It's a bad time to be a pine tree in western North America. The problem is the mountain pine beetle. Mountain pine beetles, which burrow into pine trees and disrupt the flow of sap, the lifeblood of the tree, have killed millions of acres of pine forest in the western US and Canada, and the killing goes on. You may be thinking that the pine beetle must be an invading, non-native species, but no, it turns out it's a native species. What's turned the pine beetle from a pest into a creature that's transforming whole landscapes, is human activity. For decades we've been putting out forest fires and creating global warming.
You may think that stopping forest fires is a good thing, but ultimately it's not. Forest fires are actually a vital part of forest ecosystems. They turn dead trees into ash which replenishes the soil. They kill off a lot of parasites like the mountain pine beetle, and they provide clearings where young trees have access to sunlight, which is vital to their healthy development. Without forest fires the number of dead trees goes up. Dead trees are a perfect place for pine beetles to eat and raise their larvae without the threat of pine pitch, which, in many cases, can kill them. Even for humans, putting out forest fires is turning out to be a bad policy. Most people who decide to live in pine forests do so for the seclusion and natural beauty. Now they find themselves living in forests of dead trees, through which they can readily see their neighbors. And with all of that dry wood around, these homeowners are now living in places that are severe fire hazards.
Global warming has helped the mountain pine beetle in two main ways: Before global warming, winters would include numerous snaps of extreme cold. These cold snaps would kill off large numbers of pine beetles. Now that rarely happens, so the beetle's numbers have been expanding, which has meant more dead trees, which means more pine beetles, and so on, and so on... You get the idea. Global warming is also responsible for a drought that has been going on in the west for years. This has left pine trees weakened to the point where many of them cannot fend off the mountain pine beetle by using their pitch, or by killing off their own cells which releases a substance that is super toxic to pine beetles. When a pine tree is weak and starved for water, it can't spare the resources to do either of these things effectively. This also creates more pine beetles who kill more trees, which creates more pine beetles....
So, is there any hope? Scientists say that pesticides kill off too many species to use, plus, at these scales they would be simply too expensive. Cutting down infected trees has proved minorly successful, but again, in an epidemic that is literally changing the landscape of half a continent, it's no solution. Ironically, as global warming continues, it may have a positive effect for the pine trees. The mountain pine beetle brings with it a fungus when it bores into trees. This fungus feeds their larvae, but as the average temperature goes up, this fungus is dying off. Scientists say it is possible that this trend may curb the pine beetles' numbers to the point where they're not killing off whole forests of pine trees. Another possibility is that, as climate change continues, this will become a permanent change to the landscape of western North America. Plants still grow in these areas, just different plants. Perhaps the great western pine forests will become the great high prairies of the west.
(Sources: National Geographic, phys.org, Wikipedia)