BBC has the story about Pleistocene Park, the recreation of an Ice Age ecosystem complete with megafauna, including horses, reindeer, bison, musk oxen, elk, wolves, Siberian tigers and possibly mammoths.
Here is an article in Science from 2005,
In the mammoth ecosystem, the collective behavior of millions of competitive herbivores maintained the grasslands. In the winter, the animals ate the grasses that grew the previous summer. All the while they fueled plant productivity by fertilizing the soil with their manure, and they trampled down moss and shrubs, preventing these plants from gaining a foothold. It is my contention that the northern grasslands would have remained viable in the Holocene had the great herds of Pleistocene animals remained in place to maintain the landscape.
So restocking the land with herbivores may allow grasslands to expand and be maintained, and create a Serengeti in Siberia. It is an interesting experiment. We wish them the best of luck!