For trees, diseases not always disastrous / Genetic diversity key to species survival
Published 4:00 am, Sunday, July 6, 2003
Tree diseases are hardly the stuff of engrossing cocktail party chit- chat, but they've been getting some attention lately in the Bay Area -- mainly because they've intruded on the public consciousness in a threatening way.
Monterey pines stricken with a fungal disease known as pitch canker have been toppling throughout the region in recent years, flattening cars and caving in homes. One killed a San Francisco resident last month, initiating a logging spree by public officials worried that similar catastrophes could result from the hundreds of infested pines that remain (barely) upright in the city.
A couple of years ago, a mysterious malady dubbed sudden oak death -- later identified as a fungus called phytophthora -- raised fears of statewide deforestation after it began wiping out entire stands of live oaks in Marin County. The sky-is-falling prognostications got even more strident when it was found that phytophthora also was infecting conifers, such as redwood and Douglas fir.
But while the danger to life and property from individual diseased trees is real, the scenario of a landscape rendered sere and barren by an indestructible mutant fungus is exaggerated, tree experts say.
Nevertheless, tree diseases do exert significant influence on woodlands. They can change the mix of trees and wildlife in a forest, and they can greatly increase the danger of catastrophic wildfire by turning once green, living and flame-resistant trees into desiccated stands of volatile tinder.
Pathogens can greatly affect the relative abundance of certain trees, scientists say, and sometimes they can even wipe out a specific species -- as was the case with the American chestnut, extinguished from North America by a fungal blight from Asia in the 19th and early 20th centuries.
But disease agents are rarely the sole cause of such declines. And even when they do exert a profound effect, the time scale is apt to be measured in decades or centuries, not mere months or years, experts say.
"When sudden oak death was first identified, you got this apocalyptic take on it, but no tree disease has ever spread in a way that could be termed explosive," said Dave Rizzo, a professor of plant pathology at UC Davis. "Even chestnut blight took 50 years to wipe out the American chestnut."
Rizzo said he had been heartened by some recent findings on sudden oak death. "We've been seeing resistance in some coast live oaks, which are considered very susceptible," Rizzo said. "Some of the trees flame out, but others hang in there."
"It causes some leaf spotting in redwood and Douglas fir, but it seems that's about it," Giusti said. "And even among live oaks and tanoaks, it doesn't appear to have spread much beyond the original (Bay Area) hot zones. We were waiting for the next Mount Tamalpais (where phytophthora infection is especially high) to hit, but it hasn't happened."
But couldn't coast live oaks still meet the same fate as the American chestnut? Not next year, but a century or so down the road? That's possible, say researchers -- but not probable.
One key is whether the target tree species has strong genetic diversity to begin with.
"There's strong evidence that the American chestnut had already suffered significant impacts before the blight," said Detlev Vogler, a research geneticist and plant pathologist with the U.S. Forest Service. "There was extensive logging, land conversions for farming. Basically, the blight landed on a reduced -- and probably vulnerable -- genetic base."
Voglers said experiments with tanoak -- another species highly vulnerable to sudden oak death -- showed a wide range of tolerances to phytophthora, reflecting strong genetic diversity within the species.
But there's some evidence that a lack of genetic diversity may be playing a role in the decline of the Bay Area's Monterey pines. Although Monterey pines are native to California's Central Coast, they are also one of the world's most widely utilized horticultural and timber plantation tree species. San Francisco's extensive stands are composed of cultured, not wild, trees.
"It's possible the San Francisco strains might be particularly vulnerable to pitch canker because of narrow genetic variability," said Randy Zebell, president of the San Francisco Chapter of the California Native Plant Society.
"I know problem trees must be removed, but I don't like to see the Monterey pines go," Zebell said. "They're native to California's coast, they're beautiful, and they provide valuable habitat to birds such as chickadees and nuthatches."
Monterey pines aren't the only sickly pines in the state. Almost all white pine species are susceptible to a host of fungal diseases and insect infestations. The two work in malign synergy: Bugs weaken trees and make them more vulnerable to canker infections, and vice versa.
"It's very serious in the (ponderosa pine forests of the) south state," Vogler said. "Pine bark beetles and canker have always been problems down there, but people who've been there for years say they've never seen it this bad."
Canker and beetle problems seem directly related to drought and fire suppression, Vogler said. A 15-year dry spell has seriously weakened the south state's pine forests. And eliminating periodic, low-level fires from forest management removes an efficient means of capping both pathogens and beetle populations.
By eliminating controlled fire, said Vogler, the south state's forests become far more vulnerable to catastrophic wildfire. Bugs and fungi turn once living forest into vast stretches of standing kindling.
What to do? Prescription fire is essential, say experts. But other than that -- and selective logging of dead timber to reduce fire risk -- a minimal response may be best. Vogler points out that another disease, white pine blister rust -- also Asian in origin -- ravaged both Western and Eastern pine forests in the early 20th century. Some forest managers favored an aggressive response, felling all white pines in the affected area. Others did nothing.
"The forests that were left intact maintained their genetic diversity and have demonstrated considerable blister rust resistance a century later," Vogler said. "The heavily logged forests are more susceptible -- probably because they have less genetic diversity."
Pathogens -- both native and introduced -- are a part of any woodland, say scientists, and it is difficult if not impossible to undo what they have done. Forests are not static; they evolve, and pathogens contribute to that evolution.
"At one time, the American chestnut composed a quarter of the trees in the Eastern forests," Rizzo said. "When it disappeared, oaks filled the niche. Then gypsy moths showed up (from Asia) in the 1960s. What do gypsy moth larvae favor? Oaks." Now, said Rizzo, the oaks of the Eastern U.S. are declining, and maples are moving in.
"Human beings have been changing the composition of the world's forests for millennia, either directly or through introduced diseases and insects," said Rizzo. "This is nothing new."