Plant of the Week #1 - Trembling Aspen   -   Megan Edgar, Wagner Natural Area Society, May 9, 2018

Common name: Trembling aspen, quaking aspen, aspen, white poplar

Scientific name: Populus tremuloides

Family: Salicaceae

 

If you take a walk in the forest, you may hear leaves “trembling” together in the wind. This sound originates from what is commonly known as Trembling Aspen, getting the name from the sound it makes when a breeze blows through its leaves! Aspen is a very common tree species found throughout western Canada, including Alberta, Saskatchewan, Manitoba and the Boreal and Aspen Parklands (Johnson et al., 1995).  In these regions, aspen tends to grow in dry, ridge areas, as well as rich and moist sites, which makes the aspen a widespread tree. They tend to grow best in loamy soils that are well-drained and moist, which makes certain areas of the Wagner Natural Area the perfect spot for aspen to grow.

  

Trembling aspen are typically small to medium-sized trees that have the potential to grow up to 20 m, and in rare cases can be found to be 30 m tall (Johnson et al., 1995). Aspen bark tends to be a greenish-white colour that can become darker and roughened on the lower trunk and around the bases of branches. Their leaves are oval to nearly circular and can also be rounded to square-cut at the base of the leaf with a sharply pointed tip, which can be 3.0-7.5cm long. The “trembling” sound that happens with the slightest breeze is caused by the edges of the leaves which are finely round-toothed with leaf stalks that are flattened (Figure 1).

 

The tree produces drooping catkins in the early spring, before the leaves begin to show (Figure 1). Aspens are dioecious, which means that the different sexes will appear on separate trees, essentially there are “boy” and “girl” trees. The male flowers will typically have between 5 and 12 stamens. The fruit capsules are 3-5 mm long, with very small seeds that have soft hairs present.


Figure 1: Trembling aspen leaves and catkins. 

Photo credit: Survival World.

http://www.survivalworld.com/plants/Plants%20Photo%20Pages/north-american-plant-photo-pages/trembling_aspen_photo_page.html#.WvNuRNMvy34 

Throughout my education I have learned aspen trees are one of the first species to recruit areas after a fire passes through (Johnson et al., 1995). They are the first tree species to colonize a forest succession, until they are gradually replaced by coniferous tree species. I was able to see this type of succession in Wagner Natural Area, near Atim field, as well as eventually near Post #23, where the meadow succession area is occurring. This is due to female trees being able to produce millions of seeds each year that are reliant on wind dispersal, a mode of transportation that is capable of spreading them up to 30 km in storm conditions. Moreover, the seeds can remain viable from 3 days and up to 3 weeks, which gives a larger window for successfully reproducing. Another reproductive method that they use may be considered alien-like, as they use what is called a “sucker” to self-propagate. This is done through the roots, which are able to send out underground shoots and suckers. Suckers produce trees genetically identical to the parent plant. These are typically called “clones”, which can spread and cover several hectares.

               


Aspens are a paramount species in many parts of Western Canada. They play a significant role in forest dynamics and possess a strong top-down influence on the other organisms that can be found in an ecosystem. In older aspen stands, the trees are able to have an impact on the light, temperature, wind, and structure composition in the lower layers of the forest. If you walk around the Marl Pond Trail, try to take note of the different climates that you can sense when you walk through. Personally, I find it awesome and interesting how different tree composition in a forest can so significantly change the amount of sunrays and wind that can penetrate and, ultimately, change the temperature. Trembling aspen can also contribute to the deadwood and litter present in an ecosystem, which provide many habitats for a myriad of organisms. In seasons where there is food shortage, some ungulates will feed on the aspen bark, which can leave extensive scars on the lower trunks (Johnson et al., 1995). Even more, aspen is also the preferred food of beavers and many ungulates will feed on the softer and younger twigs and leaves of the aspen. Certainly in Wagner Natural Area, the beavers have reportedly used aspen to build their beaver lodges and stock their winter caches.



Figure 2: Aspen trees in the fall, when the leaves start to turn golden-yellow.

Photo credit: Natural Resources Canada. 

https://tidcf.nrcan.gc.ca/en/trees/factsheet/58

 

While conducting monitoring around the natural area, we have also noticed that many of the aspen have white trunk rot (Phellinus tremulae). White trunk rot causes damage in the heartwood of aspens which can drastically decrease the structural integrity of the tree, making it much more prone to blowdown. Some areas within Wagner definitely support this, as there are many more trees that have fallen where white trunk rot is detectable. This is very common for aspens, and can be identified by the outward signs of fruiting bodies of the fungus, otherwise known as conks, found on the tree. By the time that the conks are noticeable on the trunk of the aspen, the rot will have been completely established. These trees, which will ultimately die, will be excellent habitat for many beetles and other insects to inhabit. I know that these are the areas that I will be watching throughout the summer season in hopes of spotting birds! Pileated woodpeckers and yellow-bellied sapsuckers, and any other bird species that love to eat insects hidden within the bark, will definitely be around these rotting aspens.  


References:

Johnson, D., Kershaw, L., MacKinnon, A., Pojar, J. (1995) Plants of the Western Forest: Boreal and Aspen Parkland. Edmonton, Alberta: Lone Pine Publishing.