The "National Audubon Society Field Guide to Trees" notes that the many oak species in North America are among the most important of the hardwood trees in many ways. Oaks provide shelter, cover and food for wildlife as well as lumber for many different purposes. Oak trees are also important landscaping trees. Identifying oaks can be problematic because of the many different kinds and the wide variations that occur between species.
The assorted features that oaks possess are one reliable way to tell oaks apart from other trees and from each other. Oaks produce a fruit called an acorn, with the sizes of the acorns, as well as their overall shape and look, differing between species. Some oaks have a darker gray bark, while others are lighter or darker shades of brown or black. Oaks are monoecious trees, with the flowers of both sexes occurring on each oak. The male flowers, called catkins, are another feature you can use in oak identification. Usually catkins emerge in the spring to complement the smaller female flowers that develop over time.
The size of an oak tree can help you to distinguish it from other species of oaks, but since many oaks are in the same general height range, this is not an absolute key for identification. For example, the oaks such as the white oak, which can be 100 feet tall, are considerably more sizable than a Gambel oak (30 feet high at most). However, oaks like the overcup oak, bur oak and post oak all can grow to be at least 50 feet tall, making estimating the height of a tree no sure way to separate species.
Red Oak Vs. White Oak
Botanists divide the oaks into separate groups: red oaks and white oaks. The leaves of the red oak species have pointed lobes tipped with small bristles, while white oak leaves have a rounded edge to their lobes. Red oak acorns need two years to mature versus one year for white oaks. The acorns of red oaks are bitter compared to those of the white oaks. While not all the oaks fit neatly into these categories, most adhere to these general descriptions.
Some oaks are evergreen, while the majority loses their leaves before winter's arrival. The live oaks, however, are exceptions to this, with species like the canyon live oak and interior live oak maintaining their leaves until replacements grow in the following spring. This gives these oaks an evergreen appearance. Another facet of some oaks is the odd shape of their leaves compared to most others. The willow oak, for instance, has elongated narrow leaves lacking lobes. Shingle oaks have leaves that are oval and more similar to a laurel than to those of an oak.
The habitat of the oaks gives away vital clues to what type of oak it may be. Most oaks have a certain habitat in which they do best. Species such as the swamp white oak grow near water, with the borders of swamps and riverbanks common sites for this tree to develop. Bear oak prefers sandy soils and rocky terrain upon which to grow. Pin oak grows more often on poorly draining soils that maintain moisture than on any other kind of ground.