Blue marks remain on several trees sitting among a tangle of forest litter and towering redwoods in Cascade Creek canyon in the Santa Cruz Mountains — an indication that they were once the target of loggers.

The paint, along with numerous, weathered springboard notches cut by lumberjacks into what are now stumps, are reminders of how vulnerable the redwood forests in the Bay Area and California have been over the past 150 years. But they are also symbols of how much things have changed.

“We got here just in time,” said Sam Hodder, president and CEO of Save the Redwoods League, gesturing toward the paint marks — at least two decades old — as he stood in the middle of 564-acre Cascade Creek, nestled between Big Basin Redwoods and Año Nuevo state parks.

The San Francisco conservation group reached an agreement Thursday to buy the picturesque canyon, a sprawling lush forest oasis with 100 acres of ancient redwood trees that have never felt the blade of a saw, the kind conservationists reverently refer to as “old-growth.”

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The league has so far raised $8.6 million of the $9.6 million needed to complete the transaction, which is expected to close May 30. The deal, once finalized, will create a continuous corridor of protected redwood habitat stretching from the Santa Cruz Mountains to the Pacific Ocean.

“This land is one of the key missing links in an extraordinary landscape,” Hodder said of the grove, which the league has wanted to buy for two decades. “This is a really important part of our effort to restore and build the old growth of the future.”

The plan is to make Cascade Creek an integral part of a larger effort to preserve second-growth trees, which are the conifers that grew up after the originals were cut down.

These fast-growing descendant trees — often found growing in a circle, called a fairy ring, around the original stumps — make up 95% of the redwood acreage across the state. Ecologists believe the reconstruction of California’s once mighty forest ecosystem is dependent on the regrowth of the previously logged giants which, at their full height and density, provide unique, valuable habitat for many rare birds, insects, reptiles, mice and other mammals.

Cascade Creek, which also contains woodlands filled with knobcone pine and madrone, was already a study plot for the league, which plans to do further research on forest growth, wildlife and the carbon-capturing properties of redwoods. The ultimate goal throughout California is to build ecological bridges connecting old-growth and second-growth stands.

“This is a terrific example of a healthy, recovering second-growth forest,” Hodder said during a recent hike through the property. “They’re popping right up out of the ancient roots that are below ground. These are the clones of the trees that have been growing here for thousands of years.”

The canyon, carved out by its namesake creek, sweeps down in a green swath from the coastal mountains on the border of Santa Cruz to the beach, where colonies of giant elephant seals bask. A 50-foot waterfall cascades amid the trees onto rocks just south of the league’s new property.

There is no shortage of ancient trees — 100 acres of old growth is hard to find in the heavily logged Bay Area forests, including neighboring Big Basin.

Researchers at Humboldt State University studied four of the largest redwoods last year, taking core samples that showed an age range of 420 to 528 years old and heights reaching 235 feet. They were surprised to find that one 100-year-old second-growth tree was actually taller than a tree more than 500 years older.

“The one that we thought was the youngest tree ended up being the oldest and vice versa, probably because the younger tree had a lot of light after the old growth was removed,” said Jim Campbell-Spickler, a Humboldt State forest canopy ecologist who often works with the Redwoods League. “This was important information because we were interested in the regeneration of trees from cut-over areas.”

The preservation of coast redwood forests is important because they are facing many challenges, including warming temperatures, increased fire danger, destruction of redwood and wildlife habitat, and the encroachment of roads, development, agriculture and illegal marijuana plantations.

Old-growth trees, which can live up to 3,000 years, once covered huge swaths of land along the California coast all the way to the Oregon border. Starting in the 1850s, loggers began cutting them down, including a huge stand in Oakland that researchers say might have contained the largest coast redwoods in the world. Those “springboard notches” held a platform that allowed loggers to get above snow and other obstructions while felling a tree.

Researchers say 95% of California’s old-growth redwoods were wiped out in the 150 years after the California Gold Rush, leaving only 113,000 acres of the oldest and largest coast redwoods.

It was a staggering loss.

Still, researchers say, while coast redwoods remain the dominant tree over much of their original range, only 22% of that ecosystem has been fully protected. There are currently 1.4 million acres of logged-over redwood groves between San Luis Obispo and Oregon.

Cascade Creek was not spared the ax. All but the hard-to-reach portion, where most of the old-growth trees are, was logged at least once when timber companies came through around 1900, again in the 1940s and finally in the late 1980s.

Larry Holmes, whose family has owned Cascade Creek since 1978, said his father did some logging on the property in 1987, but no old growth has been cut since the 1940s. Still, there are at least eight lots on the property that could be developed with houses.

“We’ve felt for many years that this is a piece of property that should be preserved as a park,” Holmes said.

Negotiations began in earnest in 2012, when the league paid the Holmes family $8 million for Peters Creek, the third-largest old-growth redwood grove in the Santa Cruz Mountains. The good feelings generated by that transaction prompted Holmes to give the Redwoods League a $1 million discount on Cascade Creek, which was appraised for $10 million.

Hodder said the league hopes to raise an additional $600,000 for restoration and maintenance through its Forever Forest campaign, a $120 million effort to protect redwood and giant sequoia groves around the state. He said the plan is to do some restoration work and eventually turn the land over to the California State Parks system.

“We’re happy that it is becoming part of the park, very happy,” Holmes said. “The whole family is.”

Hodder paused next to a giant log spanning the creek where most of the old-growth trees stand untouched and looked out in wonder.

“How unique is it that there is this little hidden valley in the heart of the Bay Area that harbors these ancient trees?” he said. “It’s true primeval forest less than 25 miles as the crow flies from the 7 million to 8 million people that live in the Bay Area metropolitan area.”

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