That's an LOD (Level of Detail) issue with the game assets. In this case the offending trees. The way games work is that objects have models with various levels of detail that get rendered depending on how far away you are from them. When you're close, the highest level of detail model gets rendered, when you move away from the object the game engine will start swapping out the model to ones with less level of detail until finally it renders a model that barely even looks like a tree, but you're so far away you can't tell.
In an ideal situation there would be enough models at various levels to cover all possible distances you could be looking at it from, but from my observations in a lot of Arma assets, especially modded ones that have been imported from Arma 2, the modders do not include a full range of LOD models. There is only the full detail model and the lowest detail model. In some cases the game will swap out the full detail model and replace it with the low detail model when you're as little as 15 or 20 meters away from it.
You can try to bump up your objects quality to ultra, but even that's not going to fix it to a point where you'll stop noticing it. Other than that there's pretty much nothing you can do about it.
