Tanoak and Coast Live Oak Under Attack

Oaks ‘n’ Folks – Volume 14, Issue 2 – August 1999

Tanoak (Liuthocarpus densiflorus)and coast live oak (Quercus agrifolia)distribution stretches more than 1,500 miles along the California and Oregon coasts. Coastal forests are not monocultures but rather full grown islands of different species that cover like ribbons the California and southern Oregon shoreline. However, since 1995 scattered patches of dying tanoaks around the Mount Tamalpais-Inverness area have provided a hint of the serious threat to their existence.

What actually does happen? Tanoaks, very resilient trees, suddenly die. The symptoms are puzzling: a mature tree’s shoots display very pale green color then suddenly droop. Old foliage changes from dark green to light green, then suddenly the whole crown is brown with leaves clinging to the branches for the remainder of the growing season. Development of these symptoms and sudden death occurs within 4 to 6 weeks. Such a progression of symptoms and rapid death of evergreen trees caused by any pathogen has never before been recorded. Only a potent chemical, such as a phenoxy-based herbicide could mimic similar rapid death. During the following growing season tanoaks lose their leaves leaving bleached twigs and branches as a testimony of the onslaught.

However, the sudden massive death of tanoak has not worried foresters, because they consider tanoak an “undesirable weed,” claiming space from other species that are economically important for lumber. Yet for people who live in the urban forest interface this tree provides shelter and other ornamental amenities. For homeowners in the California coastal region, the tanoak shades houses and shelters other tree species with its resiliency to withstand storms. For example, the coast live oak, which grows slowly in its young age but later becomes dominant, is a major benefactor of this protection, especially if the tanoak is “out of the way” in the live oak’s mature stage. The sudden death of tanoak would then be helpful to achieve more desirable live oak, but there is a worrisome development: tanoaks have become heavily attacked by oak bark and ambrosia beetles. In these rapidly dying tanoaks, the beetles reproduce in the millions, and have unleashed an epidemic of unprecedented proportions in coast live oaks grown as ornamentals and in natural forests. Again, such massive dieback of live oaks was never recorded in California and, if this rate continues, we are going to face a double environmental crisis: (a) loss of these highly valued trees from gardens and forests, and (b) serious fire hazard risk from the resulting build-up of dry fuel.

Since 1995 dying tanoaks and live oaks have been sampled to determine the cause of this unusual and rapid death. So far, only a single common relationship exists-bark beetles and the more visible ambrosia beetles are always present. However, the triggering factor, which invites these beetles to arrive and kill the tree, has yet to be discovered. Therefore, I have developed two publications that describe the current situation and offer some avenues that may help to find the biotic or abiotic factor that initiates the rapid death of tanoaks and whether a similar mechanism will operate in coast live oaks. The rate of live oak death has reached epidemic proportions in Marin County and has now spread to Sonoma County and possibly to other coastal regions.

Pavel Svihra can be reached at the Marin County Cooperative Extension Office (415) 499-4204, and email address: pxsvihra@ucdavis.edu

prepared and edited by Richard B. Standiford, Justin Vreeland, Bill Tietje