Bark, silhouettes and buds are the three keys to identifying trees in winter. My preference is buds, as they are so distinctly different from one species to the next. American basswood, or linden (Tilia americana), is a favorite. Its plump, oval, asymmetrical red buds, bearing only one or two bud scales are unmistakable.
Because tree buds tend to swell and increase greatly in size in the spring, this is often the season when we first notice them and assume that this is when trees produce them. If you look in the axils of leaves on any tree right now, you will see full-size buds that were formed this summer. These little packages of miniature leaves, branches and sometimes flowers, will remain on trees all winter, tightly closed and often protected from the elements by modified leaves called bud scales. Come spring, when trees are once again taking up quantities of water, their buds will swell, scales will fall off (leaving bud scale scars), and tiny, pristine leaves will appear. (Photo is of American beech, Fagus grandifolia, bud.)