| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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habitats Sudden Death Sentence for Oaks Three species of California's oak trees are dying by the thousands in a 350-mile stretch of coastline extending from Monterey to Mendocino. The culprit appears to be a fungus-like organism in the genus Phytopthora (Greek for "plant killer"), the grim reaper of pathogens. It is the same group responsible for Ireland's great potato blight of 1845-1850, as well as for killing Port Orford cedar trees in northern California and southern Oregon for the past 30 years. Various Phytopthora threaten oak forests in Europe and Mexico, and eucalyptus forests in Australia. In China Camp State Park the epidemic is already a disaster. Nearly half of the park's tanoaks have been hit, and the disease shows no signs of slowing. In Big Sur's Pfeiffer State Park, nine out of ten trees are either infected or dead. Will this disease, called Sudden Oak Death (sod), stop before it wipes out California's oaks, the "keystone" species in the woodland habitats that cover about 11 million acres of the state? Will it do what chestnut blight did to the great chestnut forests on the East Coast a century ago, bringing the tree to effective extinction in a few decades? No one knows yet, but that possibility is being taken seriously by the team of researchers hastily pulled together last summer to study the disease. That team-composed of about 30 botanists, geographers, entomologists, ecologists, foresters, and pathologists coordinated by the U.S. Forest Service-is frantically trying to figure out how the disease works, where it is from, how and where it is likely to spread, and how it can be stopped. Unfortunately, the team's $200,000 emergency budget is grossly inadequate to do the job. Efforts to secure more money are underway, but many fearful oak lovers and environmentalists feel that the official response to the plague has not only been far too little, but has also come much too late. So far, the disease has attacked three species: Tanoak (Lithocarpus densiflorus), really a cousin of true oaks, in a genus of its own; coast live oak (Quercus agrifolia); and California black oak (Q. kelloggii). In all three species, the disease attacks the nutrient- and water-conducting tissues by secreting enzymes that digest them. The fungus then feasts on the broken-down cells. Nobody knows for sure how long a tree takes to die after it has been infected; it may be months or years. But once a tree shows the telltale bleeding of dark red sap from the trunk, its leaves turn brown within a couple of weeks, and it is usually overrun by western oak bark beetles or oak ambrosia beetles, a sequence that appears to be a sudden death. The particular Phytopthora species found in all of the cases of sod was unknown to scientists before last July, when it was identified by University of California at Davis plant pathologist David Rizzo. The fungus still doesn't have a species name, and no one knows if it has been hiding out unnoticed here in California, or if it harks from elsewhere, brought here on someone's transplanted tree, boot, or tire tread. Another possibility, a "strong theory," according to Susan Frankel, forest pathologist with the U.S. Forest Service and chair of the California Oak Mortality Task Force, is that the new Phytopthora is a hybrid of two different species, one of which, P. lateralis, is responsible for the death of those thousands of Port Orford cedar trees. In support of the hybrid thesis is recent work by Clive Brasier at the Forestry Commission Research Agency in England. Brasier's May 11 Nature article, "The Rise of the Hybrid Fungi," showed how easily fungi like Phytopthora can hybridize and mutate. We create abundant opportunities for this kind of hybridization by frequently moving plants (and the pathogens they host) across borders and oceans, and keeping unrelated species together in nurseries. But just because the fungus hasn't been seen in California before doesn't mean it is new to the world or even new to the area. All of the 60 known species of Phytopthora are hosted by plants that have some commercial value. For the most part, scientists don't study diseases in plants without a human constituency. But there may be many, many more Phytopthora out there in the wild that usually make a living on less popular plants. It's a very scary thought that a little mutation in one of those wallflower fungi might be enough to level forests. Last summer, Steve Zack, a biologist with the Wildlife Conservation Society in New York, surveyed 8,000 tanoaks over a 350-mile stretch of California coast. In some areas, only 15 percent of the trees were infected. In other areas the number was as high as 80 percent. Zack's report also concluded that the ramifications of sod extend far beyond the deaths of the oak trees themselves. The trees are aesthetic keystones in California's landscapes, but, more to the point, they are ecological ones. Their acorns are an essential food for dozens of animals that are in turn food for many other animals. Also, the fire risk posed by the addition of millions of tons of dead wood is a major problem, says Zack. Many of the trees that escape infection, and other oak woodland species that survive the epidemic, may go up in smoke.
The impression of the epidemic given in popular press accounts is too simplistic, says independent plant pathologist Tedmund J. Swiecki. The trees don't simply get exposed to Phytopthora and then die. Some exposed trees don't contract the disease, so there must be other factors involved. Swiecki's job is to figure out what those are. A lot of the oaks struck by sod "are [in] really funky stands," says Swiecki. "Many of them were due for some problems: they are too densely stocked, and have lots of sprout-origin trees [those that have grown from stumps]. These stands are probably 80 to 140 years old. They are the generation that came up after settlers cut down huge numbers of oaks in the 1800s. They may just be reaching the point where they are running out of gas and are becoming especially vulnerable to things like this Phytopthora. Mortality among these oaks is very high now, even without this new syndrome." Various wood-decaying fungi, especially canker rot, are the primary agents associated with this non-Phytopthora mortality. When I join Swiecki on a study site on Marin County Open Space District land in Novato, he is surveying both sick coast live oak trees and healthy ones. We walk through the woods and identify the placement of trees, the temperature at the top of their canopies, their water uptake, the density of surrounding trees, the plot slope and exposure, and any evidence of tree decay or illness. Later, Swiecki will do a statistical analysis of all these data and see which, if any, turn out to coincide with cases of sod. In addition to Swiecki's and other fieldwork on the ground, lab work on the genetics of Phytopthora, and research into how the fungus is transmitted from plant to plant, scientists are also conducting aerial surveys to monitor sod's progress across the state. One major fear is that the disease will leap into more of California's 20 species of oak or perhaps make its way across the Great Central Valley and hit the oak woodlands of the Sierra foothills. "Without its oaks," says Janet Cobb, director of the California Oak Foundation in Oakland, "California would become an entirely different place. It would no longer be recognizable." Another fear, says Cobb, is that sod or some variant of it will migrate south and jump into some of Mexico's 148 different species of oak. Since no one is yet sure how the fungus moves from place to place, it is still unclear whether even extreme measures (such as quarantining infected firewood, a measure already suggested by the U.S. Forest Service) would inhibit the spread of the disease. For instance, if Phytopthora's spores can be carried by the wind, quarantines may not help. Meanwhile, in China Camp State Park, land managers are just leaving the dead snags in place. If they pose a safety danger, they are cut down and left where they fall. No one wants to touch the stuff. University of California at Davis extension horticulturist Pavel Svihra first identified the syndrome in tanoak trees in Marin five years ago, although he had no idea then if it was caused by insects, a fungus, or some other agent. Since then, Svihra has tried to get the attention of authorities, but, he says, because the affected trees were tanoaks-considered weeds by the state department of forestry, which tries to eradicate them from many fir stands managed for timber-no one paid attention. Cobb holds state and federal agencies at least partly responsible for the current crisis. "They should have taken this seriously five years ago," says Cobb. "The [forestry] department's lack of response has been across the board. If this disease had appeared in a conifer or a grapevine, or some other commercial species, they would have responded immediately. Because it appeared in a tanoak-and they hate tanoaks-for five years they didn't spend a cent. Now we have an emergency based on neglect, poor science, poor public information, and even poorer public policy." Gordy Slack is a freelance science writer and Contributing Editor to California Wild. |