Why have all the trees been dying?

Once the snow started melting around Lake Tahoe and the hiking and biking season began I suddenly came to the realization that there has been a huge increase in the number of brown trees in the forest around Lake Tahoe. Many ridges that were once a wave of green, are now mottled with swaths of brown dead trees. Even around my own house a half dozen trees have turned from vibrant green to brown in just the last six months. Why the sudden increase in tree death, and what does it mean? I decided to find out by talking to a few of our local forest experts. 

Image: Tim Hauserman

Jonathan Cook-Fisher, District Ranger for the Tahoe National Forest in Truckee said, “It is happening from the west shore to the north. The fir trees are the first to go. There are some beetles, but the primary driver appears to be overstocked forest stands with drought conditions.”

The conditions that eventually lead to the death of the Sierra conifers takes place over a number of years. “This is drought year number seven, they have been under stress for many years,” said Cook-Fisher.  While less than average precipitation can be the catalyst that leads to the trees demise, it’s the overly dense forests that create ripe conditions for the large numbers of tree deaths. 

“White fir and red fir trees in the Lake Tahoe Basin are dying off at higher levels than we’ve seen in the past. This increase over the last few years is noticeable in some higher elevation areas around Fallen Leaf Lake and on the North Shore, and can be attributed to continued drought conditions in California,” said Rita Mustatia, Silviculturist, from the US Forest Service Lake Tahoe Basin Management Unit.

“What we are seeing is caused by a few culprits including, root rot and dwarf mistletoe, a parasitic plant that deprives the tree of water and nutrients. Another parasitic plant is Cytospora, a fungus that attacks trees that have been weakened by other diseases, drought, and insects. Many of the red branches seen on various areas of the dying trees can be attributed to this fungus. Bark beetles are also responsible for some of the damage and/or mortality. With continuing concerns due to ongoing drought conditions, it is likely the mortality will increase. In addition, we’ve noticed drought impacts to Jeffrey pine as well with many of the trees infested and losing the battle with dwarf mistletoe although not causing much mortality yet,” said Mustatia. 

Overstocked forests didn’t just happen in the last few years. It has been a growing (sorry couldn’t resist) problem for the past 100 years. Sierra Nevada forests (and forests throughout the west) have adapted to regular fires that were spurred by lightning storms. Trees adapted by growing rapidly and close together in an attempt to “out-grow” the fires. The lightning caused fires stayed low to the ground, burned out the brush and those swiftly growing young firs before the stands could get too thick, leaving a forest of primarily mature, bigger trees spread out around the forest. Fewer, large trees were better able to fend off pests and drought. 

Then in the understandable desire to save the trees that we build our houses from, and those houses themselves, we began aggressively putting out all fires. Without small fires to reduce the fuel load the trees began growing very close together in thick throngs of conifers. They outgrew the amount of sunlight and water available, to the point where eventually they begin to die. 

Once the trees die they become dry, ladder fuel for the next fire that comes through. Making for hotter fires that are more likely to progress into the crowns which is when a wildfire really begins to take off and can become a conflagration like the Dixie, Caldor and Emigrant Fires last year. 

In a scientific study in the Journal of Ecological Applications on the impact of tree mortality on wildfire severity entitled “Recent bark beetle outbreaks influence wildfire severity in mixed-confer forests of the Sierra Nevada,” Rebecca Wayman and Hugh Safford found: 

“Our analyses identified prefire tree mortality as influential on all measures of wildfire severity… All measures of fire severity increased as prefire mortality increased…Managers of historically frequent-fire forests will benefit from utilizing this information when prioritizing fuels reduction treatments in areas of recent tree mortality, as it is the first empirical study to document a relationship between prefire mortality and subsequent wildfire severity in these systems.”

Image: Tim Hauserman

While the scientists went on to say that weather is still in many cases the biggest factor on how big a fire gets, forests full of dying trees is a major contributor to the size and heat of fires. 

The answer for the dying trees is the same as what is needed to reduce the number and impact of wildfires: We must aggressively thin the forest, and carry out regular off-season prescribed burning operations to increase the space between trees, and recreate the type of forest that is adapted to our climate. A healthy, more spread out forest provides more water for each remaining tree and makes it more difficult for pests to travel from one tree to the next.

“The best way to manage the stress brought on by drought is to stay ahead of it by thinning the forest to reduce tree competition for water and nutrients. There is also a need to remove trees that are heavily infested with dwarf mistletoe which will help reduce its spread to other trees,” said Mustatia. 

There is an old saying: it’s easier knowing what to do than to actually do it. While most people involved with forest management agree on the need for thinning the forest, the devil can be in the details and in relearning what a healthy forest looks like. One person’s lovely managed forest is another person’s decimated forest. But these days, we all need to learn a new definition of what makes for a beautiful forest. It is not the crowded, overly dense forest we have been surrounded by all our lives, but what our forests looked like several hundred years ago when the only humans in the area were native people, and they were not in the business of putting out fires. It was a forest where people could walk side by side through a grove of healthy large trees without touching one of them.   

Tim Hauserman is a freelance writer and nearly a life-long resident of North Lake Tahoe. He wrote the official guide to the Tahoe Rim Trail, the recently published 4th edition. He also wrote Monsters in the Woods: Backpacking with Children and has written hundreds of articles on a variety of topics: travel, outdoor recreation, housing, education, and wildfires. In the winter he is cross-country skiing and running the Strider Glider program at Tahoe Cross-Country Ski Area. Check out Tim’s website here.

Tim’s latest book: Going it Alone. Ramblings and Reflections From the Trial is now available at your local bookstore here, along with his many other publications.

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