Green Warders are cultivated by the elves of Faerûn to act as guardians of their most sacred places. Despite this, a Warder is still a plant rather than a constuct, since constructs are normally made out of unliving material, while the Warders are still living plants. The Warder is not meant to be a combatant, but a decoy. Their branches and leaves are arranged in such a way so that it looks like an elf from a distance, causing any interlopers to either turn back or follow the fake. In addition to that, the Warder can cast several enchantments to either confuse or put intruders to sleep.
When elves migrate from one area to another, they may leave the Warders behind, which will still carry out its duties. However, without the elves there to trim them regularly, the Warder grows shaggy with overgrown leaves and branches.
I know horns aren't an especially elfy thing. Well, here's what I think about elves. I ain't like 'em. Or I specifically don't like those hippie frou-frou elves that are just so calm and intelligent and beautiful and blehhh. It's kind of why I prefer drow to ordinary elves, because at least drow have an interesting trait in their general awful evilness. In fantasy, I tend to find most elves to be a race of Mary Sues with maybe a little bit of condescension for the other races.
Which means that my favourite forest elves are the Lorwyn elves of Magic: The Gathering. They're a bit closer to Fair Folk, which is how I like my elves: self-obsessed, arrogant, dangerous and in tune with the more deadly aspects of plant-life. They loathe all other non-elf creatures (they call them eyeblights) and seek to enslave others at best or hunt them at worst. Also, they have horns. Which is why I gave the Green Warder horns.