What’s killing Colorado’s aspen trees?
GRAND JUNCTION, Colo. (KREX) — Thousands head to the Colorado high country as fall transforms aspen trees to shimmering gold and fiery orange groves of color, but this year is different. While experts predict the colors will be more intense than normal across vast expanses of mountainsides, you’ll also notice dull brown patches of dead aspen groves, victims of a warming climate and beetles.
Sudden Aspen Decline, or SAD, is the widespread rapid and severe deterioration in tree health. Researchers observed this decline across Colorado nearly two decades ago, and it hasn’t let up. SAD differs from regular aspen decline because SAD happens more rapidly.
Dr. Julie Korb, a professor of Biology at Fort Lewis College tells WesternSlopeNow signs of sudden aspen decline include an orange, almost iridescent, hue to the bark. A pathogen called cytospora canker is behind the decline, separating bark from the cambium and changing its color from white to brown or black, or causing crown loss.
“What that means with crown loss, are their branches where there are new leaves that can photosynthesize, to help sustain that aspen clone.”
One other sign Korb mentioned is seeping sap, noting the most likely cause is borer beetles and cytospora canker, both native to areas where aspens grow.
“There’s a poplar borer beetle and then there’s a bronze poplar borer beetle. Those two beetles will then go into the cambium of the tree. The tree then recognizes that as a foreign entity, and so it tries to remove it.”
The tree does this by secreting tree sap to push the beetles out and to fill the spots where they tried to bore in.
But when an aspen tree is already in a state of stress, it can’t produce sap very well – or at all – and is unable to defend itself from the beetles or patch up the holes they left. This leaves the aspen trees vulnerable to the cytospora pathogen as well.
“In other words, we always have those beetles present in the system. But it’s not until those trees are stressed that we actually then start to see that the trees can no longer fight off those attacks. And that’s when the trees start to die.”
The main factor contributing to SAD, however, Korb says, is environmental.
“What’s really inciting or causing the dieback is from acute drought with high temperatures during the growing season.”
These conditions put stress on the trees, causing them to shut down and stop photosynthesizing. When this happens, the trees are forced to live off of their reserves. Once these reserves are depleted, the tree cannot transport nutrients and water to the top of the tree, causing crown loss. If this continues to happen, eventually the tree will see 100% crown loss and die.
“If we weren’t having drought and warm temperatures during the growing season, then sudden aspen decline wouldn’t even be a phenomenon.”
Another factor that determines a tree’s susceptibility to SAD is its geographical profile.
“Where we tend to see the sudden Aspen decline happening is on low elevations on south or southwest facing aspects, in areas where there’s low Aspen tree density, or these trees are physiologically at their maturity.”
Korb says while SAD remains prevalent, it’s not yet reached a critical danger point.
“I think it’s really going to be dependent upon what we see related to the climate. So if we have really dry winters, and then we have really hot, dry summers, and that keeps happening in succession, then that’s when we’re gonna have to start worrying about, you know, what’s the fate of our aspen.”
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