Austen Thibault, The Union Democrat

An unexplained but documented phenomenon could be the cause of the snapping tree that killed one camp counselor and injured four others last week near Yosemite, say local experts.

As temperatures rise, a phenomenon known as "summer branch drop" and other hidden dangers can affect certain trees, most notably oaks, which are found in nearly every ecosystem throughout the region.

"If the details I have are accurate, summer branch drop could be the cause in this case" said Dave Alley, owner of Dave Alley Tree Service in Sonora.

The Tawonga incident happened when a large oak tree - more than 70 feet tall - snapped about 30 feet above the ground at 8:25 a.m. Wednesday and crashed into a campfire circle where several counselors had gathered, according to Sgt. Jim Oliver, Tuolumne County Sheriff's Office spokesman.

Oliver said the Sheriff's Office is not investigating the incident and called it a "freak accident," as no sign of rot or decay was found on the tree.

He also said that if an arborist had a better explanation, he would be open to it.

Falling trees and sudden branch drops in the Sierra foothills can be common in summer, experts say.

"Summer branch drop" is most common in very large oaks, on exceptionally hot days. A light breeze and water expelling from the fallen limb are also associated with the phenomena, Alley said.

Groveland Community Services District 25 miles away reported a high temperature of 101 that day.

The light breeze and water expelling from the limb could not be confirmed by Camp Tawonga or responding Cal Fire officials in time for this article.

Camp officials also have not said whether any further investigation will take place.

Scientists still dispute what exactly causes summer branch drop, which is when a large, healthy looking limb snaps off of a mature tree. Some say it can be from a tree transpiring heavily during the late-summer months, while others claim its due to alack of moisture in the soil.

Alley describes the theory as a build-up of water pressure in the tree caused by the heat and possibly breeze triggering the tree to take up more water than it can handle and snapping.

The natural event has only been documented in California and England, he says. Alley says it is documented in "The Western Arborist," the magazine of the western chapter of the International Society of Arboriculture.

Alley had the rare opportunity to witness a "summer branch drop" the moment it happened while trimming a customer's trees at Pine Mountain Lake about 18 years ago. He said the branch, roughly 20 inches in diameter, suddenly fell from the tree without warning.

Alley said the noise the branch made when breaking from the trunk sounded like a "small explosion."

Oaks are multi-branching trees that grow to large sizes and can develop structural defects over time, Alley explained.

"Over many winters, they can develop frost cracks from heavy winds and snow loads," he said. "As they get older, they are more susceptible to these problems."

While he said it's more common for his company to receive emergency calls-for-service regarding fallen trees when in it is cold, certain pests and fungi can damage oak roots and cause them to fail in the summer as well.

Despite the dangers, Alley said roughly 75 percent of the trees he inspects are healthy and in good structure.

"It's good to take precautions," he advised. "Have the tree inspected by a certified arborist or ask for help from your local farm adviser."

One victim of Wednesday's accident, Elizabeth Moore, 18, was released from Doctors Medical Center in Modesto.

Another counselor, Cara Sheedy, remains in the recovery wing at Memorial Medical Center. Medical center staff would not comment on her condition or anticipated release time.

Two other victims, Juliet Ulibarri, 21, and Anya Schultz, 20, were treated for minor injuries and released the same day at Sonora Regional Medical Center.

Annais Rittenburg, 21, a student at the University of California, Santa Cruz, was killed.

No children were hurt.