What Lies Beneath (the Red Alder Trees)

Last month we had a massive windstorm, strong enough be be dubbed a "bomb cyclone." Since then, at each of our favorite hiking spots we've noticed many newly fallen trees. It's hard not to feel a pang for the downed trees, especially the grand old conifers. Sigh. Fodder for the fungi and slime molds, I guess. 

At Cougar Mountain recently, we saw some freshly knocked down red alder trees (Alnus rubra). Perhaps unfairly, fallen red alders feel less tragic, as they are pretty short-lived for a tree. Most that reach maturity will only live 60 or 70 years. They are lovely trees, though, with their pale, lichen-covered bark, their pinkish catkins in spring, or their yellow leaves in autumn.
Fallen red alder tree
As we looked at this fallen alder, I thought I spotted a slime mold or small fungi on the roots - clusters of something weird and tiny and round and orange. 
Red alder nitrogen-fixing bacteria
Definitely not slime mold - not squishy enough. Maybe a fungus? Hmm. Then it clicked! Red alder is a pioneering species, and can grow in impoverished soil. It manages that by hosting nitrogen-fixing bacteria in nodules on its roots. Plants can't use the abundant nitrogen in the air directly. However, some plants, most notably legumes like peas or beans but also alders, form a symbiotic relationship with bacteria. The bacteria infects the plant's roots and triggers the plant to grow nodules. Then, the bacteria synthesizes usable forms of ammonia from atmospheric nitrogen, which is shared with the plant. The bacteria get a nice place to live (inside the nodules), and the plant gets fertilizer. Different plants host different species of bacteria; red alders host a bacteria called Frankia alni. 

When the alder dies and decomposes, the ammonia compounds are released into the soil, and can be taken up by plants  that do not host their own bacteria (and other organisms such as fungi). Animals that eat the plants can also get the nitrogen they need to build proteins.
Red alder nitrogen-fixing bacteria
And so the red alders, and their pals Frankia alni, fertilize the forest for future generations. That's not their goal - they're just trying to make a living - but I'm grateful for their unintentional gift!
Location: GVHR+42 Bellevue, WA, USA

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