The alder tree, belonging to the genus Alnus, is a distinctive member of the birch family, Betulaceae. This group of deciduous trees and shrubs is found across the Northern Hemisphere, often favoring cool, moist environments near water. Alders are unique among broadleaf trees, possessing structures that strongly resemble the cones of conifers.
The Defining Features of Alder Leaves
The leaves are arranged alternately along the stem. They are typically dark green, simple in structure, and somewhat glossy when mature.
The overall shape of the leaf is often described as rounded, oval, or obovate, being wider toward the tip than the base. A unique feature is the leaf apex, which is frequently rounded, blunt, or slightly indented, rather than coming to a sharp point. The margins are finely toothed, often exhibiting a doubly serrated pattern.
The leaves possess prominent venation, with veins running largely straight and parallel toward the margins. When young, the leaves can feel slightly sticky due to a fine, resinous coating. Unlike many other deciduous trees, alder foliage tends to remain green late into the autumn before eventually turning brown and dropping, skipping the vibrant seasonal color change.
Bark and Trunk Characteristics
The alder’s bark changes noticeably as the tree matures. Young trees typically display a relatively smooth, grayish-green or olive-green bark. This younger bark is often marked with distinctive, pale horizontal lines known as lenticels, which are small pores used for gas exchange.
As the trunk increases in diameter and age, the bark darkens to a grayish-brown color. The smooth texture begins to give way to a rougher surface, developing shallow fissures and eventually breaking into scaly, flattened plates. The overall form of the tree is often conical or pyramidal, especially in younger, open-grown trees.
Alders often grow in a multi-stemmed fashion, particularly when they regenerate from the base, giving them a somewhat shrubby or thicket-forming appearance. The trunk is generally straight, but the rough, dark bark of an older tree provides a textural contrast to the smoother, often spotted bark of the younger branches higher up.
Cones and Catkins
The tree is monoecious, bearing separate male and female flowering structures, called catkins, on the same plant. These structures typically form during the summer and bloom early in the following spring before the leaves emerge.
The male catkins are long, yellow to reddish-brown, and hang loosely in pendulous clusters. These structures release pollen into the wind and then quickly wither and fall away. The female catkins, however, are small, initially green and oval, and stand more upright.
Following wind pollination, the female catkins mature and transform into small, hard, woody structures that look remarkably like miniature pinecones. These “alder cones,” or strobiles, are only about three-quarters of an inch long and turn a dark brown color by late autumn. They remain attached to the branches long after they have released their tiny, winged seeds, often persisting throughout the winter and into the next growing season.