Why widespread tree mortality is not prevalent in coastal forests?

December 25, 2005

During the last six months, since I have been producing Tree Tips, the most popular subject in the questions I have received has been causes and management of tree decline and mortality. Ironically, I left college believing that coastal forests were little afflicted by diseases and insects and that all of the serious problems were found in the Sierran and southern mountain ranges. While I continue to learn the numerous insects and diseases that cause decline and mortality of trees in coastal forests, coastal forests do not suffer the widespread tree mortality that shock certain communities, like Lake Tahoe and Lake Arrowhead. Why isn't widespread tree mortality prevalent in coastal forests?

Firstly, insect outbreaks become epidemic, causing widespread tree mortality, more often than disease outbreaks and insect outbreaks are more prevalent in dry regions as compared to outbreaks of disease, which are more prevalent in moist regions. Some insect genera emit pheromones, a potent chemical that attract other insects that can result in a population explosion, whereas the rate of spread of diseases is regulated by its vectors, such as wind and water, and is comparatively slower. Generally, in our coastal forests, trees die individually or in relatively small groups, resulting in little impact at the landscape level.

Secondly, single species forests are more prevalent in the Sierras. Single species forests are more susceptible to widespread mortality because a different tree species that may not be susceptible to a host specific insect is not present to break up the progression of the outbreak.

Thirdly, pines are more prevalent in the Sierran and southern mountain ranges and certain tree species, such as pine, are more susceptible to insect infestation. In fact, some of the most damaging forest epidemics are in pine forests.

Another reason that I did not learn as much about the diseases and insects that cause tree decline and mortality in coastal forests is because less is known about them. There is a comprehensive and accessible body of work about Sierran insects and diseases because the large amount of publicly owned land and visible widespread tree mortality in the Sierras has been a recipe for money, research and lands on which to do that research.

A portion of this production was gleaned from a conversation with Jack Marshall, Forest Pathologist at the California Department of Forestry and Fire Protection. If you have a local disease or insect question for Jack Marshall, he can be reached at Howard Forest in Willits at 707-459-7448.

I welcome responses as well as ideas for future Tree Tips. You can also get an email version by contacting me at thembi@mcn.org.

Thembi Borras is a Registered Professional Forester living and working in Mendocino County, CA USA.