Effects of Wildfire on Whitebark Pine

A vegetation ecologist surveys a whitebark pine tree for damage from the Sourdough Fire on September 9, 2023.

NPS

A whitebark pine (Pinus albicaulis) stand is a magical place. Perched on high ridges and rocky talus slopes, these trees act as lookouts for the rest of the ecosystem, their trunks stretching toward the sky. The smell of sun-warmed pine needles, the call of Clark's nutcrackers claiming their territories, and the subtle glow of blooming mountain flowers invoke a sense of wildness on a summer day.

Conifers, or cone-bearing evergreen trees, dominate the forests of the North Cascades, including in the Sourdough Mountain area. These forests are complex with many species of conifers, deciduous broad-leaved trees, woody shrubs, and ferns. This landscape provides shelter and food for diverse wildlife, from the small birds that nest in shrubs to the young trout that eat insects. In this ecologically diverse area, the whitebark pine is one species of particular concern during wildfires.

Whitebark pine is a keystone species of sub-alpine ecosystems in western North America. It grows in nearly homogeneous stands on harsh, dry terrain and can also be found growing with other subalpine conifers in moister, protected sites. It is often the first tree species to establish in places unfriendly to other plants. Once present, whitebark pine creates habitat that is favorable to other species, which enables forest succession and increases subalpine diversity. Whitebark pine occupies a special niche in the ecosystem and its unique habitat preferences put it at greater risk of mortality after a wildfire. Due to declining populations and persistent ecological threats, the species received Endangered Species Act protection as a threatened species in December 2022.

Whitebark pine is an important species in North Cascades National Park Service Complex ecosystems. The tree’s nuts are a dominant food source for Clark’s nutcracker (Nucifraga columbiana) and an important protein source for bears. The Clark’s nutcracker acts as a primary seed distributor for whitebark pine across the landscape, caching dozens of pine nuts in burnt areas, where it prefers to hide its meals. Sometimes these caches aren’t eaten, like in instances when the bird dies, or the bird has an overabundance of seasonal food. When caches are left, bears will often come to reap the rewards of the bird’s planning, benefiting from the high protein content. When the caches go untouched by birds, bears, or other animals; the seeds will hopefully germinate to create new stands of whitebark pine.

Populations of this species have been drastically affected by white pine blister rust (Cronartium ribicola), an invasive fungus that was introduced to North America in the 1900s. This fungus causes infections in trees that create injuries, called cankers, on branches and stems. Cankers damage the tree’s tissue and stop the transport of photosynthetic products and nutrients throughout the tree. The appearance of cankers will eventually lead to death of the branch, part of the tree, or the whole tree. Blister rust may also impact reproduction by weakening or killing cone-bearing branches.

Another existing threat to whitebark pine is the mountain pine beetle (Dendroctonus ponderosae). These beetles usually attack mature whitebark pine and can quickly decimate stands. Bark beetles lay eggs under the bark and, when the eggs hatch, the larvae mine the area beneath the bark and eventually cut off the tree's nutrient supply. The warming climate has enabled mountain pine beetles to reproduce in one-year, rather than multi-year, cycles and they are particularly attracted to fire-impacted habitat due to the presence of stressed trees. The beetles also carry a fungus that causes dehydration and inhibits a tree's natural defenses against beetle attacks.

More frequent and higher-intensity fires that have occurred in the North Cascades Range over the last decade pose a compounded threat to the area’s whitebark pine populations. Whitebark pine is moderately fire-adapted, so individuals can usually endure low to moderate intensity wildfire. However, individual trees have been weakened by stressors like the blister rust fungus, making them less likely to survive a fire. Decades of fire suppression have also put the tree at risk. Spaces between individual trees have filled with smaller, shade-resistant species that act as fuel to carry a wildfire, like the 2023 Sourdough fire, and cause it to burn hotter.

The 2023 Sourdough Burned Area Emergency Response (BAER) team included a forester, fire ecologist, vegetation ecologist, and botanist with relevant knowledge and experience, many of whom live and work in the North Cascades. These subject-matter experts researched and collected data to provide appropriate whitebark pine management recommendations to park management.

Though Sourdough Creek and Stetattle Creek drainages don’t have large or concentrated populations of whitebark pine, that makes the small number of individuals in the area even more important. The team identified where whitebark pine exists within the footprint of the Sourdough Fire and surveyed individual trees for damage by collecting data on where and how severely a tree was burnt. Some of the trees in the area were burnt, but others were spared in unburnt “islands” within the fire footprint. Scientists observed at least one tree mortality from scorch and several other burnt trees that may survive. Whitebark pine prefers direct light, so the surviving trees will benefit from increased sunshine, but these trees will see more challenges than benefits. Threats like mountain pine beetle and white pine blister rust now pose greater threats to these physiologically stressed trees.

The BAER team considered how to protect and support these small populations so that the ecosystem can fully rebound. They made recommendations to park managers, who may spend years engaged in the restoration activities recommended by the team. Ecologic effects last long after a fire is out, and the BAER team’s mission was to help provide a path to mitigate them and ensure this keystone species continues to survive in the subalpine ecosystem of the North Cascades.