The following key includes all the trees known to occur in Wisconsin outside of cultivation (native, naturalized and escaping from cultivation). It does not include species known only from cultivation, except for a few gymnosperms clearly identified as such, and it does not intentionally include shrubs. However, species of intermediate form and those variable enough to be considered either tree or shrub under certain conditions have also been included. If it seemed likely that a person encountering the species for the first time might be in doubt as to its status (e.g. Prunus nigra), it was included in this list of trees. The list of species is therefore more likely to include all the Wisconsin trees, at the risk of including a few larger shrubs of ambiguous form. Based on those broader criteria, 116 angiosperms and 12 gymnosperms have been included for a total of 128 tree species (four additional species of gymnosperm shrubs are included in the key for convenience and they are repeated in the shrub section). Twenty six species treated as trees in this key might also be considered shrubs. An additional 204 species (including the troublesome Rubus, Rosa and most of the Crataegus species) would be classed as shrubs (including 25-30 woody vines) for a total of 332 woody species. A separate treatment of the shrubs of Wisconsin is under construction and a unified key for all woody species of Wisconsin is a long-term goal.

There does not appear to be a clear basis for separating trees and shrubs. Trees tend to be taller and larger in diameter than shrubs, but even the largest trees begin life as small seedlings and must pass through at least several years during which they are no taller than the shrubs, so height alone is not a reliable character. The "trunks" of some species commonly considered to be shrubs may also reach several inches in diameter. As a guiding principle in determining the species to include in the key I have adopted a basic model defining trees as those woody species that typically maintain most of the stem mass in a single central trunk. Shrubs branch and rebranch early with no clear dominant central stem (and will be covered in a separate key at a later date). Adherence to this model, based on growth form, will make it easier to recognize trees when they are small and should avoid some difficult choices for the user, although the total range of variability is still great enough to create difficult choices.

Some popular books and environmental education venues encourage beginners to identify trees by their growth form or bark of the trunk. With experience these characters can be useful, but in fact the number of species for which this method is reliable for beginners is relatively small, and then only for mature trees. The attempt to simplify identification for beginners in this way leads to frequent errors and misconceptions as lessons learned in error are passed on to students, who ultimately teach the errors to others. Most notable in this regard are crack willow (Salix fragilis) which is often misidentified as black willow (Salix nigra) and the ashes (Fraxinus), all frequently misidentified on the basis of the bark. Growth form or characters of the mature bark are used only sparingly within this key for a few species such as Ulmus thomasi, that are otherwise very difficult to identify in the absence of flowers or fruit.

The only reliable way to regularly identify trees (or any other organisms) correctly is to learn to recognize a set of diagnostic characteristics that have been confirmed by use over a wide area and a large number of individuals. The mechanism most often employed for learning these characteristics is the dichotomous key, in which the user is presented with two exclusive alternatives (e.g. leaves opposite or alternate) and chooses the one that best fits the plant in question. That choice leads to another set of two alternatives, and so on, until the user reaches an answer. With practice it becomes second nature to quickly narrow the possibilities to a small subset of species, within which a few critical characters can be examined to conclude the identification.

This key is based largely on characters long recognized by previous botanists. It offers as refinements a set of species reduced to include only the trees of Wisconsin, numerous photographs intended to illustrate characters employed in the key, and emphasis on the specific characters that I have found to be effective in Wisconsin over the last 25 years. The style of the web-based key with only two alternatives on each page allows adequate space for more supporting information to be provided at each node of the key for difficult choices. The key is highly artificial, although I tried to keep the species of a genus together where possible. It depends mainly on vegetative characters, especially those of the leaves and twigs. The use of characters based on flowers and fruits can make the job of identification much easier, but they are typically present for only a brief period, are present only on mature individuals and may be resticted to the upper portions of the tree, beyond the reach of the botanist. Their use as primary characters has been avoided, but they are sometimes provided as supplementary characters and can be extremely helpful when flowers are present.

Key to Trees of Wisconsin