Monday, September 19, 2016

An American tragedy: why are millions of trees dying across the country?

Oliver Milman in Oahu and Alan Yuhas in San Francisco
Sept. 19, 2016


But the plight of the ohi’a is not unique - it’s part of a quiet crisis playing out in forests across America. Drought, disease, insects and wildfire are chewing up tens of millions of trees at an incredible pace, much of it driven by climate change.

Forestry officials and scientists are increasingly alarmed, and say the essential role of trees – providing clean water, locking up carbon and sheltering whole ecosystems – is being undermined on a grand scale.

California and mountain states have suffered particularly big die-offs in recent years, with 66m trees killed in the Sierra Nevada alone since 2010, according to the Forestry Service.

In northern California, an invasive pathogen called Sudden Oak Death is infecting hundreds of different plants, from redwoods and ferns to backyard oaks and bay laurels. The disease is distantly related to the cause of the 19th-century Irish potato famine, and appears to have arrived with two “Typhoid Marys”, rhododendrons and bay laurels, said Dr David Rizzo, of the University of California, Davis.

“We’re talking millions of trees killed, whole mountain sides dying,” Rizzo said.


Five years of drought in the west have not only starved trees of water but weakened their defenses and created conditions for “insect eruptions” across the US, said Diana Six, an entomologist at the University of Montana. Bark beetles and mountain pine beetles, usually held in check by wet winters, now have more time to breed and roam. The latter have already expanded their range from British Columbia across the Rockies, to the Yukon border and eastward, into jack pine forests that have never seen the bug.

The outbreak is “something like 10 times bigger than normal, I would argue a lot more than that,” Six said. “Basically a native insect is acting outside of the norm, because of climate change, and become an exotic in forests it’s never been before. We haven’t seen very good outcomes of exotics moving into native forests.”


Boosted by climate change, various beetles and the fungi they carry have already wiped out millions of acres of trees, and Six and Rizzo both warned of cascading effects. In the redwoods, Rizzo said, the loss of tanoaks and their relatives would strip away nut-producing species, leaving birds and mammals that rely on them without food. The loss of mountain pines, Six said, threatens grizzly bears and the critical snowpack that supplies water to life below.

In a few decades, Americans might not even recognize forests they see

“There’s virtually nothing you can do to stop the beetles, either, unless they’ve killed everything and run out of food,” Six said. “Or unless the climate cools, and I don’t think anyones expecting that anytime soon.”

In Hawaii, warming temperatures have helped spread four types of beetles that bore into ohi’a bark to feed. The beetles carry disease spore on their wings, in their guts and in the sawdust of burrows, spreading it from tree to tree.

The beetles are part of scolylinae, a “very destructive family” that is also decimating trees in California, according to Curtis Ewing, an entomologist at the University of Hawaii.

“They are exploding around the world due to global warming,” he said. They appear unstoppable: spraying each tree with insecticide would be time-consuming and made futile by rain, and pheromone-laced traps also appear ineffective. The university’s arboretum has started collecting ohi’a seeds in the face of a doomsday scenario that was recently unimaginable for such a common tree.


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