An arborist who examined the limb that fell from a tree and killed a student concluded last week that the branch probably fell because it became heavier after absorbing extra water for the summer.

The university had two arborists examine the tree after Pa Houa Lor, an aspiring nurse and second-year health science major, was hit on the head Aug. 27 by a tree branch that fell off of a red oak between Butte Hall and Lassen Hall. University officials announced her death later that day.

After examining the branches of each tree to ensure that no other limbs were likely to fall, both arborists certified that the tree was no longer a danger, said Joe Wills, director of public affairs and publications for the university.

The phenomena of becoming overloaded with water, known as “summer limb drop” occurs when a branch becomes so overburdened that it can no longer support itself and falls to the ground, said Daniel Efseaff, the park and natural resource manager for the city of Chico.

“In general, we tend to have lot of tree branches fall during the summer in Chico,” Efseaff said.

The fatality has prompted the university to review its tree maintenance policy, which currently consists of a periodic examination of campus trees, Wills said. The tree that the branch broke from was reviewed shortly before the incident and at least 35 trees have been trimmed since the beginning of last semester, he said.

Mature trees that are near buildings or places where people tend to congregate are flagged by tree care experts for extra scrutiny, Wills said.

With an incident that has as much emotional impact as an on-campus death, the university is taking pains to ensure that it’s doing everything it can to protect students, Wills said.

“It’s both an unusual occurrence and it’s also very affecting to everyone involved,” he said.

Nicholas Carr contributed to this article.

Benjamin Mullin and Nicholas Carr can be reached at [email protected] or or @benmullin and @nikecarr on Twitter.