More Mortality in the Sierra Nevada

Excerpts from “Why have all the trees been dying?” in a Lake Tahoe area paper [emphasis added]. I think this mortality will spread north in to the Cascades in Oregon and Washington.

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.”

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. 

In a scientific study in the Journal of Ecological Applications [2021] 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.”

4 thoughts on “More Mortality in the Sierra Nevada”

  1. I have to say, the message from the Ranger (who was a recreation and archeology specialist before becoming the ranger, if I recall correctly), is a bit more simplistic than what the silviculturalist states. The Ranger says the forest is too dense and it’s dry, so trees are dying. The silviculturlist attributes the morality to native dwarf mistletoe, annosus and cytospora (cytospora is a fungus that often infects red fir and white fir with dwarf mistletoe). There is no data to suggest that native dwarf mistletoe infection rates have been affected by past management and there is no way to rid a stand of dwarf mistletoe, other than to convert the stand to tree species that are not hosts to the type of mistletoe in the stand. Also, mechanical thinning can spread annosus and is likely one of the reasons the area has so much. Yes, dwarf mistletoe increases susceptibility to drought and beetles and I would agree that reduced density would help reduce the likelihood mistletoe-infected trees are killed by drought/beetles, but the issue is more complex than “it’s too dense and dry.”

  2. The fungi and dwarf mistletoe have always been present, but the stand density has increased, leaving trees less able to withstand these pathogens. The Sierras have been droughty, which makes it tougher for trees to survive. (FWIW, I worked on the El Dorado NF and lived in the Sierras many years ago).

  3. This “fewer larger” trees reminds me of the idea that no large trees should be cut as in the other thread.

    I also think that the idea that dead trees burn differently than living ones, and dead jackstrawed trees burn differently than standing ones, with different impacts, has been long known.


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