It's Not Easy Being Green ... Especially in Fall

Willows nearing winter. Photo by Kim Elton

When I was running in Shevlin Park recently, I admired the yellowing larches and willows, the beautiful salmon color of the chokecherry leaves, the red stems on leafless red-osier dogwood, and the glorious and golden quaking aspens.

And then there were the alders.

All along Tumalo Creek (and just about every other waterway in Central Oregon), thinleaf or mountain alders (alnus tenuifolia) stand out for being completely out of step with the fashion of the day, their jagged-edged green leaves looking much the same in October as they do in April or May.

So what gives? Why don’t alder leaves change their leaf colors as other deciduous trees do?

First, let’s back up to understand why most leaves are green in the first place. The reason is they’re filled with chlorophyll, which absorbs light that the plant then converts to energy in the form of sugar.

Alder leaf. Photo by Ed Jensen

So why is chlorophyll green and not, say, a delicate mauve? It’s because chlorophyll absorbs light from the blue and red portions of the electromagnetic spectrum, but not from the green or greenish portions of the spectrum. It reflects that green light—and that’s what we see when we look at a leaf.

Okay, so now autumn comes along and most green leaves—but not those on alders—start to turn yellow or red or other colors. Again says the kid in all of us: But why? It’s basically because as the days shorten, deciduous trees need to prepare for the long, cold season ahead. Just like the chipmunks and squirrels scurrying around their bases to collect seeds, the trees need to “collect” nutrients to survive winter, and some of those nutrients are contained in their own leaves.

That’s why, after the leaves have done their job of photosynthesizing like mad all through summer, in autumn the trees pull reusable nutrients like chlorophyll (and proteins, nitrogen, phosphorus, iron, and sulphur) back into their trunks. Then, since those leaves are going to fall off soon, the trees also fill them up with unwanted substances like chlorine, silicon, and heavy metals. Yep, the trees use the leaves as a dumping ground for their own waste products!

Without chlorophyll, or with less of it, the leaves of fall are no longer green. Instead, they show off other previously concealed pigments, including red, orange, and yellow.

Which brings us back to those stubbornly green alder leaves.

Alders live along streambanks, which isn’t an easy thing to do because the soil isn’t very good there. The alders’ roots have special adaptations to deal with the constant wetness, and the roots are special in another way: They fix nitrogen. That means they take nitrogen and convert it, using bacteria nodules attached to their roots, into an organic form the tree can use. So, effectively, the tree fertilizes its own soil.

Alders along Tumalo Creek. Photo by Ed Jensen

Now, along comes fall in Shevlin Park, and all the other deciduous trees are sucking into their trunks the chlorophyll, nitrogen, and other nutrients from their leaves because they need the nutrients to survive winter. But the alder? It’s getting plenty of nitrogen from its roots, so it doesn’t need to spend energy pulling nutrients (including chlorophyll) from its leaves.

And that, dear friends, is why most alder leaves stay green right up until they fall to the ground in late autumn. Alders may not look great in your fall photos, but their adaptations are beautiful indeed.