Among the Hidden
What helps to hide Luke? The woods. What is being destroyed at the beginning of the book? The woods.
Luke spends most of his life being hidden by the woods, so it's no surprise that they're being taken away at the beginning of the book. (How else are we going to get some conflict?) But this doesn't just mean the end of his almost idyllic childhood; it means that he might "never be allowed outside again" (1.2). The woods were almost like a security blanket for Luke. As long as they were there, he was safe and hidden. But now "their absence made everything look different" (1.12). And when people start seeing things differently, people start acting differently—like escaping from their prisons and striking up friendships with the neighbors.
See, just as much as the woods symbolize Luke's protection, they also represents his imprisonment. Without them, he becomes painfully aware of the drastic limitations of hiding. Luke notices that without the trees "everything was brighter, more open. Scarier" (1.12). Later on, when Luke decides to sneak out, he hesitations when confronted with the wide open spaces that "he'd been taught all his life to fear" (13.5). Facing this fear is especially hard for Luke since it comes with the added possibility of being seen.
Nature vs. Nurture
But the woods don't just represent Luke's protected childhood. They also represent nature: living, procreating, raising food, and just generally being free and independent as opposed to living under the Government's oppressive thumb. After all, Luke isn't the one eating potato chips and playing on the computer. When he's left home alone, he bakes bread and does chores. Meanwhile, his neighbor, the third child of a government official, acts like she's never even seen a tree.
We're not saying that Haddix wants us all to run off from our oppressive cities and live in the woods, but we're not not saying that, either.