Breeding for Success

Of Whitebark Pines and Blister Rust

On a recent hike to a pass above Esmeralda Basin, I came across a whitebark pine (Pinus albicaulis) tree bearing a sign that read: “Select Tree for Genetics - PLEASE PROTECT.” One of my favorite tree species, whitebarks are denizens of the high country, found in subalpine and alpine zones, particularly along the Cascade crest. They grow singly, in stands, and as krummholz, the stunted and fierce trees able to withstand some of the region’s harshest growing conditions.

In the words of ecologists, whitebark’s are a foundation species, one whose structural and/or functional attributes are essential to the health and function of their ecological community or ecosystem. Whitebark’s meet this definition by being a nurse tree, particularly in disturbed, high elevation sites, where they are often the first to grow. After establishment, the trees provide shade and a windbreak and retain soil and snowpack, which combine to create ideal microsites where other species can colonize and thrive.

Whitebarks are also one of the more threatened species in the Cascades. The threats come in three sizes. From smallest to largest, they are white pine blister rust (Cronartium ribicola), mountain pine beetles (Dendroctonus ponderosa), and humans (Homo sapiens). The first two are indiscriminate killers via direct intercession and the latter, an indirect killer, due to how we have changed the climate, as well as to our suppression of fire in low elevation forests. The sign I saw was directly related to the first concern.

White pine blister rust is a non-native fungal pathogen that arrived in the United States in the late 1800s and primarily impacts five-needled pines, such as the whitebark. The blister rust kills trees by creating cankers that can girdle the stem and limbs. In young trees, cankers can kill rapidly whereas larger, older trees may persist for decades, although functionally dead, because the top half of the tree, which contains the majority (sometimes all) of the cones, has died. Blister rust can cause up to 90 percent mortality, making it a primary killer of whitebark pines.

Fortunately, some whitebark pines have genetic resistance to blister rust, which is where the sign comes in. Over the past decade or so, researchers have gone out in the field, harvested whitebark pine seeds, brought them back to a nursery, planted them, inoculated them with blister rust, and tracked their growth. When researchers found a seedling resistant to blister rust, they knew that the parent tree had to be resistant, too. And, that it had to be protected because disease resistant whitebarks could be the source for future generations of trees adapted to this pathogen.

To further protect the trees, researchers also staple a green packet to the resistant trees and to other whitebarks growing nearby. Each packet contains verbenone, a naturally occurring chemical known as an “anti-aggregation pheromone,” which helps repel attacks from mountain pine beetles. Native to the region, the beetle’s larva eat the inner bark, or phloem, and create an extensive gallery of tunnels, or mines, which cuts off the movement of water and nutrients. Long present and long innocuous across the western United States, mountain pine beetles have become problematic in recent decades because of catastrophic outbreaks, compounded by climate change.

After researchers locate disease resistant whitebark pines, they regularly go out and harvest the seeds and grow them in the nursery. They then take those seedlings and plant them in the wild. The hope, and plan, is that those rust resistant trees will spread, aided primarily by their relationship with Clark’s Nutcrackers, birds that relish whitebark pine seeds; a typical nutcracker can harvest about 90 seeds at a time (stored in a small pouch under their tongue), which they then carry (anywhere from inches to up to 20 miles away) and bury, or plant, as a food source for later eating. Because the birds fail to harvest all of their buried seeds, many start to grow; if the nutcrackers harvest seeds from the disease resistant trees, the birds will spread these trees and help create healthier forests.

Biologists know that this will not be a quick fix. It may take a century or more for expansion from the core to outlying areas but ultimately the new trees and forests will have resilience to blister rust and climate change. One thing that inspires these ecologists, is that they know of a similar process of whitebark spread. Research shows that after the last Ice Age, Clark’s Nutcrackers carried whitebark pines seeds from refugia out into terrain previously covered by ice, ultimately forming the modern distribution of the whitebark pine ecosystem. If it happened once in the past, it can certainly happen again, at least that’s the plan. So keep your eyes open for the Select Trees and treat them and their surroundings well.

Thanks to Helen Lau for providing me information about these trees and for your work in protecting them.

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Upcoming Talks and Walks

Sept 9 - 11am - Redmond Library - I’ll be talking about Spirit Whales and Sloth Tales.

Sept 12 - 6:30 PM - Lacey Museum - I’ll be talking about Puget Sound: A Maritime Highway. Available via Zoom, too.

Sept 21 - 10:30am - Field Trip Society - I’ll be leading my new walk, Georgetown: Beer, Burials, and Rails.

Oct 5 - 10:30am - Field Trip Society - I’ll be leading my very fun and popular Who’s Watching You walk, exploring carved and terra cotta faces in downtown Seattle.