One of the so-called problems in recent years in many forests in the western United States has been larger “attacks” of various species of bark beetles, which feed on the trees, introducing fungus as they do so that eventually kills the trees; the fungus converts tree tissue into food for the beetles. When there is an unusually large growth in beetle population, many acres of pine trees can die in fairly short order.
It is a normal process that some years, there will be a lot of beetles in one area; however, the total area being affected by beetles has grown over the years, due to global warming. This, we are told, increases the risk of terrible fires for the rest of the forest around the beetle-killed area, and provides justification for the US Forest Service (part of the Department of Agriculture) to do preventative thinning, which of course benefits timber companies and their supporters that can’t stand to see any forest not converted into dollar signs.
Now, however, there is some evidence that beetle-killed forest areas are no more likely to burn than other forests – what increases the risk of a section of forest catching on fire is the terrain and whether or not the region is in a state of drought, because drought means overall drier conditions, and if things are drier, well, they burn more readily.
What is even more interesting is what trees the beetles don’t kill:
A healthy tree can usually beat back invading beetles by deploying chemical defenses and flooding them out with sticky resin. But just as dehydration makes humans weaker, heat and drought impede a tree’s ability to fight back — less water means less resin.
Researchers have found evidence that trees that have genetically adapted to warmer, dryer conditions are more able to resist being eaten by beetles.
Millar was comparing tree-core measurements of limber pines, a slight species found in the eastern Sierras of California that can live to be 1,000 years old. After mountain pine beetles ravaged one of her study sites in the late 1980s, certain trees survived. They were all around the same size and age as the surrounding trees that the beetles tore through, so Millar looked closer at tree ring records and began to suspect that, though they looked identical on the outside, the stand in fact had contained two genetically distinct groups of trees. One group had fared well during the 1800s, when the globe was still in the Little Ice Age and average temperatures were cooler. But this group weakened during the warmer 1900s, and grew more slowly as a result. Meanwhile, the second group seemed better suited for the warmer climate, and started to grow faster.
When beetle populations exploded in the 1980s, this second group mounted a much more successful battle against the bugs. After surviving the epidemic, this group of trees “ratcheted forward rapidly,” Millar explains. When an outbreak flared up in the mid-2000s, the bugs failed to infiltrate any of the survivor trees in the stand. The beetles had helped pare down the trees that had adapted to the Little Ice Age, leaving behind the ones better suited to hotter weather. Millar found similar patterns in whitebark pines and thinks it’s possible that this type of beetle-assisted natural selection is going on in different types of trees all over the country.
One of the arguments against human-directed “thinning” of forests, to try and improve their health (reduce the risk of fire, reduce beetle outbreaks, etc., all of which is, naturally, heavily supported by anyone who wants to support the timber industry) is that we cannot tell which of those trees being cut down might contain some really great genetic material that, if left to reproduce, would lead to a better long-term health of the forest. Ecosystems, however, have built-in processes that remove members of the population that are not well-suited to current conditions, and we are currently in conditions that are just the beginning of what is to come.
Despite the “humans are smarter than wild systems” biases, it could be that the best thing to do for forests, to ensure their ability to adapt to and survive global warming, is to leave them alone, and let other elements of the ecosystem help those best-suited for hotter, dryer conditions have more space in which to grow.
(Edit, June 11: for more on wildfire and bark beetles, check the wildfire tag, I found some new stuff!)