Section 1.2: Trembling Aspen 101

Trembling Aspen

Trembling aspen is one of the most widely distributed tree species native to North America, ranging from the tip of northern Canada through the United States and into Mexico. The scientific name for North America's trembling aspen is Populus tremuloides and they are a deciduous tree, meaning their leaves change colour and fall off in the autumn season. A single trembling aspen tree typically does not survive more than 150 years but it can live for more than 200 years and grow up to 20-25 m (65-80 ft). The diameter of the trunk is usually around 20 to 80 cm (8 inches to 2 feet 7 inches). The tallest trembling aspen recorded was 36.5 m (119 ft, 9 in) and 1.37 m (4 ft, 6 in) in diameter.

Trembling aspen are called as such due to their leaves. When you closely inspect a trembling aspen leaf, you should be able to notice how narrow the stem of the leaf is. When the wind blows, these leaves can easily be influenced to move and as such, you hear the trembling sound and see the leaves quivering in the wind. Aside from the trembling aspen title, they are also identified by many other names (see the table below). There is a closely related aspen species across the sea known as Populus tremula, whom European peoples have also referred to as quaking aspen because of the leaves’ tendency to quake or tremble.

How to Identify Trembling Aspen

Trembling aspen are characterized by a large variety of traits found in their canopies, trunks, roots, growing habits, and seasonal changes. It is important to note that they have a close relative, the black poplar or Populus nigra, and it can be easy to mix up the two. Distinguishing characteristics of black poplar trees are that their bark is most often dark brown but can appear black and is thick. Sometimes it fades to white, the higher the tree grows. As well, black poplar leaves are similar to white poplar but have more of a spade shape and are usually larger. See Figure 7 for an image of a black poplar tree. The following lists describe characteristics of parts of the white poplar tree, which is the tree this program will focus on.

How do Trembling Aspen Reproduce?

Trembling aspen reproduce through seeds or through root sprouting.

Seeds

Seeds develop on flowers called catkins (refer back to Figure 11) and the wind takes them to find the opposite reproductive organ. Reproduction through seeds produces trees with unique genetic makeup but is the least successful method due to the following reasons:

• Pollen from male catkins must use the wind to find and fertilize female catkins.

• Seeds from the fertilized female catkins use the wind to find soil with just the right conditions (water, elements, sunlight, etc.) to allow for germination (the growth of a plant from a seed).

• Seeds lack protection or stored nutrients, so they are viable for a very short time.

Root Sprouting

The most common and successful way for trembling aspen to reproduce is by root sprouting. In this process, new aspen trees sprout from the roots of a mature aspen tree. These trees have a shared root system, which enables a process called root sprouting. Root sprouting produces numerous trees or clones with identical genetic information, leading to a colony of trembling aspen. Colonies can consist of a few to hundreds of identical trees.

What is a Stand?

In forestry, stands are areas within the forest that have been measured, mapped, outlined, and described as a distinct group of trees. Stands can be large or small, ranging from a few acres to hundreds. Trees within a stand will share similar characteristics such as age, size, condition, and species. Often, stands are defined by species dominance. For example, in an area with an abundance of trembling aspen trees, this becomes known as a trembling aspen stand.

A trembling aspen stand is what appears to be a group of individual trees, but are all connected via the same root system. Scientists have described trembling aspen stands as clones, or clonal stands, because of their reproductive tendency to sprout new stems from the same lateral roots, thus creating a shared root system with genetically identical trees (see Figure 15).

Trembling aspen stands can grow as a single-species stand or in a mixed-species stand. As stands with a single species age, more shade tolerant species may grow within that stand. This occurrence produces a mixed species stand that can include white spruce, black spruce, balsam fir, white birch, balsam poplar, and jack pine.