A mountain hemlock burned in the 2019 Swan Lake Fire, pictured June 29, 2021. (Photo by Matt Bowser/USFWS)

Refuge Notebook: When hemlocks burn

When the weather deteriorates in the mountains, I head for those deepest green patches of forest at tree line. These groves of hardy, resilient and slow-growing mountain hemlocks hang on to the shoulders of our mountains.

The trees grow together so thickly that they provide excellent shelter from the wind and rain. You can always find a dry patch somewhere in a hemlock stand.

After the 2019 Swan Lake Fire reached high into alpine areas where mountain hemlocks live, I wondered how these slow growing, long-lived trees respond to fire. A comparatively rare forest type on the Kenai National Wildlife Refuge (31,000 acres or 2% of the refuge) that seldom burns, we have no historical information on fire in hemlocks on the western Kenai Peninsula.

The Swan Lake Fire offered a unique opportunity to learn about the relationship of fire and one of my favorite kinds of trees.

Few shrubs grow under the dense shade and in soil acidified by fallen hemlock needles, leaving the understory open, dry and inviting. The leaves are welcoming.

Try alternately hugging a spruce and a hemlock, and you will quickly learn that stiff, sharply pointed spruce needles prick your skin painfully while the supple, rounded tips of hemlock needles make their branches soft to the touch.

The rich, dark green of their foliage and the often-irregular shapes of their crowns make mountain hemlocks easy to recognize, even from far across a valley.

As such obviously ideal places to take refuge, people chose to establish official and unofficial campsites among hemlocks along our popular hiking trails. Rusted cans, broken glass and scars of fire rings in hemlock stands far from trails testify to less regular use by hikers and hunters.

If you take a look around at the contorted, often fantastical shapes of hemlocks near the treeline, you should be able to see that these trees have not had easy lives. Heavy snow loads, avalanches, violent gusts and desiccating winter winds have broken, crushed, and withered trunks and branches.

Unlike most conifers, where damage to the growing tip of the tree often leads to an untimely death, mountain hemlocks withstand their tops being broken off, redirecting upward growth to their remaining branches.

Insects and fungal diseases damage hemlocks, but we have not seen evidence of whole stands of hemlocks succumbing at once in our area like the familiar outbreaks of bark beetles in spruces. These resilient trees live to be the oldest trees we have cored on the western Kenai Peninsula, some of them having started as seedlings in the 1500s.

Because mountain hemlocks prefer moist sites, fires seldom happen in these forests. Six hundred to over 1,000 years typically passes between fire events. When fires do come, these trees are not fire-resistant.

Their thick bark might offer some protection, but hemlocks grow close together so that fires move easily through their flammable foliage, burning whole groves at once. Mountain hemlocks are most susceptible to fire where they blend into the spruce forest, which typically burns more frequently.

On the refuge, we know of few fires in the hemlock forest. However, a lightning strike on the southeast side of Skilak Lake in 2005 started the Irish Channel Fire, which burned for months and grew to 1,000 acres, much of it in a hemlock forest.

At least one small group of hemlocks along the Funny River Horse Trail burned in the 2014 Funny River Fire. In 2019, the Swan Lake Fire made its way up the mountain slopes east of Mystery Creek Road, including often visited hemlock groves on the Skyline Trail and Fuller Lakes Trail.

The Swan Lake Fire also burned 31 vegetation plots established and monitored by the Department of Agriculture Forest Service’s Forest Inventory and Analysis Program, part of a vast grid of plots over Southcentral Alaska. Since 2004, Kenai refuge biologists have also sampled this same grid of plots as part of our Long Term Ecological Monitoring Program.

In June and July 2021, we revisited 26 of the plots within the fire to look at how severely the fire burned and to document the plants growing after the fire.

Two recently burned plots were in a hemlock forest, providing a rare opportunity to see what happens when hemlocks burn in places where we know what the forest was like before the fire. Based on counts of tree rings from cores taken at these plots from 2001 to 2017, the trees ranged from 70 to 200 years old at the time of the fire in 2019. However, the forest was likely much older.

When we revisited these plots two years after the fire, we found that all the mature hemlocks had died. The fire burned off much of the upper parts of the soil and consumed almost all of the understory plants. In their place, fireweed had grown in, mostly from seeds blown in after the fire. Tiny hemlock seedlings had already gotten started. We also saw a few spruce and birch seedlings.

Based on our recent observations and other information we have on how mountain hemlocks respond to fire on the Kenai Peninsula and in other parts of North America, we can make some predictions about future conditions. We expect these burned plots will return to mature hemlock forest in 100 – 400 years as long as another fire does not roll through.

After having learned a little more about these groves of old mountain hemlocks where I like to take shelter, even for a quick snack, I appreciate that they are part of a dynamic landscape. We live at the intersection of biomes where long-range seed dispersal, fires, insect outbreaks and climate oscillations interact so that most of the forest is constantly changing. Hemlock stands in the mountains might be more stable than spruce forest, but they change, too.

Matt Bowser serves as Fish and Wildlife Biologist at Kenai National Wildlife Refuge. For more on fire and mountain hemlocks in Alaska, see https://www.fs.fed.us/database/feis/fire_regimes/AK_mountain_hemlock/all.html. Find more Refuge Notebook articles (1999–present) at https://www.fws.gov/refuge/Kenai/community/refuge_notebook.html.

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