One of the most frequently asked questions in my Yosemite workshops is some variation of, “Why are there so many dead trees?” My standard answer has always been a summary of what I’ve learned from talking to Yosemite rangers: The drought has stressed the trees and made them more susceptible to the bark beetle. This morning I read an excellent summary of the problem on the NPS Yosemite site explaining the problem, and adding to what I already knew, and I thought I’d share what I learned.
As someone who has been visiting Yosemite for (pretty much literally) my entire life, the tree death in Yosemite Valley in the last five years has been staggering. Yosemite Valley, once a carpet of green, is now stained with large patches of rust-brown dead or dying trees. Scenes I’ve photographed for over 40 years are suddenly marred by these trees.
Going through my portfolio of Tunnel View images, I chose two with very similar compositions that illustrate the tree death. The first, my rainbow image from 2009, shows the green valley floor I remember. The second is a winter scene from 2016, and the tree death is obvious. And sadly, in the year-and-a-half since I took the 2016 image, I guess that at least twice as many trees have died.
The drought has clearly taken its toll on Yosemite’s trees, both by killing the thirstiest outright, and by weakening many others until they become easy targets for a very opportunistic bark beetle. But the problem is not just about weak trees—it’s also about healthy beetles, a lot of them. Consider that while the 2016 image was taken in late January, there is absolutely no snow in Yosemite Valley. Of course the drought has something to do with that, but the lack of valley snow in recent years can also be attributed to warming temperatures. As Yosemite’s climate warms, much of the precipitation that once fell as snow now falls as rain.
Snow doesn’t kill the bark beetle (it’s still not cold enough), but an extreme freeze does. But as the number of sub-freezing days in Yosemite decline, the mechanism that kept the bark beetle in check gets out of whack. While Yosemite’s evergreens have no problem handling an extreme freeze, each freeze kills many bark beetles. But fewer freezing days each winter means more bark beetles, and more bark beetles makes even healthy trees more prone to attack.
And finally, America’s long-time knee-jerk fire suppression policy has taken its toll. By thinning growth, consuming dead wood, and enabling regeneration, fire is a natural part of maintaining forest health. But for over a century, fires in Yosemite (and pretty much every other national park and forest) were doused as soon as they ignited because they were inconvenient, and they (temporarily) scarred the scenery.
Thankfully that misguided policy is largely behind us, but its legacy remains. We’re left with too many trees competing for the available water. Some die of thirst, while many survivors lack the resources to stave off a beetle infestation.
What’s being done
The National Park Service has undertaken the monumental task of removing dead and dying trees. Because it’s impractical to remove all of them, the emphasis is on those trees that pose a hazard to people and property. Also, in developed areas the NPS has started prophylactic application of a (naturally occurring) pheromone that discourages the beetles from attacking susceptible trees.
No one knows for sure, but it’s possible that the tree death will stabilize, or even start to decline over the next few years. While the current mitigation efforts might help stem the tide, the primary hope is that an equilibrium will be reached as the most susceptible trees die and forest health is restored through better management. Fingers crossed.
From the horse’s mouth
Here’s the link to the NPS tree mortality article.
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