In the chapter Of the Rings of Power and the Third Age in The Silmarillion, after the final battle of the War of the Last Alliance, the following passage occurs:
And northward, beyond the Falls of Rauros and the Gates of Argonath, there were as yet other defenses, powers more ancient of which Men knew little, against whom the things of evil did not dare to move, until in the ripening of time their dark lord, Sauron, should come forth again. And until that time was come, never again after the days of Eärnil did the Nazgûl dare to cross the River or to come forth from their city in shape visible to Men.
If you look at the maps in The Lord of the Rings, the forest of Fangorn is just north (and west) of the Falls of Rauros and the Gates of Argonath, which is where Treebeard and the other Ents live; Ents are ancient and Men know little about them, so Ents could be the "defenses".
However, the Elven kingdom of Lothlórien also fits the description, both in terms of the location and in terms of the powers there being ancient and mysterious to Men. So maybe the "defenses" are the Elves of Lothlórien.
Or maybe the "defenses" are something else entirely.
Is there any way to determine what "defenses" Tolkien has in mind here?