The answer – genetic diversity – may unlock secrets for managing this important tree species under climate change.
Dramatic aerial photos show patchworks of green conifer forests interrupted with large swaths of dead, burnt-orange trees: evidence of the mountain pine beetle outbreaks that are killing masses of trees in parts of western North America. Though mountain pine beetles are native to these forests, a combination of factors including climate change are leading to more severe and devastating mountain pine beetle outbreaks.
Whitebark pine, an important species in high-elevation ecosystems of western North America, including parts of Washington, Oregon, Idaho and Montana, is not immune to this tiny but mighty beetle. Whitebark pine is declining across its range due to stressors including mountain pine beetle outbreaks, which are worsening with climate change. But some whitebark pine trees are surviving beetle outbreaks, and scientists are wondering why.
NW CASC-funded research explored how individual whitebark pine trees’ genetic traits, growth rates and climate sensitivities influence their ability to survive mountain pine beetle outbreaks. They found evidence of significant genetic diversity among whitebark pines, which increases the likelihood these trees can adapt and persist in the face of both mountain pine beetle attacks — and climate change.
Whitebark pine are more at risk from mountain pine beetles under climate change.
Whitebark pine grows in subalpine zones, where cooler temperatures usually protect it from beetle attacks. In periods of warm droughts, however, beetles are able to move into higher elevations and prey on mature whitebark pine until colder conditions return. Warmer conditions brought by climate change are allowing the mountain pine beetle to persist in subalpine zones, causing outbreaks and death of mature whitebark pine.
Since whitebark pine has, until recently, been buffered from mountain pine beetle attacks in its subalpine habitat, it may have fewer defense mechanisms than tree species at lower elevations that have experienced more consistent pressure from the beetle over time. This is cause for concern that whitebark pine could experience severe declines and local extinctions where the beetle is now present.
Some whitebark pine trees are surviving beetle attacks. But why?
Despite these threats, some mature whitebark pines have survived severe mountain pine beetle outbreaks, suggesting that adaptation may be occurring in response to increasing selection pressure from the beetle. But little research has been done to understand tree adaptation to climate-induced insect outbreaks — and even less on how genetics and climate sensitivities of individual trees influence their resistance to the beetle. Instead, most bioclimatic models assume that all individuals of a tree species respond similarly to insects and usually incorporate only broad ecological parameters.
Certain traits keep whitebark pine trees safe from attack.
Mountain pine beetles use both visual and chemical cues to decide which trees to attack, and there are certain traits that make some trees less attractive to beetles than others. To better understand these survival traits, NW CASC researchers compared surviving, mature whitebark pine (mature trees are more prone to attack), with small whitebark pine that escaped predation (used as a proxy for populations without beetle predation). Since some tree traits are determined by genes, while others are determined by a combination of genes and interactions with the environment, researchers looked at genetic and chemical profiles, along with growth rates and climate data.
By comparing these traits, researchers wanted to know whether recent mountain outbreaks had led to rapid adaptive selection for trees with genes and physical traits associated with survival. The researchers expected the mature, surviving trees to have similar genetic and chemical profiles that were distinct from the smaller whitebark pine, and for the surviving trees to be less sensitive to drought and temperature. They were surprised to find considerable complexity within and among populations of whitebark pine, making it hard to point to specific traits that defined tree resilience. Different cohorts of trees in each population exhibited different genetic and physical profiles as well as different responses to climate.
Some patterns did emerge. They found growth to be the trait most clearly associated with survivorship. In their study, surviving trees experienced fast growth in early years, followed by slower growth prior to the outbreak. But how do beetles differentiate between trees with different growth rates? There may be a connection between growth rate and how trees spend their energy strengthening their defenses, like chemical signals, which beetles can detect. They also found that the trees’ sensitivity to drought and temperature was not predictive of survivorship, although tree responses to climate variables across the sites suggest that genetic differences within whitebark pine populations increase the diversity of tree responses to climatic conditions, which may increase forest resilience to changing conditions over time.
The take-home message: diversity is key to adaptation.
This study found evidence of significant genetic diversity among white bark pines. The ability of whitebark pine trees to persist in the face of stressors like climate change and mountain pine beetle attacks will rely on genetic diversity, which increases the potential for adaptation. To help enable whitebark pine to adapt to changing conditions, maintaining genetic diversity within and among populations should be prioritized while practices that reduce diversity should be avoided or approached with caution.