The major way in which Pres. Taft dissapointed reformers who cared about conservation was in his firing of Gifford Pinchot. Gifford Pinchot was the highly respected head of the Divison of Forestry. Pinchot engaged in harsh and public criticism of his boss, the Secretary of the Interior (Richard Ballinger). Ballinger had opened public lands to private development. This angered conservationists like Pinchot.
Because of Pinchot's repeated criticisms of Ballinger, Taft fired Pinchot. This made conservationists angry and it also annoyed Theodore Roosevelt, who had appointed Pinchot when he was president.
By angering these people, Taft helped split the Republican Party, leading the party's defeat in the 1912 presidential election.