Alberta has 2 endangered tree species: whitebark pine (Pinus albicaulis) and limber pine (Pinus flexilis). These species are endangered because they have been declining rapidly across their ranges. They grow slowly, only starting to produce cones around age 40 (limber pine) and 50 (whitebark pine).
Reversing their decline is a long-term commitment. Without healthy populations of these keystone trees, their ecosystems would no longer provide the habitat and values that animals and people depend on, including:
- slope stabilization
- a rich source of food for birds
- bears and small mammals
- headwater streamflow control
- subalpine tree island formation
- windswept scenic beauty
The bird pines
Sustaining and conserving these trees means focusing on more than just the tree itself. Whitebark and limber pine both depend on the Clark’s nutcracker, a bird in the jay family, to reproduce.
Eons of co-evolution driven by the nutcracker's role in seed dispersal have led to the whitebark and limber pines’ both producing very large, wingless seeds with heavy seed coats. While these characteristics render the trees completely reliant on the nutcraker to open its cones and disperse ripe seed, they provide crucial support for seed development after germination and help the seeds establish in areas with little soil and nutrients.
The nutcracker pecks the cones open, extracts the seeds (which are very high in protein and fat) and caches them in the ground with its beak. The nutcracker can store about 100 seeds at a time in a special throat pouch. Each bird plants about 100,000 seeds and digs up about 30,000 seeds each year, from memorized locations, as its main source of food. Surviving seeds grow into seedlings.
Unlike those of the whitebark pine, limber pine cones open, but only seeds cached by nutcrackers germinate and grow into trees.
Four main threats affect these species. Reversing or lessening the damage caused by these threats is key to their recovery.
White pine blister rust
White pine blister rust (Cronartium ribicola) is a deadly human-introduced fungus from Asia that affects 5-needle pine trees and has spread through North America for over a century. The fungus needs both 5-needle pine trees (primary host) and currant bushes (alternate host) to complete its life cycle, although it does infect some other plants. Efforts to eradicate the rust by removing these alternate host plants have failed. The fungus infects needles and grows into the main stem where it cuts off vital water and nutrients, killing the tree. When a large tree is infected, it may take years to die. When the crown starts to die cone production stops and its key ecological function is lost.
Rare 5-needle pine trees have been found with natural tolerance or resistance to the disease. These trees must be tested to prove their seedlings are inheriting disease resistance, a process that takes 7 years in a controlled facility, or longer in the field. Seeds from these special trees are collected and used for restoration. Cuttings collected from these trees, called scion, are grafted to make copies of the resistant trees for gene conservation. Scion from mature trees keeps its physiological maturity, so these grafted scion can produce seeds decades earlier than waiting for seedlings to mature conventionally. Grafts are planted in genetic archives called clone banks, preserving extra copies of the original trees. They are also planted in seed orchards, similar to a fruit orchard but managed to produce abundant seed crops.
Mountain pine beetle
Recent mountain pine beetle (Dendroctonus ponderosae) outbreaks have killed far more trees than in the past, and spread into higher elevation forests. Beetle populations have expanded outside their historic range due to recent fall, winter, and spring temperatures being too mild to kill overwintering larvae. Extensive mature pine forests allow the mountain pine beetle the opportunity to spread widely.
Plant-derived compounds can protect valuable blister rust resistant trees from mountain pine beetle attack. One such compound, vebenone, sends out scnets that mimic a fully attacked tree’s chemical signals, which helps repel attacking beetles. Others, such as green-leaf volatiles, send signals to insects disguising the tree as a type that is not a host for beetles. Landscapes can also be managed to reduce the chance of beetle attack by changing the composition of species and age classes, and by removing trees recently attacked by beetles that contain overwintering larvae.
Wildfire suppression and succession
Whitebark and limber pine trees have thin bark, making them vulnerable to fire. But regeneration of these slow-growing pines can also benefit from open habitat created by fire. Fire suppression promotes shade tolerant species like Engelmann spruce and subalpine fir, which form dense stands that hinder whitebark and limber pine growth.
Identified rust-resistant trees are identified as high-value resources and protected from fire by the Alberta government’s Wildfire Management Branch. Some areas benefit from wildfires or prescribed fire through reducing fuel buildup and creating open caching sites for Clark’s nutcrackers.
Whitebark pine and limber pine live at the environmental limits of tree growth. Warming is moving species higher in elevation and latitude – whitebark pine may be at risk of being forced off mountaintops as species shift upwards. Moving northwards is another option that has been evaluated but, if suitable habitat is already occupied by other species, whitebark pine is a poor competitor. Planting these pines in novel habitats (often called assisted migration) can only succeed if there is enough high-quality alternate food to sustain Clark's nutcracker populations for decades until the migrated pine populations mature. No habitat in Canada appears to meet these conditions. Conversely, climate models predict limber pine will benefit from more suitable habitat and favourable climates, especially in lower sites.
Studies that use climate and habitat models, test assisted migration and establish provenance trials to observe how seedlings originating from different populations grow in different environments are providing information on management options for these species as their habitat changes. This research is useful for assessing the appropriateness of certain seed sources for restoration planting. Slow growth and centuries-long life spans show they have adapted and grown under a wide range of conditions.
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