Air fungus

 

Each year, in early May, a pilot and a researcher fly  low, long hours over the Oregon coast range, sweeping back and forth in transects two miles apart. Below the small aircraft, a rugged, uneven carpet of mature and regenerating forest unrolls: a landscape scarred by logging, but still dominated by Douglas fir. They're in search of discoloration — tracks of spindly, jaundiced fir that signal the presence of a fungal disease, Swiss needle cast, beleaguering the species.

Seldom deadly, the pathogen (Phaeocryptopus gaeumannii) simply slows a Doug fir's growth, causing it to shed needles. But as they sift one by one to the forest floor, those needles presage a potential shift in the composition and carbon sequestration capacity of the Northwest ecosystem, and in the ability of clear-cuts to recover. A slow-growing tree may struggle to hold its ground against other species like western hemlock, and, of course, will convert less CO2 into woody mass — a significant consideration as carbon emissions climb, and biomass is explored as an energy source in the Northwest.

Recently, tree core samples examined by Oregon State University scientists showed the extent to which Swiss needle cast impairs Doug fir, adding to our understanding of the epidemic affecting roughly 300,000 acres along coastal Oregon (click here for an abstract). In one study plot near Tillamook, the lateral growth of diseased trees slowed 80 percent, and their overall growth by 23-50 percent. A more average lateral growth decrease for an infected fir is 20-30 percent. In the light-and-dark ring sequence of years, the disease's arrival is obvious and abrupt. Dendrochronology — the study of tree rings — has been used to monitor tree diseases in the past, especially infestations of insects, but apparently this is one of the first times it's helped assess the impact of a foliar blight.

Monocultures of Doug fir planted in the wake of clear-cuts are especially prone to outbreaks and, to date, Swiss needle cast research has focused on trees less than 30 years old. The Tillamook cores, however, come from naturally regenerated trees at least 70 years old, on unlogged terrain, which shows that older trees are vulnerable to Swiss needle cast, too. True old growth may have inherent buffers from the fungus, like airier crowns and groves, but 400-year-old firs need to be sampled more widely to see if the disease has a long, periodic history. For a baseline of growth in their study, the researchers compared the fir with neighboring western hemlock, whose growth has remained steady (or even increased slightly, in response to lagging Doug fir).

Despite its name, Swiss needle cast is thought to be a Northwest native. It was discovered on Doug fir imports form the Northwest to Switzerland and Germany in 1925. By 1984, the fungus was impacting fir across the Northwest in "unprecedented severity, at least in the context of the past eighty years." Since 1996, when aerial surveys first began, the amount of acreage with symptoms has increased steadily. The disease appears most marked at low, south facing elevations near the coast, and is possibly linked to climate change:

"We now know that weather is a driver in the epidemiology and spread of this disease," said Bryan Black, an assistant professor of forestry based at OSU's Hatfield Marine Science Center. "We can't say yet whether climate change is part of what's causing these problems, but warmer conditions, milder winters and earlier springs would be consistent with that."

A spore sifts through the air, lands on a young, moist needle, and germinates. Before long the fungus creeps inside the needle through a pore, colonizing it with a crisscross of filaments, or hyphae, that, under a microscope, look like chewing gum stretched from wall to wall. The fungus blocks the pores, or stomata, through which the needle exchanges gases necessary for photosynthesis, and the needles soon bleach golden and drop away. More spores float on the breeze.

Several thousand feet above the ravinous country, an ailing slope of Doug fir looks somewhat like a head of bruised and browning broccoli. The researcher gazes out the window of the plane, then down at an iPad-like computer in his hands, which displays a scrolling topographic map with their GPS position. His stylus sweeps across the touch screen, tapping the location of yellow-umber patches. "S" or "M," they're labeled — severe, or moderate. What those patches of disease will mean for the Northwest in the long run is still up in the air. For now, be sure to check out this gallery of related, rather alarming photos.

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