The tallest redwood tree in Muir Woods — a giant that was assumed to have sprouted up to 1,500 years ago in the Middle Ages — is a measly 777 years old, a puerile sprig in the hallowed halls of old growth, an analysis of tree-ring data has revealed.

The study, by a Humboldt State University scientist, is the first definitive determination of the age of trees in Muir Woods. The findings by Allyson Carroll, a tree-ring specialist, mean that a 249-foot-tall coast redwood known by the lackluster name of Tree 76 sprouted seven centuries later than originally believed, at the beginning of the Medieval Inquisition in the early 13th century.

The new birth date is curiously apropos since the tree is in a place called Cathedral Grove, presumably acceptable under the Inquisition, which was characterized by an effort by the Catholic Church to suppress heresy. The date nevertheless means the oldest and biggest tree that researchers could find in the Bay Area’s most famous redwood forest is a babe in the woods compared with the giant old-growth trees farther north.

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“It’s one of the largest redwoods in Muir Woods, so it probably represents one of the oldest,” said Emily Burns, science director for San Francisco’s Save the Redwoods League, which is documenting the age, size, health and tree-ring history of California's last remaining old-growth redwood groves as part a statewide project known as the Redwoods and Climate Change Initiative.

Carroll, a biologist at Humboldt State and a consultant for the League, came up with 777 years by comparing the tree rings from Tree 76 to a database of core samples taken from redwoods across California, including from Samuel P. Taylor State Park.

Specific dates can be fleshed out by comparing the size and thickness of the tree rings, which are larger during wet years and smaller during dry years.

The samples were taken in Cathedral Grove in March 2014. Although that was one of dozens of redwood study plots in California, the research represented the first significant scientific study of the tree canopy at Muir Woods.

The plan over time is to identify tree-ring patterns, or markers, that are consistent throughout the coast redwood range and figure out how the trees react to climate change. As of now, the collective redwood tree-ring record in California can reliably be traced back to the year 328, revealing drought years and other major weather events, Carroll said. Scientists must have data from multiple trees in different locations to compare traits, which is why the reliable record goes back only 1,687 years. However, individual trees have been found that are older.

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Growth accelerating

Dendrochronology, as tree-ring science is called, was used to document a coast redwood in Redwood National and State Parks, near Crescent City, that is 2,520 years old. The oldest giant sequoia, a redwood species that grows in the Sierra, is 3,240 years old, according to the record.

Scientists have found that the growth trend of redwoods has accelerated over the past few decades, and they have pinpointed 1580 and 1739 as particularly parched years in California. The tree-ring record has also been used in cultural history projects. Carroll all but confirmed that a wood remnant at Trinidad Head, north of Arcata, was part of a cross erected by the Spanish in 1775. The redwood altar at Grace Cathedral in San Francisco came from the Humboldt area, but there wasn’t enough information to pinpoint the age of the tree it came from or the date it was cut, she reported.

Dendrochronologists are now trying to determine the age of the old adobe found at the historic Presidio Officers' Club by dating its redwood support beams. The adobe, hidden for years under a covering of wood planks and plaster, is believed to be the last remnant of the original Spanish fort El Presidio de San Francisco, established in 1776.

Until now, nobody had ever definitively calculated the age of the trees in Muir Woods, but scientists have suggested that the tallest trees were probably 1,200 to 1,500 years old. Besides Tree 76, Carroll calculated the ages of two fallen redwoods in Muir Woods, further bolstering the tree ring record in the grove.

Catastrophe centuries ago

The Vortex Tree, which fell in 2011, was 693 years old, according to her report published in April. The Solstice Tree, which fell in 2012, was 536 years old, she concluded.

Burns said the relative youth of the three trees means the grove itself is probably younger than was believed. She suggested a fire, flood or some other catastrophe might have struck the area many centuries ago, forcing the entire forest to regenerate itself from scratch.

“We know redwoods can get older than this, and some redwoods in the Bay Area were logged. But we don't see any evidence of logging,” she said. “That leads me to believe that there may have been some sort of disturbance.”

Peter Fimrite is a San Francisco Chronicle staff writer. E-mail: pfimrite@sfchronicle.com. Twitter: @pfimrite