On managing California bay laurels to improve oak health

16 07 2011

Removing young bay laurels and burning the remains. Photo by Lee Klinger.

Several friends and tree professionals have contacted me about my thoughts on the following article by Peter Femrite that recently appeared in the San Francisco Chronicle:

Saving oak trees by chopping down bay trees

Workers began chopping down 250 California bay laurels this week in the Santa Cruz Mountains so that 49 signature oak trees might be saved from the infectious scourge known as sudden oak death.

The tree-removal project is an attempt by the Midpeninsula Regional Open Space District to prevent the spread of the tree-killing pathogen, which uses bay trees to scatter spores in the forest.

(h/t to R Zingaro for alerting me to this article)

First, there are important points here with which I agree. The bays are clearly major vectors for sudden oak death disease. I do believe that selective removal of bays will lower the incidence of sudden oak death (SOD). However, I am bothered by the singular focus on the disease. I would rather the focus of efforts be made toward promoting the overall health of the forest ecosystem.

In matters of removing a mature tree of any species from the land, my advice is that the decision be well informed. Also, it is always best to get a second opinion before removing a mature tree.

The California bay laurel is a major element of the forest ecosystem here in Big Sur. Historically, the bays were tended by the Native Americans for their nuts, which when roasted make an excellent coffee-like beverage, for the medicinal properties of their leaves, and for countless other uses.

Ecologically, the shade-tolerant bays tend to replace oaks in the succession of forest communities in coastal California. The aromatic leaves of the bays are extremely volatile and when exposed to fire are quickly consumed by flames. The burning practices of the Native Californians kept the bays at bay (sorry, couldn’t resist) and encouraged the oaks, which provided acorn – a major staple of their diet.

Now with the help the some of the world’s best fire fighters, the forests here in California are changing rapidly due to the fire-free conditions that have not been seen here for many thousands of years. And so the forests are responding accordingly, the fire-adapted oaks are dying out and the bays, firs, pines, and redwoods are replacing them in the inevitable process of succession.

The problems of disease and insect pests in the oaks are directly attributable to the dramatic successional changes resulting from fire suppression. Indeed, the bays are on rise and spreading rapidly. Yes, the bays spread disease, particularly young bays with disease-bearing leaves that brush up against the trunks of the oaks. But that is not the only way they harm the oaks. Bays growing adjacent to oaks will compete for water and nutrients in the soil. There are only so many resources to go around, so more bays (and other woody plants) mean less healthy oaks. Eventually, the young shade-tolerant bays grow up through the branches of the oaks and become large bays. Mature bay trees can easily overtop mature oak trees and will eventually shade out oaks. Thus, oak-dominated forests become bay-dominated forests through the course of succession.

Of course, in California it is only a matter of time before the forests will burn. Where bay trees are abundant fires burn hotter and are more likely to spread to the crowns of the mature oaks, often killing them.

In short, there are undoubtedly many more bay trees now in coastal California than this region has likely seen for many thousands of years. If there are more bays, then there needs to be less of something else. In this case that means less oaks.

For these reasons I do often support the removal of bay trees as part of a broader “fire mimicry” approach in restoring oak forests. However, not all bay trees are candidates for removal. When considering the removal of bays here are the factors I weigh:

1)    Young bay trees growing near the lower trunk of an oak are more likely to be direct vectors of disease than are large bay trees.

2)    Do the bay leaves show signs of carrying SOD? (see photo below)

3)    Bay trees growing under or near the canopy of oaks will likely increase the intensity of a wild fire and provide ladder fuel for the fire to spread to the crown.

4)    Bay trees growing under, over, or near oaks take away resources (water, nutrients, light) needed to sustain the oaks.

5)    Bay trees growing near structures increase chance of damage from fire.

6)    Individual large bay trees growing in more open, woodland settings are significantly less problematic for oaks than bays growing among oaks in a closed canopy forest.

7)    Bay trees growing on steep hillsides may be important for slope stability.

8)    Once bay trees are removed there should be a plan to keep the bay from growing back, which they will do if left unattended.

Bay leaves showing symptoms of infection by Phytophthora ramorum, the "causal agent" of Sudden Oak Death. Photo courtesy D. Schmidt, Garbelotto Forest Pathology Lab, UC Berkeley.

In situations where bay are removed there is the issue of what to do with the remains. If possible, they should be kept on site. Where fires are permitted I recommend piling the remains in brush piles and burning them. The ash from the fires is a good fertilizer and can be then spread on the soils around the oaks. Alternatively the remains can be chipped and piled for a period of time to reduce the viability of SOD innoculum, then used as a mulch. More information on managing bays with SOD can be found at the Sudden Oak Death website.

In a landscape with a periodic fire regime, oaks and bays can happily and healthily co-exist. In lieu of fire we must resort to fire mimicry, which involves, in part, the removal of some bays. Selective removal of bays will improve the health of the nearby oaks and significantly reduce the fire hazard. Of course, the removal of the bays and other dense overgrowth is only one of a suite of practices (e.g., moss and lichen removal, fertilization of soils with minerals and compost, proper pruning, etc.) that I use in the restoration of oak forests.




One response

16 07 2011
Ralph Zingaro

In my opinion, this information confirms that P ramorum and sudden oak death are native ecosystem organisms. There is no definitive scientific proof that P ramorum was introduced into the California ecosystem. In fact many old foresters in Northern California have told me that “the bay trees kill the oaks”. So the entire premise of exclusion and nursery “host plant” eradication and monitoring is a total waste of taxpayer money.
Also, the infected nursery plants could have been infected from forest trees, not the other way around. What we do know is the forest ecosystems have been degraded due to multiple anthopogenic factors like acid rain and forest trees are in a weakened state. Rather than concentrate on trying to exclude and kill a single pathogenic organism, we should be concentrating on ways to strengthen the forest’s immune system. This work has already been successfully demonstrated by the US Forest Service site at Hubbard Brook in New Hampshire , where they aerially applied calcium to entire forest ecosystem in 1999. Since that time the data has shown multiple benefits to declining forest trees like sugar maples.
This data backs up the claims on this site that the forest ecosystem is more important than any single forest pathogen. The native oaks of california are being predisposed to this infection and the pathogen most likely has been in the forest ecosystem for thousands of years.
Here in the east we have asian longhorned beetle, emerald ash borer, hemlock wooly adelgid and beech bark disease (another Phytophthora infection). Interestingly enough Beech Trees and California live oaks are in the same family “Fagaceae”. They are dying on both coasts from Phytophthora infections.
In the book “The Dying of the the Trees” by Charles Little we can learn that even President Ronald Reagan could sense that there was something wrong with the forest when he was travelling to Camp David in the early 1980’s. At that time 80% of our native dogwood trees were dying on both coasts from an anthracnose infection. A cursory examination of the literature yields some clues about dogwood trees- ” they have the highest calcium requirement of any forest tree”. This was and represents today the proverbial canary in the mineshaft. We as scientists just need to pay closer attention.

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