Whitebark Pine After the 2023 Sourdough Fire, North Cascades

A vegetation ecologist surveys a whitebark pine for damage from the Sourdough Fire, September 9, 2023. Credit: National Park Service.

A stand of whitebark pine (Pinus albicaulis) is a special place. Perched on high ridges and rocky talus slopes, the trees stand like lookouts over the rest of the ecosystem. On a summer day there is the smell of sun-warmed needles, the calls of Clark's nutcrackers staking out territory, and the glow of mountain flowers in bloom.

Conifers dominate the forests of the North Cascades, including around Sourdough Mountain — complex forests of many conifers, broad-leaved trees, shrubs and ferns that shelter wildlife from shrub-nesting birds to insect-eating young trout. Among them, whitebark pine is a particular concern when fire comes.

A keystone of the subalpine

Whitebark pine is a keystone species of subalpine ecosystems in western North America. It grows in nearly pure stands on harsh, dry ground, and with other subalpine conifers on moister, sheltered sites. Often it is the first tree to take hold where other plants struggle, and once there it creates habitat for other species, allowing the forest to develop and making the subalpine more diverse. Its particular habitat, though, leaves it more likely to die after a wildfire. With populations falling and threats persisting, it was protected as a threatened species under the Endangered Species Act in December 2022.

Its nuts are a main food of Clark's nutcracker (Nucifraga columbiana) and an important source of protein for bears. The nutcrackers are the tree's main seed spreaders, caching dozens of nuts in burned areas, where they like to hide food. Some caches are never retrieved — if a bird dies, or has more food than it needs — and bears often raid them. Seeds left untouched may sprout into new stands.

Threats that stack up

• White pine blister rust (Cronartium ribicola), an invasive fungus introduced to North America in the 1900s, has hit populations hard. It causes wounds called cankers on branches and stems that damage tissue and block the movement of nutrients and the products of photosynthesis, eventually killing the branch, part of the tree or the whole tree. It can also weaken or kill cone-bearing branches.
• Mountain pine beetles (Dendroctonus ponderosae) usually attack mature trees and can quickly wipe out stands. Their larvae tunnel beneath the bark and cut off the tree's nutrients, and the beetles carry a fungus that dehydrates the tree and weakens its defences. A warming climate lets them breed in one year instead of several, and they are drawn to burned areas full of stressed trees.
• Fire. More frequent and more intense fires in the North Cascades over the past decade add to the danger. Whitebark pine is moderately fire-adapted and can usually survive low- to moderate-intensity fire, but trees weakened by blister rust are less likely to. Decades of fire suppression have also let smaller, shade-tolerant trees fill the gaps between pines, fuelling fires like the 2023 Sourdough Fire and making them burn hotter.

After the Sourdough Fire

The 2023 Sourdough Burned Area Emergency Response (BAER) team — a forester, a fire ecologist, a vegetation ecologist and a botanist, many of whom live and work in the North Cascades — studied the fire's effects to recommend how the park should care for its whitebark pine.

The Sourdough Creek and Stetattle Creek drainages have no large or concentrated stands, which makes each tree there more important. The team mapped whitebark pine within the fire's footprint and surveyed individual trees, recording where and how badly each was burned. Some had burned; others survived in unburned "islands" inside the fire area. At least one tree had died from scorching, and several burned trees may survive. The survivors, which like direct light, will get more sun — but they face more challenges than benefits, as blister rust and pine beetles now threaten these stressed trees more than ever.

The team made recommendations to park managers, who may spend years on the restoration work it proposed. A fire's ecological effects outlast the flames, and the team's goal was to chart a way to ease them so that this keystone species keeps its place in the subalpine ecosystem of the North Cascades.

Sources

Based on "Effects of Wildfire on Whitebark Pine," published by the National Park Service, North Cascades National Park Service Complex; rewritten in hubnx's own words.