So I noticed that for many forest living animals such as tigers, Asian elephants, and gorillas, they are exposed to the sun in their exhibits. I'm sure they're fine with it but I think there's a way to make an exhibit look like it has a canopy without having trees that will be killed within a fortnight.
My idea is to simply have a reinforced mesh over the exhibit and have vines or ivy grow on it. That way, you get the illusion of animals being under a jungle canopy.
IMO zoo exhibits do not, in general, really replicate the dense forests where these animals live for several reasons:
1. It is too hard to find a tiger in the dense understory;
2. In temperate zone zoos a dense tree canopy usually results in a bare ground layer. So.... dirt floor;
3. These animals can be very destructive of groundcovers and shrubs, especially in moderate-size exhibits. So again, dirt floor;
4. Asian elephants can be very hard on trees and on the root systems of trees so planting lots of live trees in an Asian elephant exhibit will prove to be a waste in a year or two.
So back to your idea:
If you create a large, high unclimbable structure which will simply provide shade
1. that is a large expense just to provide some shade (especially over an entire elephant enclosure);
2. Animals are more easily spotted and photographed in brighter light;
3. When we see these animals in the wild it is not the light level that we first notice but rather the huge trees and lushness of the plantlife. Placing an awning over elephants would just feel like an awning over elephants.
4. There are some excellent exhibits that suggest animals-in-the-forest. The gorilla enclosure at Disney Animal Kingdom comes to mind.
5. Any vines grown over the mesh will be destroyed by the elephants, primates or any other folivore
6. Your idea brings to mind this
Islands - Rotation Exhibit - ZooChat