Three days before Cal's football team opens its season at home, UC Berkeley today began erecting a chain-link fence around the oak grove next to Memorial Stadium where a half-dozen tree-sitters have been roosting since December. Five to 10 protesters remain in the oak and redwood trees, where they will be surrounded by the fence. They vowed to remain in the trees as 80,000 Cal and Tennessee football fans filter past the grove Saturday to and from the 5 p.m. football game, which will be nationally televised. The tree-sitters are protesting university plans to remove several dozen of the trees to build a sports training facility. The City of Berkeley, a neighborhood group and a tree-advocacy foundation have filed a lawsuit to stop the project because of its proximity to the Hayward Fault. The case will be heard in Alameda County Superior Court Sept. 19.