^ The information you want is here (
PDF link).
The total number of trees in Jackson Park is unknown, but a little more than half of the park has been surveyed in recent years and contains just under 5000 trees. For the sake of argument, assume 9000 trees in the park total. Out of that, the Obama center and all the roadway projects are projected to remove 789 trees, or around 9% of the trees in the park.
All removed trees will be replaced at a 1:1 ratio. Stony Island will also be widened into a boulevard north of 65th, which will include new trees on the east and west side parkways and in the median that are outside of the park boundaries but functionally part of the same urban forest. There are also new trees being added in the Midway between Stony and the Metra tracks which will offset the loss as well.