Is the Bark Stronger Than the Bite? The Use of Models to Predict Bark Beetle-Induced Tree Mortality

Article: Robbins, Z. J., Xu, C., Aukema, B. H., Buotte, P. C., Chitra‐Tarak, R., Fettig, C. J., … & Scheller, R. M. (2022). Warming increased bark beetle‐induced tree mortality by 30% during an extreme drought in California. Global change biology, 28(2), 509-523. https://doi.org/10.1111/gcb.15927

Beneath the Bark

The calm forest can seem like a respite from our hectic daily lives. Indeed, the trees seem to be a calming presence, stoic and watchful. However, they themselves may be fighting their own chaotic battles. In the Western United States, conifer forests are especially at risk of perishing due to bark beetle attack. Bark beetles can be aggressive opponents and unfortunately, can overwhelm large numbers of trees. Before attack, the beetles congregate by secreting special airborne chemicals called pheromones. Then, all at once, they will descend on the trees, burrowing through the bark into the inner layer of the trunk. Once there, they will feed, and the females will lay eggs. This feeding activity disrupts the flow of nutrients and may strip the tree of its food and water, thereby eventually killing it.

As with most arthropods, bark beetles, such as western pine beetle (WPB), have multiple life stages. After hatching from eggs in the inner bark, larvae begin to eat and migrate toward the outer bark. Temperatures beneath the bark are critical for the larvae to begin transforming to the adult stage. After they become adults, they still must stay underneath the surface as their exoskeletons hardens. Finally, external temperature again plays an important role determining when adults emerge and synchronize flight. If the temperatures are either too hot or two cold, life stages will change accordingly. Thus, bark beetle populations in any given generation are dependent on previous seasons’ temperatures. Higher-than-normal temperatures result in voltinism, or an increase in the number of generations per year. Voltinism can have catastrophic consequences for host trees. A higher number of beetles will increase pressure on the pine trees when the ambush occurs. More trees will fall victim, and even healthier trees will perish if enough beetles attack.

Bark Beetle Buffet

Unfortunately, climate change has only served to worsen the severity of bark beetle outbreaks. In fact, over the past two decades, approximately 4.3 million hectares in the United States have suffered mass tree mortality induced by bark beetle attack. Warming temperatures lead to droughts that are longer lasting and increasingly frequent. This, in turn, causes an increase in conifer susceptibility to beetle attack as host tree moisture correlates with the tree’s ability to combat the pest. The drier and weaker the tree, the more attractive it is to a bark beetle. In order to better inform management and control decisions, modeling is used to predict bark beetle outbreaks and resulting host tree mortality. However, models must account for environmental changes as well as the subsequent reactions in both bark beetles and their host trees. Robbins et al. sought to create an accurate mathematical way to consider all these aspects; they accomplished this by developing two models. They then demonstrated the effect of warming on bark beetle phenology and the effect of drought on beetle-induced host tree mortality.

Photo caption: Taken in 1926, this photo represents the damage done beneath the bark of a ponderosa pine. The western pine beetle burrows into the area that moves water and nutrients up the tree. This may ultimately kill the tree. Photo credit: F.P. Keen, Source: Flickr

Math, Models, Mortality

The scientists used the western pine beetle (WPB) and its preferred host, Ponderosa pine (PP) as the study system. They built two complex statistical models to test two hypotheses: 1) Current air temperatures that are higher than those recorded historically will result in increased WPB-induced PP mortality, and 2) Higher tree mortality is caused by more bark beetle generations per year (voltinism) rather than a reduction in overwintering mortality. The study area included a mixed conifer forest in the Sierra Nevada area. This area suffered a massive PP mortality event because of WPB attack during a severe drought. The scientists used field data and observations to validate their model before simulating data and predictions.

The first model created by the scientists is called the insect mortality and phenology model (IMAP). This model encompasses the temperature-sensitive life stages of the bark beetles to calculate the proportion of the beetle population that will ultimately take flight under various temperatures. Expedited maturation, and therefore voltinism, as well as decreased overwintering mortality due to higher temperatures contributed to the model’s final output. The second model uses this proportion to determine how successful the beetles are at attacking and killing host trees. In other words, it will provide information on host mortality, based on host size and density as well as any negative impacts trees might experience due to drought stress.

After comparing model outputs when using historical temperature data versus contemporary temperature data, Robbins et al. discovered that PP mortality from WPB attacks increased 29.9% under our current warmer temperatures. Even worse, during one year, bark beetles accounted for the loss of roughly 45% of all PP biomass in the area. The second hypothesis was supported as well. An increase in voltinism led to an increase in PP mortality by 26.9%. A reduction in overwintering WPB mortality due to warmer temperatures led to only a 6.4% increase in PP mortality. This is certainly severe as well, but increased voltinism had a greater effect, as the researchers hypothesized. An increase in development rate and therefore more generations per year will lead to more egg-laying adults. The model estimated that the number of WPB that completed development was 35.1% higher in contemporary times at the start of the drought than the population under historical temperature simulations.

Climate change, as a key factor in drought frequency and intensity, directly affects the relationship between WPB and PP. Warming temperatures will produce more generations of beetles per year, and droughts will weaken even large, healthy trees. Protecting the majestic trees of the forest from bark beetle attack is a challenge, and efforts include thinning to reduce density and increasing biodiversity by introducing other species of trees (non-hosts). Models, such as the ones produced by Robbins et al. are useful tools to employ if we are to stay one step ahead of the beetles in protecting our forests. Forests not only serve as havens for recreation, but also reduce human-caused carbon dioxide emissions by sequestering carbon. Mass tree death will worsen climate change, as more carbon dioxide will remain in the atmosphere. Saving the trees from foes is critical in saving

Reviewed by: