Somewhere in a forest in the mountain West, a female beetle no bigger than a grain of rice releases pheromones, hormone-like chemicals that attract other beetles to mass-attack ponderosa pine trees. If successful, they start chewing their way into the bark and lay their eggs. These beetles commonly carry spores of a blue-staining fungus that will also infect trees and could disrupt their water transportation systems.

If it’s been a rainy, snowy year and some other calamity hasn’t stressed the tree, it fights off the attack by pushing resin out through the beetle-chewed hole, blocking more beetles from entering and preventing the fungus from girdling the tree and cutting off water and nutrients. A subsequent cold winter would kill a large number of beetles, putting the beetle invasion on ice.

In drought, it’s a different story. Stressed by the lack of water and unable to push resin out against the invaders, ponderosas struggle to mount a defense. More and more female beetles lay their eggs in the grooves of the bark, where the larvae feed on the soft, tasty inner bark and the phloem, the veiny material that passes sugars down from the leaves.

Attracted by the wind-borne pheromones, male and female beetles swarm to the tree. The invading females release more pheromones and attract even more beetles. If the drought-stricken tree cannot repel the attack, the beetles overwhelm it in a synchronized mass attack, and it will die within a year. If a large number of new adults fly from the tree and are not disrupted by heavy weather, which prevents other beetles from detecting the pheromones, they will attack another tree, another, and another. How quickly the beetles spawn new generations influences the number of trees that will suffer.

Our team of scientists from Los Alamos National Laboratory and several other institutions wanted to see if increased temperature during a drought makes beetle attacks worse. Warmer-than-normal droughts have become increasingly frequent in the West and New Mexico under global warming. Using Los Alamos supercomputers, we modeled bark beetle dynamics and ponderosa pine die-off during the extreme drought of 2012-15 and earlier periods in California’s Sierra Nevada. Then we studied field observations of maximum and minimum temperature, precipitation, the density of forests, how many trees died, and when fully developed beetles left their tree of origin. We also examined lab studies of the rate at which the beetles develop to maturity and fly from their home tree.

In warm droughts, trees lose more water to the atmosphere through their leaves and needles. In warm winters, beetles in greater number survive the cold, while an earlier spring gives beetles a head start on their reproductive cycle. Under these climate-change conditions, beetle population growth accelerates quickly as tree defenses wither.

That one-two punch devastates forests, but how badly? Our team found nearly one-third more ponderosas die during warm droughts because of the accelerated rate of development of western pine beetles and, to a much lesser extent, because fewer die in the winter under warming conditions. That’s on top of the tree-mortality rate caused by drought alone.

Our research showed that even slight increases in the number of annual generations of bark beetles due to warming can significantly increase tree mortality during drought. We had previously thought the beetle needed an additional whole generation each year to substantially impact tree die-off. In the Sierra Nevada, though, we saw only about one-third more generations per year, and yet that really amplified ponderosa mortality.

So a small bump in population growth among western bark beetles and their relatives can spur catastrophic mortality in forest systems during abnormally warm droughts in the Sierra Nevada and throughout the Western United States, including New Mexico. An increasing number of outbreaks in the past two decades have devastated forests, striking nearly 11 million acres nationwide and threatening the basic structure and ecological processes of many forests.

This trend could accelerate global warming. Forests worldwide absorb almost a third of the fossil-fuel carbon released into the atmosphere, so they have a dampening effect on global warming. Because dead trees don’t absorb CO2 but instead release it to the atmosphere through decomposition, beetle infestations could substantially reduce forests’ capability to absorb CO2. But that’s an area that needs further study, as insect dynamics have not yet been incorporated in current generation of Earth-system models.

We still have much to learn about the complex interactions among climate, trees, beetles, a variety of other ecological factors and climate. For instance, increasing temperatures might also put the brakes on beetle population growth, since the tiny bug thrives within a kind of Goldilocks zone of temperatures: not too hot, not too cold. It will take more research to find out.

Zachary Robbins is a researcher at Los Alamos National Laboratory, graduate student at North Carolina State University and lead author of the recent paper “Warming increased bark beetle-induced tree mortality by 30% during an extreme drought in California.” Chonggang Xu, a senior scientist at Los Alamos, simulates forest-vegetation dynamics in his research and is a coauthor of the paper.

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