"Leave it as it is. You cannot improve on it. The ages have been at work on it, and man can only mar it." That's what Theodore Roosevelt said about the Grand Canyon in 1903. But "improving" a piece of land can mean a lot of different things. In the case of this Industries chart from 1890, it meant "cultivating." Huh?
According to the Online Etymology Dictionary, one early definition of "improve"--to turn land to profit--was used in the American colonies. Well, now that makes sense here. It adds another layer of meaning to Roosevelt's "You cannot improve on it," too. He may have simply meant You can't make this gorgeous place better! as it seems to us as readers now, but maybe this land-use meaning was intended too: Don't try to profit off of this place! Just don't do it!
Anyway it's nice to imagine a time when only about 12 or 13% of California had been "improved."