As I was heading south on Highway 29 between Yountville and Napa the other day, I noticed   something unusual that prompted me to pull off the highway and take a closer look. A huge valley oak had collapsed, destroying a large live oak next to it and leaving a pile of logs, limbs and brush on the railroad tracks.

Fortunately, as far as I know, no-one was hurt. 

An interesting aspect of this incident is that the valley oak revealed no obvious cause of the failure. Granted, I did not see the tree before it failed, but the remaining, standing section of the trunk looked like sound wood to the center: solid, no cavity, no discoloration. The upper end of the break showed sharp splinters, a typical pattern for sound wood.

So, what caused the tree to fail?

Checking a little further, I looked at the weather statistics (www.wunderground.com) and found that the maximum temperature the day of the failure was                77 degrees, and the winds around Yountville were 20 miles per hour, with maximum gusts to 41.

Winds gusting to           41 miles could have caused the tree to fail, especially if it was heavy with foliage, as many local trees are this year. That’s not conclusive. It just a plausible explanation.

Another possible explanation of the failure is the phenomenon known variously as “high temperature limb drop,” “sudden limb drop” and “summer branch drop.” A fairly common occurrence, here’s the typical scenario: On a hot, windless summer afternoon, a large limb suddenly fails with a loud noise. 

I witnessed this where I used to work in the Menlo Park area: A huge valley oak limb failed on a hot afternoon. I can still picture the massive limb swinging like a huge battering ram from the tree cable, which was supposed to have prevented it from failing. The cable didn’t fail, but the limb failed in spite of the cable.

Arborist Nelda Matheny, speaking at the California Tree Failure Report Program in January 2006, gave a state-of-the-art summary of what is known about this strange but common phenomenon. She reviewed everything she could find on the subject and concluded that we still don’t know what causes it.

She did isolate some critical aspects that arborists and tree managers should keep in mind.  Temperatures do not need to be very high; over 70 degrees may do the trick. The failure may occur in the afternoon, evening and, sometimes, at night. The failure point tends to be a “brash” break, i.e. blunt, not sharply splintered. Brash wood can be a result of “incipient” (the early stages of) decay. Reaction wood — wood that is formed in response to mechanical stress — may also be brash.

Her description of typical trees and limbs exhibiting summer branch drop included old, slow-growing trees and horizontal limbs with an upsweep extending beyond the rest of the canopy of the tree.

Sudden branch drop is not limited to valley oaks. It’s been recorded  in many tree species in California, New York, Texas, British Columbia, England, Australia and South Africa.

The take-home message is this: Trees can fail for a variety of reasons or no apparent reason.  Sometimes, trees fail without showing us any warning signs. The best we can do is to be aware of warning signs, take action when necessary and acknowledge that living with trees is living with some degree of risk.

With the onset of hot weather and the unusually heavy growth of foliage in many trees around the valley this year, it is critical for tree owners and property managers to take a close look at trees under your stewardship, consider the level of risk you are willing to live with and take action. If it looks suspicious to you, it probably warrants a professional opinion.

In many cases, judicious pruning for thinning, which leaves the tree looking nice and natural, is all that is needed to reduce the failure potential of a tree or limb to an acceptable level of risk.

Bill Pramuk is a registered consulting arborist. Visit his Web site www.billpramuk.com, e-mail questions to info@billpramuk.com, or call him at 226-2884.