Are Survivors Different? Genetic-Based Selection of Trees by Mountain Pine Beetle During a Climate Change-Driven Outbreak in a High-Elevation Pine Forest

Increased mortality of forest trees, driven directly or indirectly by climate change, is occurring around the world. In western North America, whitebark pine, a high elevation keystone species, and lodgepole pine, a widespread ecologically and economically important tree, have experienced extensive mortality in recent climate-driven outbreaks of the mountain pine beetle. However, even in stands experiencing high levels of mortality, some mature trees have survived. We hypothesized that the outbreak acted as a natural selection event, removing trees most susceptible to the beetle and least adapted to warmer drier conditions. If this was the case, genetic change would be expected at loci underlying beetle resistance. Given we did not know the basis for resistance, we used inter-simple sequence repeats to compare the genetic profiles of two sets of trees, survivors (mature, living trees) and general population (trees just under the diameter preferred by the beetles and expected to approximate the genetic structure of each tree species at the site without beetle selection). This method detects high levels of polymorphism and has often been able to detect patterns associated with phenotypic traits. For both whitebark and lodgepole pine, survivors and general population trees mostly segregated independently indicating a genetic basis for survivorship. Exceptions were a few general population trees that segregated with survivors in proportions roughly reflecting the proportion of survivors versus beetle-killed trees. Our results indicate that during outbreaks, beetle choice may result in strong selection for trees with greater resistance to attack. Our findings suggest that survivorship is genetically based and, thus, heritable. Therefore, retaining survivors after outbreaks to act as primary seed sources could act to promote adaptation. Further research will be needed to characterize the actual mechanism(s) of resistance

Using ISSR Markers to Study Genetic Diversity in Whitebark Pine (Pinus albicaulis)

Whitebark pine (Pinus alibicaulis) is a highly endangered tree species in North America. It is experiencing high mortality due to white pine blister rust and mountain pine beetle outbreaks, as well as the effects of climate change on it’s habitat. It is unknown how this massive mortality has altered the genetic diversity of whitebark pine populations. These large die-offs can act as strong selection events, removing individuals with lower fitness. I am looking at whether survivors of mountain pine beetle outbreaks have different genotypes than those killed, and what effect severe beetle outbreaks have on the genetic diversity of populations. I am using Inter-Simple Sequence Repeats (ISSR), a method that detects high levels of genetic polymorphism, to test two hypotheses: 1) surviving trees differ genetically from those that are killed by the beetle, and 2) the outbreak has reduced overall genetic diversity in affected stands. I will compare the genetic diversity in two whitebark pine stands—one that has experienced a beetle outbreak with scattered survivors, and one that has not experienced beetle kill. I collected needle samples from both populations and isolated the genetic material from each individual tree. I am currently screening primers and performing gel electrophoresis to analyze genetic differences between individuals. This study will develop a more informed approach to whitebark pine restoration. If I can identify trees with drought tolerant, beetle-resistant genotypes, managers can avoid use this information to replant trees with favorable genotypes. Whether I am able to detect markers associated with survivor genotypes or not, I will determine whether beetle outbreaks reduce genetic diversity in this tree. This will aid in the understanding of how this tree will adapt to climate change in the future

Genetic differences in whitebark pine after a mountain pine beetle outbreak

Whitebark pine (Pinus albicaulis) is a high elevation tree in serious decline.. It is experiencing high mortality due to white pine blister rust, an exotic disease, and outbreaks of the native mountain pine beetle, as well as the effects of climate change. It is unknown how this massive mortality has altered the genetic diversity of whitebark pine populations. Furthermore, large die-offs can act as strong selection events, removing individuals with lower fitness. It is possible that survivors of mountain pine beetle outbreaks have different genotypes than trees killed and may be better adapted to current and future warmer drier climates. This project uses Inter-Simple Sequence Repeats (ISSR), a method that detects high levels of genetic polymorphism, to test two hypotheses: 1) surviving trees differ genetically from those that are killed by the beetle, and 2) the outbreak has reduced overall genetic diversity in affected stands. Needles were collected from surviving trees after a beetle outbreak and from trees just under the minimum diameter beetles attack. The smaller trees were used a surrogate for the ‘general population’ pre-beetle selection. DNA was extracted from the needles and screened with three ISSR primers using PCR and gel electrophoresis to analyze genetic differences between individuals. Survivors and a few general population trees clustered distinct from other general population trees indicating distinct genetic differences among survivors and those selected by the beetles. This suggests that selection may be occurring in these populations as a result of the beetle outbreaks. Information gained in this study will help develop a more informed approach to whitebark pine restoration and forest adaptation to climate change