Ohio Buckeye

The first tree to leaf out in the park is the Ohio Buckeye, Aesculus glabra. Most of them are quite small - there are clusters of young trees at the end of the driveway coming out of the Ornithology Center, another patch just before the sharp curve at Fisherman's Cove, and near the end of the Cabin Loop trail by the EDC. I only know of one big tree in the park, although there are probably others scattered around.
.
The European version of the Buckeye is known as the Horse Chestnut, Aesculus hippocastanum, and is sometimes planted as a landscape tree. Despite the name, this is not the chestnut of "roasting on an open fire" fame - the nuts produced by both of these trees are actually toxic to humans, although deer and a few other animals can eat them without harm. Edible chestnuts come from the American Chestnut, Castanea dentata, a member of the beech family, which sadly has disappeared from most of our forests due to an introduced fungal disease.
Although it may look like I'm holding five leaves here, in actuality there is only one compound leaf in the picture, a single leaf composed of five leaflets. Buckeye leaves are also palmate - shaped like a hand with all of the leaflets connecting at the center. Some compound leaves, like those of the walnut, are pinnate - shaped like a fern, with the leaflets along a central stem. The new leaves are dark red, but they'll turn green as they get larger.
Here's a photo of a Buckeye from earlier in the year, which shows how the buds/leaves are arranged in an opposite pattern on the twig, rather than alternate. Opposite leaf and bud arrangement is an important clue when it comes to tree ID, as there are only a few tree groups that have opposite leaves: Maple, Ash, Dogwood, Horse Chestnut, and Buckeye. MAD Horse Bucks is an easy way to remember the opposite-leaved tree groups. I've also seen MAD Cap Horse - the "Cap" stands for the family Caprifoliaceae, which includes shrubs like honeysuckle, viburnum, and elderberry, all opposite-leaved.