Sudden Oak Death's impact on urban forestry and arboriculture

Susan J. Frankel, Plant Pathologist, USDA Forest Service, Pacific Southwest Region, State and Private Forestry, Vallejo, CA

Sudden Oak Death (SOD) was first recognized in wildland-urban interface forests of Marin, Santa Cruz, and Monterey Counties where more than 7 million people live. These urban forests have become a public safety issue and have sustained ecological changes. Tree care workers, including arborists, face a number of SOD- related issues: continually changing treatment recommendations due to a lack of scientifically based pesticide and woody-debris-disposal treatments; accusations of pathogen spread on equipment; and increased hazards to workers from limb and bole failures. The plethora of dead trees has increased business for tree service companies, particularly for tree removals, but has hampered utility arborists in line-clearing programs. As a newly identified forest pathogen, Phytophthora ramorum is challenging the abilities of science to keep up with the real-life needs of people that live and work in the urban forest.

Public safety in the SOD-infested urban forest

Tree failure
The die-off of wildland-urban interface trees due to Phytophthora ramorum threatens homes, buildings, roads, power lines, trails, and watercourses, particularly in Marin and Santa Cruz Counties. Considered dangerous if within striking distance of inhabited buildings, thousands of dead trees needed to be removed immediately. The hazard posed by infected trees with green crowns has been harder to determine. On oaks and tanoaks, P. ramorum causes a disease of the bark but does not decay wood; secondary organisms cause infected trees to deteriorate. Typically, tissues weakened by P. ramorum are rapidly colonized by bark beetles and Hypoxylon thouarasianum, a sapwood decayer. Canker rots (Inonotus), Armillaria root disease, and other agents that commonly decay oaks, but that aren't recognized by most people, often accelerate limb and bole failure.

Preliminary results indicate that trees with Sudden Oak Death symptoms fail at a higher rate than historically seen in the affected woodlands. A study led by Ted Swiecki, Phytosphere Research, with four study locations (total 6.72 ha), in forests infested with P. ramorum in Marin and Sonoma Counties found, on average, 15 SOD-related tree failures per hectare. The study areas include 870 Quercus agrifolia and Quercus kelloggii trees lacking any significant failures and 258 that have had a significant failure within the past 10 years. Of these failures, 61% involve trees with SOD symptoms. Bole and scaffold failures, the most likely to cause damage, were found on 9% of the oak population among symptomatic trees and on 4.8% of the oak population among trees lacking symptoms. Forty percent of the SOD-related failures occurred in trees that were still living; almost all are trees with H. thouarsianum and/or beetle damage in addition to P. ramorum. (Ted Swiecki, Phytosphere Research, personal communication). This failure rate is representative for residential areas.

Tree failure caused the temporary closure of the only campground in China Camp State Park, in and adjacent to the City of San Rafael (Marin County). In fall 2001, after several near misses from trees failing in the campground, all 30 sites were closed for about a month, along with adjacent trails, and 150 trees were removed. The park used prison crews to remove the trees but couldn't afford to remove the debris, so for several months, piles of tree trunks and branches filled the space once occupied by stately trees.

Fire hazards
Both dead and live Phytophthora ramorum -infected trees increase the risk of fire. While both coast evergreen and redwood/tanoak forests are considered relatively fire resistant, large amounts of fuels created by dead trees are generally recognized as hazardous, particularly in steep canyons, dotted with wood-shingle roofs and wood decks and accessed by narrow roads. A safety advisory for Sudden Oak Death was issued by the Marin County Fire Department and the California Department of Forestry and Fire Protection in February 2001 stating that live trees infected with P. ramorum have a 20 to 40% lower moisture content than healthy trees. Also noted were the additional hazards to firefighters from accelerated trunk and branch failure in areas with SOD.

To evaluate fire behavior in the wildland-urban interface, the Marin County Fire Department, in cooperation with the California Department of Forestry and Fire Protection, set off a test burn in 100 acres with Sudden Oak Death. Under the controlled conditions of the burn, ignited on September 19, 2001, firefighters reported a slight increase in crown fire and fire intensity in the area with SOD when compared to healthy areas. However, firefighters did note that if fire conditions were extreme, and located in a dense residential neighborhood, the incremental increase in intensity could result in a catastrophic fire (Ray Moritz, consulting arborist, Marin County, personal communication).

Ecological changes in the urban forest

The native vegetation die-off caused by Phytophthora ramorum has created a habitat ripe for exotic weed invasion and soil erosion, has degraded wildlife habitat, has increased hazardous fire conditions, and has decreased property values. Recreation lands have been irreparably altered. Openings have also increased sunlight penetration and enhanced views, considered by some to be a positive change.

Landscapers redesigning gardens have difficulties making planting recommendations since Phytophthora ramorum infects most woody species in the coast evergreen and redwood/tanoak ecosystem and the epidemiology on each species is not fully known. Replanting with even large nursery plants can't repair the damage created by the loss of overstory trees. The loss of cherished oak trees has changed the character and ambience of yards and neighborhoods. In addition, removing dead or hazardous overstory trees that once provided screening, noise protection, and shade can be emotionally upsetting and very costly. Removals alone can cost from $500 to $5,000 depending upon the proximity of the tree to structures and the level of difficulty involved in its removal. Typically, several trees have to be removed on a property, which then needs to be treated for erosion and exotic weeds.

Impacts on arborists

Sudden Oak Death has created conflict, confusion, and increased work for arborists. There is a tremendous demand for a registered pesticide for treatment of ornamental, high-value trees. Media reports announcing successful preliminary pesticide trials have frustrated arborists eager for a treatment. Although wanting to help their clients immediately, arborists must wait until a pesticide is registered (after field trials are completed and replicated) before they may legally use the pesticide. There is also disagreement about insecticide recommendations and the role of insects in tree mortality. Some entomologists recommend insecticide for trees infected by P. ramorum to prolong tree life. Trials to demonstrate the effect of insecticides are being set up but will take at least another year to complete.

Sudden Oak Death has increased business for arborists in affected counties. The 2001 California Sudden Oak Death legislation (AB62) provided $1.2 million to counties for hazard tree removal, assessment, and restoration. Most counties use private tree care companies under contract for tree removal. Ken Bovero, owner of Marin County Arborists, estimates that business for arborists has increased 10 to 15% due to SOD. His company removes an average of five trees per week that have been killed by P. ramorum . Ray Moritz, a consulting arborist in Marin County, estimates a $3 million/year increase in work for tree companies in Marin County. He looks at three to five P. ramorum-infected trees each week.

Sudden Oak Death has also changed the way trees are removed. Standing dead trees that were killed by Phytophthora ramorum are extremely dangerous to climb. Where possible, arborists are climbing healthy trees next to dead SOD removals and using the healthy trees to tie in for stable support, reducing the risk to climbers (Ken Bovero, personal communication).

Impact on utility arborists

Utility arborists face additional problems because of Phytophthora ramorum. In several neighborhoods, residents have refused to allow utility-line-clearing crews on to their property for fear that the crews would introduce the pathogen into their trees. Resolving these issues is difficult, however, because there are no scientific studies to document whether the pathogen would readily enter trees via pruning wounds during certain seasons or environmental conditions. There continue to be problems with disposal of pruned branches in quarantine counties; the requirement to leave woody material on site is undesirable to some residents who consider plant debris unsightly and flammable. Stem failure from an infested tree has ignited at least one fire, when a tree intercepted and severed high-voltage wires. There are concerns that the pathogen could be transported long distances on utility arborist machinery as the crews move around the state. (This is a concern for other industries as well.) In response to these concerns, Pacific Gas and Electric issued SOD sanitation and best management practices for their crews. Unfortunately, without scientific studies to support their best management practices, line-clearing operations continue to be hampered by these concerns and problems.