A 47-year-old woman was taken to a hospital in serious condition Wednesday after a large branch hit her as she sat under a tree on Kalakaua Avenue, the heart of Oahu’s tourist district.

At about noon Wednesday, witnesses said the woman, described as a visitor, was resting on a concrete street planter under an autograph tree when a 15-foot-long limb split off and fell on her.

The planter sits in front of the Nani Aloha Shirt Outlet store near Kealohilani Avenue. A nearby lifeguard and police officer assisted the woman before paramedics arrived.

The Emergency Services Department said she was transported to a trauma center in serious condition.

In an emailed statement, Jon Hennington, spokesman for the Department of Parks and Recreation, said Division of Urban Forestry workers are investigating the cause of the fallen limb.

“The initial inspection shows no clear point of failure, nor any signs of decay,” he said. “The investigation will include a forensic examination of the branch by certified arborists.”

A city crew cut down and removed the entire tree by 1:45 p.m.

“The autograph tree is one of the trees under DUF’s maintenance jurisdiction, and was last pruned by a city contractor March 30,” Hennington said.

On June 25, a city crew removed a cracked branch hanging from the tree.

“An inspection of the tree at the time showed no signs indicating that further pruning was needed,” he added.

Kana‘i Sarkissian, a groundskeeper with the Department of Parks and Recreation who was a few minutes away from the scene when the branch fell, said he believes the limb was top heavy due to thick foliage.

That, combined with the wind, caused it to split and fall, he said.

David Cox, a sales associate at the GNC vitamin and supplement store next to Nani Aloha Shirt, said there are strong winds from time to time.

While inside the store Wednesday, he heard the tree branch shear. The sound was sharp like “sheeooh,” Cox said.

“There was no way to run away. … It was too fast,” he added.

In 2012, a California couple was injured when an 8-foot branch fell on them from a tree at the old International Marketplace.

The autograph tree, Clusia rosea, native to Florida and the Caribbean, was imported as an ornamental plant.

The tree’s informal name derives from the practice of scratching words, often people’s names, on the dark green outer layer of the leaves, which exposes a lighter green inner layer. The etched words last as long as the leaf stays on the tree.