Rhetorical: Alma is referencing his earlier statement in verse 35 that his people should "bring forth works of righteousness, and [they] shall not be hewn down and cast into the fire…" Alma is emphasizing the major point of the sermon. His people must change their actions.
Translation: This verse very clearly echoes Matthew 3:10 (and Luke 3:9):
Matt. 3:10 And now also the axe is laid unto the root of the trees: therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire.
The language is so close that there can be no question that the New Testament phrases were the model for the phrases in the Book of Mormon. However, as has been noted several times before, this does not deny that a similar meaning was on the plates. Indeed the imagery of cutting down unproductive trees would have been sufficiently familiar to a New World audience.