). One email per tree will be emailed to Paul Shannon, who will extract and collate the data. A grid (of dimensions to be decided) will be applied to each region, marked with flags on the ground and/or laid out on a web-accessible map. Each assessed tree will receive a temporary marker, removed at the end of the study, so that completeness and non-reduncancy of the survey can be checked.

In 2021 Marianne Elliott suggested that we adopt the basic form of the Dwarf Mistletoe Rating, in which each tree is (visually and approximately) divided into vertical thirds, after which an infestation score is assigned to each section. We used this method in 2021, and use it here again, with some refinements:

• We use a scale of 0-3, for ascending health, assigned from the (still evolving) criteria listed below.
• Hemlock needle health, aggregated over their branch, is the fundamental measure.
• Standardized reference photos of the 4 categories in the 0-3 scale are in preparation.
• Western hemlock is a shade tolerant tree which, in normal circumstances, maintains a full complement of densely-needled branches, from ground level to crown, in all compass directions.
• In addition, healthy hemlocks will often self-prune their branches in response to environmental conditions. Sunlight appears to be the primary driver of this process – cladoptosis.
• Regarding water response: western hemlocks are adapted to the PNW Puget Lowland Mediterranean climate of cool wet winters, and warm dry summers. It is commonly said that their shallow roots make them more susceptible to drought and heat than co-occurring tree species (Douglas-Fir, Red Cedar, BigLeaf Maple). Soil moisture measurements throughout each site, taken throughout the year, may correlate with the contrasting mortality at the two sites.
• Hemlock cladoptosis is most vividly seen in mature hemlocks which have broken through the canopy, reaching full sunlight. These mature trees generally have naked boles for two-thirds or more of their full height. (City street mature hemlocks, growing in open sun throughout their height, often have healthy, fully-needled branches at all heights.)
• We often see self-pruning in younger apparently healthy trees as well growing entirely below the canopy, in limited light, with needle and branch loss enriched on the north, sunlight limited side. Some of this, we believe, indicates tree decline, though this alos occurs in apparently healthy trees.
• In general, these younger trees – with crowns within the canopy – have well-needled branches all the way to the ground, often radiating a full 360 degrees.
• In our 2021 survey, perhaps do to summer heat, some of the lower branches had obvious distress, failure and needle drop. The black excrescence pictured above accompanied the needle failure.
• The current 2024 survey takes place in winter and early spring, when needle abundance and health is likely to be static, unchanging. Therefore we will assess this category of lower branch ill-health by relative sparseness of needles. In some cases, needles will be entirely absent; in this case, the persistence of very fine branchlets will be interpreted as recent needle loss. Fine branchlets persist for very few years..
• We wish to distinguish normal branch loss (cladoptosis) due to sunlight effects, and canopy breakthrough cladoptosis, from needle and branch loss due to ill-health. At present we do not have a rigorous assay for this. Nonetheless, a gestalt perception and consensus tends to emerge among observers examining any single tree. Distinguishing a healthy from a failing tree, in semi-quantitative terms, is usually straightforward.
• Western hemlock decline usually occurs from the bottom up. This contrasts with typical drought effects upon foreat trees, where transpiration failure is seen first at the top of the tree, later progressing downwards. This bottom-up hemlock phenomenon, when seen in understory trees, is presumptive evidence of poor tree health. Sparse and/or discolored needles, and moss-covered branchlets, often accompany tree decline.

By mid-February 216 trees have been measured and mapped in the two sites. The interactive map and data table can be viewed here.