The long-awaited purchase of a vast grove of giant sequoias once targeted for homes and a ski resort has been completed by the San Francisco conservation group Save the Redwoods League, which plans to open trails and eventually make it a federal park.

The league raised $15.65 million to buy the 530-acre Alder Creek Grove, a picturesque hillside forest in Tulare County that includes the 3,000-year-old Stagg Tree — the fifth-largest tree in the world.

It was the culmination of 20 years of negotiations, said Sam Hodder, president and CEO of Save the Redwoods, which received donations from 8,500 individuals and foundations in all 50 states and 30 other countries after announcing in September that it planned to buy the largest grove of sequoias still in private hands.

Now Playing:

“It really was an extraordinary and inspiring outpouring of generosity,” Hodder said. “This is the best of what’s left. This is a truly magical place, and it comes at a time when people needed some good news — something that protects the beauty of the world.”

The transaction, which was finalized Dec. 31, saves from logging or development 500 old-growth trees nestled in the midst of a diverse, high-country forest virtually unknown to the general public.

Owned since 1946 by the Rouch family, Alder Creek is surrounded by Giant Sequoia National Monument and Sequoia National Forest. It contains 483 giant sequoias that are at least 6 feet wide. The ancient stand is about the same size as Yosemite’s famous Mariposa Grove of giant sequoias, which was established in 1864 and helped inspire the creation of the National Park System.

The 34.7-foot diameter Stagg Tree, named in 1960 after legendary football player and coach Amos Alonzo Stagg, is older and bigger than the Mariposa Grove’s Grizzly Giant, which is 30 feet in diameter.

The Redwoods League now plans to spend another $4.75 million on ecosystem studies and restoration work, including clearing dense stands of trees and underbrush. Hodder said trails will be built and a public access plan will be developed over the next five to 10 years.

Once that is done, he said, the land will be conveyed to the U.S. Forest Service for incorporation into Giant Sequoia National Monument.

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