Red Alders: Forest Succession and the Weed Tree that Makes it Possible

We had an arborist out to our house recently, because a hemlock tree nearly 80 feet tall and not a dozen feet from our house was clearly in the process of dying. Dead and dying trees are, of course, a critical part of forest ecosystems, both as habitat and providers of nutrients, but those dozen feet from our house are not a natural forest, and so that tree was a hazard that had to go.

It was located off the corner of our house where the most open part of the yard is. And so, there too was our garden — a couple raised beds that had produced a few papery heads of lettuce and some stubby carrots last year, along with radishes and rutabagas that never bulbed out, and peas that were nibbled by rabbits before we had a chance to see whether they would climb — we have some work to do on our soil and our fencing.

Also in that corner are the three young fruit trees we got from a local nursery and planted, and a fig tree that a neighbor dug up and gifted to us when they forgot to prune theirs for too many years — where one of the branches grew long and dropped down to touch the ground, it had set new roots. This reminds me of holly, but I decided to forgive the fig its similarities. Western redcedar is known to do this too; it’s no one’s fault that holly is the first tree I noticed this predilection in. I worried about the fruit trees though, and that the lettuce would remain papery even if we did improve our soil because, next to that dead or dying hemlock was also a red alder large enough to put most of the area in shade for much of the day.

And since the arborist was there anyway, we asked him about removing the alder too. He didn’t blink twice, calling it a weed tree and declaring that it had to go. This may not seem surprising, coming from someone whose living depends on homeowners wanting to remove trees, but I wonder: is it possible to get into a profession like being an arborist without also loving trees, or at least learning to respect them? He did seem to: there was a large madrone tree nearby that he praised, and some mushrooms on the ground (fall in the Northwest) that launched us into a discussion about fungal networks and their symbiotic relationship with trees. He clearly noticed and found joy in the natural world.

And yet: alders as weed trees. They are prolific, I suppose, and have a tendency to die from the top, which makes them look nearly menacing at times, leafless clawing branches reaching to the sky above still-thriving lower limbs. Even Jonathan Drori, in his book Around the World in 80 Trees, a book written for the express purpose of extolling the various attributes of different trees and their impacts on human history, geography, and culture, starts the section on Alder trees by saying “On the surface, there is little to distinguish the alder.” High praise, indeed.

Distinguished or not, Alder trees are a critical part of an ecosystem. More specifically: a critical part of a specific type of ecosystem in a specific phase of its lifecycle. Nitrogen is essential to the growth of trees and anything leafy green. It is absorbed from soil through the roots. But while nitrogen makes up 78% of the earth’s atmosphere, it can’t be used directly by trees or plants in this form; trees can only take it up from the soil, where it cannot go on its own — it’s one of the most common fertilizers gardeners apply.

In mature forests in the Pacific Northwest, most of the nitrogen is captured into the soil via a lichen called lobaria (all lichens are actually a symbiosis between two distinct organisms, an algae that can photosynthesize sharing resources with a fungi that cannot, a fact that delights me), which is able to extract nitrogen from the air. Then, when lobaria falls from the tree branches where it spends most of its life and decomposes, the nitrogen it absorbed through its life becomes available to trees in the soil, sharing this superpower with everything around it.

However, lobaria aren’t found just anywhere there are trees. It doesn’t appear at all until a stand reaches about one hundred years of age, and doesn’t become abundant for up to another one hundred years after that. So where does the nitrogen that these trees need to become hundreds of years old come from?

Enter alder trees. Alder trees have a symbiotic relationship with a bacteria called Frankia alni, which is found on root nodules of alder trees and fixes nitrogen back into the soil. This makes alder trees a perfect early-successional species.

When an area is disturbed, whether by logging, wildfire, landslide, clearing for development or road-building, unless it is paved or a structure built on top of it, something will grow back (and even when it is paved or a structure put on top, something will eventually grow through or around, it just stretches the time scale a bit). Land typically goes through stages as this growing back occurs, called successions: grasses and flowers give way to shrubs, which give way to deciduous trees, and then to conifers. These will eventually become what people call “old growth.” This process of succession takes hundreds of years and the phases are not sharply delineated — plants from different phases can coexist and land will be set back to the beginning if at any point disturbance comes again. As each of these layers dies off, its nutrients return to the soil, laying the literal groundwork for the next, building up the soil so that organisms with greater requirements can find what they need.

Alders, in their hundred or so years of dominance early in this process, can leave enough nitrogen in the soil to support the re-growth of the conifers that will eventually (eventually) become the old growth that people like to rhapsodize about, until the lichens can take over their nitrogen-producing work. In order for that to happen of course, the alders must not be treated as weeds and summarily removed, they must be allowed to grow and support their bacterial pairs, and die in their own time. The Washington State University extension office, in an article singing the nitrogen-fixing and carbon-sequestering praises of the red alder, even wrote a pithy statement to this effect: “What was once considered a weed may end up being just what we need.” Even this makes alders sound like a means to an end, not something that might itself have value.

It says nothing of the bug and bird species an ecosystem full of alders can support in the meantime, their snags serving as perfect feeding ground and nesting cavities for several species of woodpecker and other birds (that predilection towards dying from the top not entirely a bad thing if you are a woodpecker). Red alders are part of a whole from which we don’t get to pick and choose and still maintain the whole.


Dalan, Garrett, Alder: A Weed, or Just the Tree We Need? (Washington State University Extension, Washington State Department of Natural Resources)

Drori, Jonathan, Around the World in 80 Trees (Laurence King Publishing, 2018)

Luoma, Jon R. The Hidden Forest: The Biography of an Ecosystem (New York: Henry Holt and Company, 1999)