As others have said, sticks are not safe chew toys. Dogs chew for 3 main reasons:
1. Because they are bored
2. Because they are stressed
3. Because they enjoy it
You can rule out #1 with more exercise and play. You may also be able to tackle #2 if you think hard about it - what might be causing them stress and how can you alleviate it? You could try a couple of drops of rescue remedy in the drinking water for example. I think this might be especially beneficial for Gracie. #3 can only be solved by providing them with plenty of things they ARE allowed to chew on and making them as appealing as possible. Have you tried raw meaty bones, kongs that kind of thing? (I mean have you tried offering them to the dogs; I don't expect you've tried them yourself!) What self respecting dog would want to chew on a bit of wood if there was a juicy bone available???? Have you tried using Bitter Apple or would this harm the trees?
Block the dogs' access to the trees, every time they do it it is reinforced even more, regardless of whether you scold them or not, so you have to disallow it.
"Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life." (Pratchett, Jingo)
"I used to look at [my dog] Smokey and think, 'If you were a little smarter you could tell me what you were thinking,' and he'd look at me like he was saying, 'If you were a little smarter, I wouldn't have to.'" - Fred Jungclaus