The hollering of "Timber" is almost the trademark of the lumberjack, or mountain jack.

In the late 20th century, however, a new meaning crept into this honorable term "mountain jack", with the introduction of a new fangled device which borrowed the name: a hysroulic machine designed for making mountains out of molehills.

Lumberjacks and other mountain people holler across hollows, which are also called hollers in the vernacular. However the etymology of the two is quite different, one stemming from the ancient verb "halloo" (as in fox hunting) and the other from the word "hollow", IM not-so-HO.

"1699, Amer.Eng., var. of hollo (1542) "to shout," especially "to call to the hounds in hunting," related to hello. As a style of singing (originally Southern U.S.), first recorded 1936." (online Etymological Dictionary)