Felled trees that were killed by bark beetles. Photo credit: US Forest Service.
Wandering through a post-fire forest can feel like walking through a graveyard. Blackened skeletons loom over the soft ash.
Natural disasters like severe wildfires are some of the most extreme illustrations of the ways in which climate change can transform a familiar landscape. But while ecosystems can recover from short-term disaster, warming temperatures are pushing ecosystems everywhere toward a new normal. Can nature keep up?
In recent years, wildfires in the American West have gotten stronger, more severe, and more frequent. Poor fire management, rising temperatures, and drought are some of the most well-known causes of the longer fire season. But the changing climate is also changing ecosystems’ makeup in a few surprising ways.
Problem #1: Zombie Forests.
New research at Stanford University recently named the phenomenon of “zombie forests.” “Zombie forests” are forests made up primarily of conifers like pines, sequoias, and firs where, due to climate change, it’s become too hot for conifers to survive, so the conifers die. When conifers die, they dry out. Their naturally flammable sap makes them great fuel for the fires. And because they remain standing, their death does not create room for new trees to grow.
Historic vs. current conifer habitat; regions that are purple on the right and orange on the left are home to “zombie forests.” Credit: Avery Hill and Stanford Data Science.
This is part of a broader trend in climate change called vegetation transition. Animals can migrate in reaction to their habitat getting too hot or too dry; plants can’t. Instead, they remain standing, though dead
It’s estimated that 2,000 square kilometers of the Sierra Nevada mountains are zombie forest – roughly 18% of all conifer forests in the region. The town of Paradise, CA, which suffered devastating damage after a wildfire in 2018, is surrounded by zombie forests. Though the Paradise fire was sparked by a power line, zombie forests fueled the inferno.
Problem #2: Bark beetles
Bark beetles are another forest scourge whose spread is aided by climate change. Since the 1990s, bark beetles have been infesting forests at epidemic levels all over the western U.S. and Canada.
Bark beetles have existed for millenia, and are a typical part of the forest ecosystem. At healthy levels, they can even be productive: usually bark beetles infestation, which target older trees, help make room for new growth.
Mountain pine beetles are tiny – between ⅛ and ¼ of an inch long – but these miniscule insects can take down a giant tree through sheer numbers. They live in and eat through the layer under the tree’s bark, the phloem, and create tiny tunnels that are visible where the bark has fallen away.
By studying tree rings, ancient pollen samples, and beetle remains, scientists have reconstructed ancient infestation patterns. What they found is that current invasion patterns are happening at a higher intensity, over a wider geographic range, and more frequently in multiple forests at the same time than they have before. Why?
It turns out warmer temperatures are great for beetles – they have fewer die-offs in winter and higher reproductive rates, and they even grow up faster when it’s a few degrees warmer.
In addition, droughts stress trees, which weakens their defenses against beetle attack. Injured trees are especially vulnerable – after a severe fire, bark beetles kill up to 25% of the trees that survive in succeeding years.
Forest after a beetle kill; red trees have died. Photo Credit: UBC Micrometeorology.
Climate change and fire worsen beetle attacks. Unfortunately, the inverse is also true. Beetle Invasions can kill up to 90% of trees in a stand, leaving them vulnerable to fire. In addition, the death of larger trees leaves a forest more vulnerable to high winds, which can blow fires further.
Will the Sierra’s Forests Survive?
At the end of the day, a lot about how forests will function in the future remains mysterious. The forces mentioned here don’t doom conifer forests; if anything is constant about forests, it’s their ability to regenerate and change.
But it is concerning that vegetation transition exists already at such a large scale (18% of conifer forests in the Sierra Nevada) and that bark beetle invasions have increased so dramatically in the past 30 years alone. If enough forests die past their ability to regrow, it’s possible that aspens and firs may take over where pines now reign, and many species will lose crucial habitat.
This is not just an environmental concern. Severe fire comes at higher costs to the humans living nearby, as the case of Paradise makes clear. Monitoring these issues with greater precision demands the attention of scientists.
As a citizen scientist, you can contribute by going into the woods and tracking evidence of beetles, as well as identifying plants that may signal to scientists that vegetation transition is happening. Apps like iNaturalist are incredibly useful for aggregating data; even for those outside California, data on species distribution is always helpful for ecologists. As ecosystems evolve, zombie forests and killer beetles will likely be just the first in a long line of strange phenomena that accompany our warming forests.