As this long, hot summer heads toward fall, trees will be doing their best to place some new offspring on the landscape. Some tree seed gets a lift on wind or water to new growing locations. Some seed will count on being eaten and then excreted by a bird. We are perhaps most familar with the activities of squirrels in gathering and storing seed like acorns, hickories, and walnuts. Foresters are rather fond of gray and fox squirrels - they collect seed and bury it in scattered locations around their home range for later excavation and use. What the forester likes is the squirrels don't always re-find all the buried nuts, so a new tree may result. If you have an interest in growing trees, or just want to get in touch with the world of squirrels, you could consider gathering and planting some tree seed. Oaks, walnuts, and hickories are a good starting point for using seed to grow new trees. Start scouting for seed on the trees in your neighborhood, local parks, or even cemetaries. Be sure you have permission to pick up seed if it is private property. Some seed is already being eaten by squirrels or falling from trees, so now is a good time to start looking, although prime seed collection time is usually mid September to October. Acorns, walnuts, and hickory nuts will drop from the trees when ripe, so keep an eye on the ground. Before collecting a lot of seed, check the quality of the seed by cutting or cracking a few sample seed. You may need pruning shears or, in the case of walnuts and hickories, a hammer to open the seed. Viable seed should be firm and white or light tan inside with no signs of insect infestation. Weevil grubs are fairly common in oak seed, so discard seed that has obvious small holes from weevils or with the cap still attached(with& the exception of bur oak, which retains its caps). Figure a percentage of good seed from your test lot to see how many total seed you need to collect to get the number of good seed you want. Keep your collected seed cool and moist until you plant it. Plant as soon after collection as possible. A good general guide for planting depth is one to two times the diameter of the seed, so a walnut would be planted one to two inches below ground surface. Squirrels can locate burried seed, so you may want to protect seed from squirrels by covering the planting spot with wire mesh sheets or cones anchored to the ground. Another easy protection method is a tin can with one end completely removed and an X cut in the middle of the other end with the corners pulled back to create a hole. Dig a hole approximately the size of the can. Place the seed and soil in the can and then push the open end of the can down until the cut end is flush with ground line. The seedling can then grow out through the hole in the cut end.
The screens and cans can be removed at the end of the first growing season once leaves have fallen, or if they are steel, simply left to rust away. Aluminum screen or cans will not rust away, so they must be removed. If you are willing to share with the squirrels, just plant a lot more seed. You can also start seed in containers. Half-gallon or gallon milk jugs with a drain hole cut in the bottom would work, as would deep planters. Over winter, you should bury or heavily mulch the pots to insulate them from rapid temperature fluctuation.
Many other tree and shrub species may be grown from seed including persimmon, redbud, dogwoods, maples, and tuliptree. The particular requirements for best germination and growth for tree seeds varies by species. A great reference for growing almost any native or common introduced tree species from seed is the Woody Plant Seed Manual produced by the US Forest Service. An on-line version is available at http://www.nsl.fs.fed.us/nsl_wpsm.html.
Purdue Forestry and Natural Resources/Extension and the Hardwood Tree Improvement and Regeneration Center offers several publications that can help answer your seedling questions.