California tree mortality

Last Friday, the Forest Service released the results of their latest field research on the great California tree die-off: New Aerial Survey Identifies More Than 100 Million Dead Trees in California, and the news is very, very grim:

the U.S. Forest Service has identified an additional 36 million dead trees across California since its last aerial survey in May 2016. This brings the total number of dead trees since 2010 to over 102 million on 7.7 million acres of California's drought stricken forests. In 2016 alone, 62 million trees have died, representing more than a 100 percent increase in dead trees across the state from 2015. Millions of additional trees are weakened and expected to die in the coming months and years.

Who knows how you even count 102 million dead trees. I suspect they use techniques like those used to estimate the size of crowds during parades: divide a very large-scale picture into small sections; pick a handful of those sections at random; zoom in on that tiny image section and count the trees (by eyeballing them); assume an even distribution across the overall image and multiply by that factor.

That is, random sampling of large scale data.

Which can be quite sensitive to inaccuracies, but still, it's at least some data, collected as rigorously as their budget would allow, I suspect.

I was up in the mountains in August, for my back-packing trip, and again in October, for our visit to Mammoth Lakes and Yosemite. The route we took, both times, was CA 108, known as the Sonora Pass road.

From what I can tell, Sonora Pass has been roughly the dividing line between mild mortality and severe mortality, with the worst damage being to the south of Sonora Pass, while areas to the north have been significantly less affected.

However, the Forest Service cautions that even this not-as-bad-as-it-might-have-been news is changing quickly:

The majority of the 102 million dead trees are located in ten counties in the southern and central Sierra Nevada region. The Forest Service also identified increasing mortality in the northern part of the state, including Siskiyou, Modoc, Plumas and Lassen counties.

Those bark beetles are hungry, it seems, and once they ate all the healthy bark around Yosemite they flew off to find more trees to chew on.

On our October trip, we noticed a lot of the results of the efforts to collect and remove the astonishing amount of dead trees in the various national forests, an effort the Forest Service has been championing, noting that:

"These dead and dying trees continue to elevate the risk of wildfire, complicate our efforts to respond safely and effectively to fires when they do occur, and pose a host of threats to life and property across California," said Vilsack. "USDA has made restoration work and the removal of excess fuels a top priority..."

The San Francisco Chronicle has a slideshow-style view of photographs of the current situation.

And I can tell you, from our October experience, that those pictures are very accurate.