Sunday, Aug. 31, 2008

Killer oak trees: Their falling limbs able to take a life

Tipsy drivers, jammed highways and smog aren't the only health hazards families will face this weekend as they head out for Labor Day fun. Add killer oak trees to the list.

In Yosemite National Park and along Highway 140 in Mariposa County, authorities have been taking reports of heavy limbs -- even entire trees -- suddenly crashing to the ground.

It has become such a public safety threat that the Mariposa County Resource Conservation District put out an advisory this week with the headline: "Watch out for killer oaks."

The phenomenon of falling oak limbs has been known to happen across the Sierra, particularly in dry years. Naturalists say it's a result of a defense mechanism oak trees use to survive when water is scarce.

They believe that in prolonged dry heat, dehydrated oaks compensate by hoarding water. This makes limbs heavier and more likely to fall.

And sometimes they don't just fall -- they explode off the trees, experts say.

With more people expected to picnic or just rest in the shade of mountain oaks this weekend, Yosemite and Mariposa County officials are warning people to be careful.

"Common denominators seem to be hot, calm, late summer afternoons, oak trees, and long, horizontal or leaning limbs or stems," said Yosemite Park Forester Brian Mattos.

No serious injuries have been reported this year, but in September 1985, a 6,000-pound oak limb fell onto an open-top tram in Yosemite Valley, killing two vacationers and injuring 12 other passengers.

Earlier this month, two workers in Yosemite Valley narrowly escaped when the top 12 feet of a large oak broke away and crashed onto a housekeeping building, heavily damaging it.

That was among several "large oak failures," including some around El Portal and Midpines along Highway 140 between Mariposa and the park entrance, Mattos said.

In Mariposa, California Highway Patrol Sgt. Edward Greene said he tells his officers patrolling Highway 140 to watch out for people parked in the shade of oak trees along the prime route into Yosemite and warn them of the danger.

The Mariposa County Board of Supervisors last week talked about the Labor Day weekend danger to unwary out-of-area visitors. This weekend's Mariposa County Fair will bring visitors to the foothills town, Supervisor Brad Aborn said.

Aborn has had some personal experience with falling limbs. This month, an oak tree on his property that he estimated to be about 200 years old split down the middle and fell "in a cloud of dust." There was no warning, he said.