2024 Hemlock Mortality Study


We did a preliminary study in the summer of 2021, four of us working for six weeks, 12 hours a week. Faced with dramatic ongoing mortality, and Jerry Franklin’s claim – which we have by anecdote – that lowland hemlocks die in their second century, we begin a more rigorous follow-up study in January 2024. We hypothesize that mortality is affecting all ages of hemlock, with pronounced differences across otherwise similar sites in Seward’s 120 acre forest.

You will find the original study’s final report here.

Our goal is to locate, measure, and assess the health of as many Western Hemlocks as we can, perhaps exhaustively, perhaps in two contrasting areas, perhaps using random site selection as well. We will use the same protocol as in 2021 (see the final report) with some refinements.

Foliage loss as an indicator of hemlock health

Many diseased trees lose foliage from the top down, probably because the water transport system is compromised, so that lifting water to the crown becomse increasingly difficult.  By contrast, our Western Hemlocks are almost all dying from the bottom up, losing needles first on their lower branches.

In 2021 we used needle loss as our principal metric of disease, awarding a number (0-3) for needle presence in each vertical third of the tree.

This time we want to be a bit more nuanced. In most species of tree, branch shedding (“cladoptosis”) is a normal response in shaded branches in the growing tree.   But this is much less common in hemlocks, because they are a shade tolerant species.  Nonetheless, some large, older and mostly healthy hemlocks at Seward have no branches in the lower third of the tree – shedding them, apparently, as do doug firs and big-leaf maples.  

In this year’s study, we want to allow for this. Bare branches with many small fine remaining twigs have probably died recently, suggesting disease. If only stubs of large branches remain in the lower portion of the tree, or if no branches are present, that absence is long-standing, and no indication of ill-health.