What are those green garbage bag looking things around the base of young trees?

They are called Tree Gators. You fill them up with water and they slowly release the water into the ground, there by watering the young tree. Its important to keep the trees hydrated so that they can become established and stay healthy. In your yard, you could set a soaker hose or such on it, but in public areas where landscaping crews are not on hand to water them everyday, the tree gators work quite nicely. You can buy them for your yard too if you ever transplant a tree and don't want to spend alot of time watering- check your local garden center for them. -Neb

They hold water and act as a slow drip irrigation system. It takes 2-3 years for roots to become established. Once the roots dry, tree dies.

Now go drink a beer and relax

I think it has something to do with protection from extreme weather conditions.