Alnus rubra – Flora around Nanaimo

Family: Betulaceae

Common name: Red alder

E-flora BC: https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Alnus%20rubra

Wikipedia: https://en.wikipedia.org/wiki/Alnus_rubra

Red alder is found in moist places, along streams, lake shores, and low lying areas of the landscape where water collects. It’s also a ‘pioneer species’, meaning that it’s often the first one to colonize a recently cleared area, e.g. after a landslide or forest fire.

Red alder is significant in that it engages in a symbiotic relationships with nitrogen-fixing actinomycete bacteria of the genus Frankia. The bacteria take up residence in the root system of red alder. The tree provides their bacterial symbiont with a protected habitat and carbohydrates for food, while the bacteria provide their host tree with nitrogen in the form of ammonia. Useful forms of nitrogen (ammonia and nitrate) are typically in short supply in the soil, so this symbiotic relationship gives red alder an adaptive advantage. Eventually, the nitrogen fixed by Frankia and used by the red alder dissipates to the rest of the ecosystem. While most trees are careful about breaking down proteins and retrieving all the precious amino acids and other nitrogen-containing compounds from their leaves before dropping them in the autumn, red alder can afford to drop its leaves while they’re still green and rich in nitrogen-containing compounds. As the alder leaves are degraded by microbes and fungi in the soil, ammonia and nitrates are released and made available to other plants in the habitat. In that sense, red alder (with assistance from Frankia) helps the growth of other plants around it.

Root nodules on a related alder species (*Alnus incana*). Symbiotic actinomycetes, *Frankia*, reside in these nodules. *Alnus rubra*, red alder, makes similar root nodules. Photo credit: Jesse Bellemere via Twitter

Red alder leaves, dropped while still green in the fall. Note the bigleaf maple (*Acer macrophyllum*) leaves which were yellow-brown when dropped, reflecting the fact that proteins have been degraded and nitrogen-rich molecules retracted into the trunk of the bigleaf maple tree for use again next year.

While walking through the woods, pale grey trunks with whitish splotches of lichen growth alerts you to the presence of red alder. Photo credit: Staffan Lindgren

A close up of a red alder trunk, showing the light grey color of the bark, and the white patches of lichens growing on the surface. The short horizontal raised marks on the bark are called ‘lenticels’. Lenticels are porous areas of the bark, allowing oxygen to diffuse into, and carbon dioxide to diffuse out of the living, respiring tissue under the bark. Photo credit: Staffan Lindgren

Why is the species called ‘red’ alder? The name comes from the color of the freshly cut wood, as shown in the photo below. The red pigment is especially intense in the inner bark, and has traditional uses by First Nations people in our region to, e.g., dye red cedar fibers used for weaving. Alder wood is considered one of the best for smoking salmon.

The freshly cut wood of red alder is, as the name suggests, quite red. The colour pales as the wood dries and ages. Photo credit: Willamette Biology via Flickr

The foliage of red alder appears quite shiny and dark green. Photo credit: Staffan Lindgren

Twig of red alder, note the alternate arrangement of leaves along the stem. Photo credit: Staffan Lindgren

Note the characteristic venation of the red alder leaf, as well as the toothed edge. Notice that the edge is wavy, and also finely toothed. If you turned the leaf over, you’d notice that the edge looked as it had been very carefully rolled under. Photo credit: Staffan Lindgren

Red alder is a flowering plant, but its flowers are not the typical shape that we tend to think. The cone in the photo below is an old female inflorescence, a cylindrical spike of many female flowers. And the longer hanging spike is the male inflorescence.