Here's the December update on Sudden Oak Death-related research and management affecting the North Coast. Remember, your questions and comments are welcome. Happy reading!
Severe Impacts in Sonoma: Are We Next?
Tanoak trees have been dying in very large numbers in Sonoma County for several years now. As we’ve mentioned in this newsletter before, residents who had been watching trees die by the dozens in their yards and in steep canyons behind their homes finally got together in 2006 to attract some money from the county and from the USDA Forest Service to address the issue. That initial work soon sprouted into a grant from the Bureau of Land Management to fund removals of dead tanoaks near homes and in neighborhoods to provide clearance, breaks in fuel continuity, and hazard mitigation.
Now that program is getting underway, according to Caerleon Safford of the county Department of Emergency Services and Fire Safe Sonoma, with the first projects set to begin any day now. Awards have gone to individual landowners as well as coordinated neighborhood groups, and the intensity of the work to be done varies according to the topography, the nature of the vegetation, and the amount of mortality on each property. For more information about the program, call Safford at (707) 206-5467 or email her at csafford@mcn.org. You can also visit the program web site at http://www.firesafesonoma.org/whats_new.htm.
Mendocino and Humboldt County residents would do well to examine Sonoma County’s experiences. Although the wave of tanoak mortality has slowed in much of Humboldt County over the past two years because of the dryness of the late winter/early spring periods, the next wet winter could set off another wave. Parts of Humboldt County look a lot like the heavily infested areas of Sonoma County, with many people living in steep canyons that are covered by forests very similar to the ones in Sonoma County. These areas are heavily vegetated with tanoak and have difficult access and egress.
If things go the way they have down south, within a year or two after the next wet winter/spring period, Humboldt County residents could see increased numbers of dead tanoaks near their houses and along the roads. For example, one Sonoma County resident reported 137 dead tanoak trees within 150 feet of his house, according to Safford.
For a sense of the magnitude of the problem in Sonoma County, you can check out the Sonoma County Sudden Oak Death Strategic Response Plan prepared by the county outreach coordinators with help from their colleagues in the county and presented to the Board of Supervisors early this year. Altogether, the plan estimates that the county faces over $4 million in immediate needs across various county departments related to mitigating Sudden Oak Death impacts, with over $3 million in costs over subsequent years. You can access the plan at http://cesonoma.ucdavis.edu/Custom_Program193/Sonoma_County_SOD_Strategic_Response_Plan_-_January_2008.htm.
Tanoak mortality at one Sonoma County property (Photo courtesy of Caerleon Safford)
More Sonoma County tanoak mortality (Photo courtesy of Caerleon Safford
Banana Slugs and Sudden Oak Death
For a long time, people have suggested that wildlife may carry P. ramorum, serving as vectors for Sudden Oak Death and P. ramorum infection of non-oak host species. But it’s hard to figure out how to study that. Look for spores on turkey feet? Detain some deer and send them through an infected forest?
Researchers from Oregon State University and UC Davis have done pretty much exactly that—on a miniature scale. Observing that the spores of some other species of Phytophthora were known to have improved germination rates after passing through the digestive systems of snails, these researchers decided to feed two species of slugs common in the Pacific Northwest—the banana slug, Ariolimax Columbiana, and the gray garden slug, Deroceras reticulatum—chlamydospores of P. ramorum.
Chlamydospores are the “resting spore” stage of P. ramorum. They are big, relative to the other spore stages, and tough. They presumably take a lot of energy for the organism to produce, and they appear to function as one possible “oversummering” mechanism, allowing the pathogen to survive our long, dry summers and autumns. In the laboratory, they germinate at unpredictable times, with no well-known, specific triggers to make them do so.
After the researchers fed the slugs pure cultures of P. ramorum or strawberries infected with P. ramorum, they observed numerous chlamydospores in the slugs’ feces. Many of these chlamydospores germinated to form new germ tubes, and others germinated to form structures bearing other spore types. The scientists observed the same phenomenon when they let the slugs loose on a prepared “litter” of infected bay, tanoak, and rhododendron leaves: the slugs excreted lots and lots of chlamydospores in their feces.
Banana slug fecal samples caused lesions, indicative of P. ramorum infection, on tanoak and rhododendron leaves to which they were applied. Also, when slugs that had grazed on infected litter crawled over intact tanoak logs (blemish-free bark), two of nine logs became diseased.
Although the researchers heavily cautioned readers not to read too much into the experiment, noting that field conditions can be quite different from laboratory conditions, they have apparently uncovered one possible animal vector (albeit an awfully slow-moving one) for Sudden Oak Death and related diseases. Slugs, of course, are very active in the wet months when P. ramorum inoculum is being produced and transported in water and wind. The researchers are seeking funding for more long-term studies of slugs and P. ramorum. Stay tuned.
Banana slug (photo courtesy Joseph O'Brien, USDA Forest Service, bugwood.org)
New Research on Fire and Sudden Oak Death in the North Coast
Many people have speculated that increased oak and tanoak mortality from Sudden Oak Death might contribute to an increase in extreme fire behavior in the coastal ranges. Especially worrisome to fire behavior analysts is the time period from tree death to the shedding of brown, dead leaves. This period can last from one to three years (more in some cases) depending on the weather and exposure of the tree. Analysts wonder whether this persisting fine dead material can increase ember production and spotting behavior, potentially contributing greatly to fire spread, given the right conditions. Other concerns center on the buildup of surface fuels in infested stands.
Studies have begun to test these speculations. For example, a new research partnership plans to visit sites in Sonoma, Mendocino, Humboldt, and Del Norte Counties to inventory ground fuels in Douglas-fir-dominated stands that contain large amounts of tanoak and that show varying levels of impact from Sudden Oak Death. They also plan to visit stands in which the tanoak was killed with herbicides varying numbers of years ago, to see if these stands are viable surrogates for stands that have been impacted by Sudden Oak Death over the long term. Since we have only observed the effects of the disease in California forests for about a decade, it is important to try to get a sense of what the impacts will be over a broader time horizon. Will surface fuels decay quickly? Will they build up and subside in waves, as the disease cycles through in wet years and then dies down again? This group consists of partners from UCCE Humboldt-Del Norte, CAL FIRE Humboldt-Del Norte, the Department of Forestry and Wildland Resources at Humboldt State University, and the Rizzo Laboratory at UC Davis. The project was funded by a grant from the USDA Forest Service Pacific Southwest Research Station.
Another study related to fire and Sudden Oak Death is ongoing right now in Humboldt County, where Humboldt State University graduate student Howard Kuljian, advised by Morgan Varner from the Department of Forestry and Wildland Resources, is studying foliar moisture in the leaves of tanoaks that are declining or dead from Sudden Oak Death. Howard presented preliminary results from his moisture measurements, and they were surprising: although foliar moisture levels did not decline significantly in the initial stages of P. ramorum infection, foliar moisture was much lower than expected in dead tanoak leaves and twigs. Howard will continue his measurements monthly through March 2009, completing a year of observations in this infested stand.
Additional fire-related studies are underway in the large areas burned by fires in 2008 in Big Sur, where the Rizzo Lab from UC Davis had already established hundreds of plots in P. ramorum-infested forests. Altogether, these studies should help fill in some large gaps in our understanding of the relationship between fire and Sudden Oak Death and enable us to better plan for managing the impacts of this disease.
