Northern Arizona is famous for its aspen trees, and a multi-decade study looks into how they’re being affected by climate change. Among the findings: There are a good number of smaller, younger aspens and a good number of older ones, but not all that much in the middle.
Mike Stoddard, a forest ecologist with the Ecological Restoration Institute at Northern Arizona University, joined The Show to discuss, starting with the fact that there’s this kind of missing middle among the aspens.
Full conversation
MIKE STODDARD: The great observation, that was one of the kind of like the surprising findings that we found in this study. Aspen are known for their resiliency to disturbance, and they're often described as a disturbance loving species. But not all disturbances are equal. Aspen respond well to some types of disturbance like fire, which can clear the way for new growth. However, other disturbances like drought may stress the trees without the benefit of renewal.
And so what we're seeing is with this prolonged drought and these other disturbances, we're getting a lot of aspen regeneration. But something's keeping them from growing up into tree status.
MARK BRODIE: Well, what are the implications of that if those younger sort of smaller saplings aren't reaching maturity?
STODDARD: Well, without a balanced age structure or size structure, the force lacks resiliency. Mature trees can provide stability, but they won't last forever. And if there are not enough younger trees to provide that replacement over time, then we've got a sort of a long term problem in terms of like renewing these aspen forests.
BRODIE: So you mentioned that aspens generally don't necessarily mind disturbances, but not all disturbances are created equally. Is this just one disturbance that's causing this, or is this more of like a perfect storm of disturbances that are sort of conspiring together to cause these problems?
STODDARD: I think you hit the nail on the button there. There's a lot of things kind of interacting together. In the 2000s, we experienced probably one of the most extreme droughts in the Southwest in modern history. And then with the lack of fire to stimulate that regenerating growth for aspen, so you don't have fire, you have extreme drought, and then you have you have grazing. So you have an abundance of grazers like elk and deer that are restricting those new aspen suckers is what they're called into reaching tree size.
BRODIE: Well, so what can you do about any of those? I mean, obviously there's not much humans can do about drought, but the fire and and the wildlife, I would imagine that there is something that can be done, right?
STODDARD: I think one of the crucial elements that can be done is just having adaptive strategies. So, in this particular unique setting, it's a wilderness setting. So there's not a whole lot you can do in a wilderness, but you can manage fire, you can manage herbivores outside of the wilderness. And so when you do have disturbances, you can basically adapt to the aspen needs following these disturbances.
For instance, we just had a fire in 2022 called the Pipeline Fire. Super devastating to the community of Flagstaff and the peaks. But one of the silver lights following this fire is the recruits that are coming, aspen recruits that are coming following the fire. We already have 6 to 8 feet tall aspen stems. And likely a lot of that has to do, you know, having this fire that stimulates new growth, and then past management of elk and deer.
BRODIE: One of the factors we haven't really discussed is climate change, and I wonder what role that might be playing specifically again looking through some of the research, something called oyster scale, an insect that kind of sucks up tree sap that maybe is having more of a run on these trees than it has in the past.
STODDARD: Sure, so we're lucky on the San Francisco Peaks. We haven't observed oyster scale yet. They seem to not like elevations above 9,000 feet, and I think a lot of it has to do with the cold. It's still cold enough to deter them from living beyond their normal life cycles.
But in the elevations below, and definitely on these more extreme southern aspects, they're being inundated with these oyster scales. I guess you call them a pest, but they're native. They've been around. They just, with climate change, as things become warmer. They're hanging on longer, and so they're having a more prolonged effect on aspen and the recruits.
BRODIE: How optimistic are you or what are the chances that aspens will be able to adapt to these disturbances, maybe as they have adapted to other kinds that maybe they're more resilient toward.
STODDARD: You know, I'm, I'm one of the, I'm one of the people that love to consider the glass is definitely half full. I feel like these ecosystems are a lot more resilient than we give them credit for. I believe in the last 20 years, management has definitely come together as a cooperative unity for solutions, working together, finding solutions, adapting, I think is the way that we're gonna move forward and it's gonna definitely help these forest ecosystems.
BRODIE: Well, so when you mention the word management, that kind of raises the question that, you know, these ecosystems are resilient, but do they need human intervention to be as resilient as they need to be or can they do it on their own?
STODDARD: Well, that's a great question, a loaded question that is. So just taking the example of the pipeline fire, looking at the response that we're seeing. Yes, it seems like aspen are doing what they do best following disturbance without human intervention. How long that lasts, how long it's just gonna be time and a lot of monitoring to see how these respond.
