Introduction to Tree Identification

Gymnosperm leaf arrangements: Coniferous trees vary in how they carry their needle like leaves. Some, like the firs (Abies), carry their needles individually, each attaching individually to a point on the branch. Two of the more distinctive conifer arrangements may be seen here. Fascicles are bundles of needles held together at the base in such a way that the entire unit may be pulled away or fall from the tree together. The Pine (Pinus) genus bears their needles in such fascicles, with different species having different numbers of needles in the arrangement. In contrast, trees bearing needles in whorls have their leaves clustered but still individual (when that needle is removed from the bundle it will come away on its own). This may be seen obviously in Tree One on the tour, the Deodar Cedar, but may also be observed in less traditional conifers such as Tree Eighteen, the Ginkgo.

Angiosperm Leaf Arrangements: Broadleaf trees have their own distinctive leaf arrangements, the most common of which are Alternate, Opposite, and Whorled. In alternate growing pattern leaves grow in an ascending pattern, with a single leaf out of a single side at each node on the branch. Opposite leaves, on the other hand, grow across from one another at each node in pairs. Whorled leaves also have multiple leaves per node but in numbers greater than two so they are not opposite but rather are whorled. 

Types of Broadleaves: A simple leaf structure is fairly straightforward, each single stretch of leaf tissue is a leaf. A Bigleaf Maple (Tree 72 on the tour) is an example of a simple leaf. Sometimes, however, it is necessary to identify which unit on the specimen is the leaf and which is a component on the leaf. While this may sound straightforward, the existence of compound leaves can make it more difficult. Compound leaves are leaves made up of multiple leaflets, some of the types of which are shown here. Leaves themselves are complete units, forming as one from the buds of a tree. Leaflets are components of leaves which were created by the tree along with the rest of the leaf (rather than as an individual). Sometimes trees will have multiple levels of complexity, such as in the case of the Silk Tree (#23 on the tour), which is bipinnately compound, with two levels of compound perpendicular leaflet structures.

Leaf Shapes: Overall shaped of leaves vary significantly between species, families, and, in some cases, even between branches on the same tree. Some differences are obvious, most conifers, for example, have needlelike leaf shapes (think pine needles), but even amongst these needlelike leaves there is variation. The needles of the coniferous Monterrey Pine (Tree #3) have the cylindrical pointed Acicular sharp which denotes many of the Pinus genus. The Western Hemlock (Tree #80) is another conifer, yet its flattened needlelike leaves conform far more to a linear shape description. One important leaf shape (which arguably could be a margin trait but is more useful at this higher level of classification), is the palmate leaf shape, i.e. leaves which have a hand shape due to lobes such as the Bigleaf Maple and others of the Acer genus.

Leaf Tip and Base Shape: There are a variety of smaller distinctions in leaf shape as well. Some leaves taper to abrupt points (the acuminate in this graphic), others have notched tips (the emarginate in this graphic and the fir (Abies) genus species on the tour. Bases also vary in shape significantly. One important note on leaf shape is that it is not uncommon for members of a plant family or genus to share common traits and phenotypes. Observing, for example, that a leaflet has a spatulate shape and the full leaf grows in a compound palmate formation would be a great indicator that it may belong to the Aesculus genus (Tree #40), and is likely a member of the buckeye family. The species within this genus all share this common feature, which is more unusual in other families. Such a note would narrow the search for the specific species name when identifying a tree of these features.