Whitebark Pine (Pinus albicaulis) is an endangered subalpine keystone species growing in the western North America and Canada. It plays a significant role in the mountain ecosystem by moderating snow and soil conditions and allowing the establishment of diverse plant communities. The highly nutritious seeds constitute an important food source for a diversity of wildlife such the Clark’s Nutcracker (Nucifraga Columbiana), grizzly bears (Ursus arctos), black bears (Ursus americanus) and red squirrels (Tamaisciurus hudsonicus) for whom the seeds are a crucial food source. There is an interesting mutualistic relationship between the Whitebark pine and the Clark’s Nutcracker as the Whitebark pine relies exclusively on the nutcracker to plant its seeds. The nutcracker caches the seeds for its fall and winter food supply and within its lifetime, a single nutcracker is estimated to cache 98,000 seeds in up to 7,500 places where it consumes around half of those, leaving the others a chance to germinate.
One of the greatest threats to whitebark pine populations is blister rust (Cronartium ribicola). It is an introduced pathogen, introduced from Eurasia in 1910 that now occurs in whitebark pine throughout its range in Canada. Blister rust infects the needles of white pines and disrupts the movement of nutrients and sugars by growing into the branches and trunk of the tree. Cankers erupt on the surface of the bark, damaging the tissues of the tree. Rodents like to feed on the sugar-rich rust and can cause further damage to the tree; although in some cases rodents can also kill the rust by consuming it. Blister rust causes death by the loss of vascular tissue and weakening of the tree, allowing secondary pathogens to infect it. Low occurrence of rust resistant individuals has been documented in wild populations and this is key to enabling the regeneration of whitebark pine populations.
Another threat to the whitebark pine is the mountain pine beetle (Dendroctonus ponderosae), which is a native beetle species whose survival, growth and reproduction have been enhanced by changes in climate. The management of forests, mainly the increase in lodgepole pine within the age class susceptible to mountain pine beetle attack has played a role in the beetle becoming epidemic in British Columbia. It generally only kills whitebark pine weakened by blister rust infection, though under epidemic condition it will kill healthy trees too.
Other threats to whitebark pine include fire exclusion and climate change. The reduction in low intensity fires has reduced the suitable habitat for whitebark pine and increased competition with shade tolerant species. Climate change is predicted to result in reduced suitable habitat for whitebark and a shift in habitat location. The ability of the whitebark pine to colonize new habitat is unknown.
Blister Rust Screening
Attempts to suppress or eradicate blister rust in North America have failed and the persistence of white pines will depend on fostering co-existence with the disease in populations. This can be achieved by screening seedlings produced from trees thought to exhibit some resistance. Seedlings are exposed to rust spores and monitored over time for infection. Seedlings that become infected and survive exhibit some tolerance or resistance of the disease. This process allows introduction of an increased frequency of individual trees that can withstand infection within wild populations; something that is unlikely to develop in wild populations without intervention. It is critical to address the most imminent threat to the survival of whitebark pine and contributes to the long-term conservation of the species.
Seed Collection
Whitebark pine cones are collected for blister rust screening, restoration activities and ex-situ gene conservation. The aim of seed collection from whitebark pine is to collect seeds from putatively blister rust resistant trees, screen the seedlings and monitor their health, and identify the potential level of rust resistance that may be attributed to a parent tree. Seed collections need to be well documented, considering location and methods of collection over time. A tree that appears to produce rust resistant seedlings may be revisited to collect further seeds and scion material to establish a seed orchard.
Whitebark pine is a masting species and cone crops vary between years and locations. Assessment of immature conelets allows prediction of the next year’s crop. Cages are placed around cones during the summer months to protect them from predation and allow them to mature before harvest in the fall.
Cone Production
Currently, seed collection occurs in unmanaged stands where suitable trees are selected. The goal is to collect from selected stands, seed production areas and seed orchards in the future.
Selected whitebark stands have a history of good cone production, easy site access, harvestable cones and rust resistant traits. Typically, these would occur where blister rust infection is high. Stands may be managed to improve the quantity and quality of seeds at a site. These include seed production areas (SPAs) and seed orchards.
Seed production areas are natural stands of whitebark pine of seed-producing age. Non-target and competing trees are removed to increase the productivity for whitebark pine. Diseased trees may also be removed. This management method may allow increased seed production at a reduced time. It is a cost effective method of obtaining seed, since it reduces collection costs and long-term monitoring may assist with the selection of rust resistant trees.
Seed orchards are established from seedlings or grafts of scion grown from rust resistant parent trees, with the goal of producing rust resistant seeds more rapidly. Due to the slow growing nature and lack of experience with grafting of whitebark pine seed production from orchards in British Columbia is currently limited. Established orchards will not have seed production for some time.
Seedling Culture
Whitebark pine seeds are sown in a heated greenhouse in April-May. Once they germinate, they remain without heat in the greenhouse until the second year and usually planted in the summer of the third year, after bud-set.
White Pine Blister Rust in British Columbia
Data on blister rust has been collected in B.C by individual research, seed collection and restoration programs and long-term monitoring plots have been established by government. Long-term monitoring of changes in rust infection over time in a stand aids in the understanding of rust trends over time.
83 long-term blister rust monitoring plots and transects were compiled in British Columbia, with 67 established by National Parks, 11 by provincial government and 5 by industry. The plots are not distributed evenly over the range of whitebark pine in BC and there is a need to establish more plots across the province to improve the ability to monitor blister rust infection dynamics and allowing collection and restoration activities to be prioritized.
A collation of a 20 year dataset from 573 temporary and permanent plots mean rust infection rates were lowest in the south west of British Columbia and highest in the south east and north west. Further data collection is required on blister rust infection and other impacts on the whitebark pine population such as the mountain pine beetle, conifer encroachment and cumulative impacts of anthropogenic activity.
- Source Don Pigott, Randy Moody, Alana Clason. Promoting Whitebark Pine Recovery in British Columbia. Prepared by sern = society for ecosystem restoration in north central British Columbia for the Ministry of Forests, Lands and Natural Ressources.