We learn from Der Spiegel:
“The trees provide roughage that helps keep the animals regular.”
“The zoo received 300 trees from traders this year for its three fully grown African elephants, and they have finished devouring them.”
“We fed them one or two trees each per day,” said Kauffels. “If we gave them 10 at a time they’d get picky and would only eat the tips.”
http://www.spiegel.de/international/zeitgeist/0,1518,605502,00.html