I guess keeping live trees and plants, accessible to the animals, in elephant enclosures is only possible if the enclosure is REALLY big by zoo standards. Apart from the example below though, I am sure that I have seen it be possible in one US zoo. Unfortunately I can´t remember which one, but I know that I was pleasantly surprised by the size of the enclosure. Perhaps something like 3-4 acres (12-16 000 square meters). The trees etc there did not seem to be protected by hot-wire. The grass was growing high, too - suggesting that the elephants simply could not eat it all.
Then there is the elephant enclosure at Disney´s Animal Kingdom which is lush to the extreme! But there is probably something fishy about that, Disney being Disney....