Can scientists predict how forests will fare under increasing threats?

In the absence of crystal balls and time machines, scientists use natural records like ice cores to understand what happened on the planet before we arrived, and math and computers to predict whether humans can survive the changes ahead. It’s not a perfect approach, but it’s the best available outside of sci-fi films and fairy tales.

In Arizona, the science ecosystem change has a lot to do with trees.

Donald Falk is an associate professor with the University of Arizona’s Laboratory of Tree-Ring Research. He uses trees and forests as a lens to study how southwestern ecosystems respond — in the past, present and future — to challenges such as the rising average temperatures, worsening drought and more intense wildfires linked to climate change.

In March, Falk and colleagues published a review paper in the journal Forest Ecology and Management titled “Mechanisms of forest resilience.” It is a comprehensive academic look at the ecological processes behind what, in the Southwest, is plain to see: The forests are dry, brittle and burning at unprecedented rates. Sometimes they don’t come back as forests.

In 2020, more than 32 million people visited Arizona, even as the pandemic put a sizable dent in tourism. As any nature-loving Arizonan already knows, many of the state’s most popular destinations are forested: the north rim of the Grand Canyon, Sedona, Flagstaff, the Sky Islands and the White Mountains.

The Arizona Republic spoke to Falk about whether Arizona will still have Douglas firs and Ponderosa pines to visit in 2122, how resilient the state’s forests might be to increasing threats and what people can do to protect these beloved landscapes. He conjured up his tree ring time machine and his Rip Van Winkle crystal ball to give his best predictions.

Dr. Falk, you study the history of wildfire in the Southwest to better understand its future. Can you explain how that works?

I’ve been studying wildfire in all its aspects in the Southwest since the late 1990s. My Ph.D. was on fire history in New Mexico. You can think of what I do in a past, present, future framework. We study the past mostly with tree rings. In the present time, we study how forests recover following wildfire and we watch how ecosystems are resilient. And then, on the future side, I’m part of a group that does simulation modeling. We try to wind the clock forward and ask, “What’s the Southwest going to look like in 100 years?”

The past has a lot to teach us about how ecosystems are resilient, because we have records going back many centuries all over the Southwest and they show us that wildfires, historically, were something the forest was very well adapted to. We have individual trees that have 35 fire scars on them, meaning that fire passed by that tree at least 35 times and left an imprint in the growth of the tree but clearly didn’t kill it. That tells us something very important about fires that were happening in the past – they were frequent, but generally not the kind of burn-down-the-forest conflagration that we’re seeing today.

How have you seen wildfire change Arizona landscapes?

After the year 2000, it’s like a switch was flipped. Wildfires are 10 times as large. We are seeing ecosystems change, and I would say that the pace of change has really picked up. We’re seeing a lot of mortality of trees, from wildfire and just because it’s getting too hot and dry for the kinds of forests that we used to have to survive.

But nature abhors a vacuum. So, something else is going to come in and take its place.

We see oaks replacing pines, we see shrubs replacing trees, sometimes we see grasses replacing everything. Many of these grasses are invasive, brought in by humans, and are what we call pyrophilic, or fire-loving. You’re not going to burn them out, and it’s tough to push back that tide. So the landscape is converting to different kinds of vegetation. And once those changes are in place, it’s very hard to turn around and go back to where we started.

What have been the major drivers of these changes?

I think it’s reasonable to think of the combination of climate, wildfire and insect pests as being the immediate drivers. But clearly, the most important thing that happened was the onset of the multi-decadal drought and associated high temperatures, and that’s not showing much sign of letting up.

Droughts do occur periodically in North America and all over the world, driven by background climate variability. But the intensity of this drought and the duration right now is making it exceptional. It’s one of the worst droughts in the last 2,000 years.

Now we could step back from that, of course, and ask, “Well, what’s driving those drivers?” The signs are that human alteration of global climate is driving the climate part of the equation. And one of the direct effects you get from that is these longer, deeper, hotter droughts. Our attempts, for more than a century, to keep fire out of ecosystems that need fire has also contributed to the severity of wildfires. And then insects are, in a way, responding to both of those because a dense forest with weakened trees is ideal habitat for many insects.

Unfortunately, we have only ourselves to blame for a lot of what we’re seeing now in terms of the destructive impacts of climate and wildfire on our forests in the Southwest.

In your recent review paper, you define resilience as consisting of three parts: persistence, recovery and reorganization.

Can you explain this process and why it matters?

Resilience is something everyone is thinking about. It’s our way of asking ourselves, “how are we going to get through this stressful period and are these things that matter to us still going to be there?” So we wanted to make sure we understood what we actually mean by resilience.

We looked at hundreds of studies and found an interesting pattern. The first thing a forest seems to do is try to persist, or ward off change. Healthy trees can tolerate a certain amount of stress. A big, old Douglas fir or Ponderosa pine, we might see in the pattern of the growth rings that some years were probably dry and hot and not optimal for growing. But the tree didn’t die, it simply grew less that year.

The second line of defense is called recovery and that’s not at the level of the individual tree but of the entire forest. Let’s say a fire comes through and kills an entire acre of trees that had been weakened by insect pests. Seeds from nearby trees find these open patches and become established and, over many decades, the forest recovers.

Persistence and recovery are what most people generally think of as resilience. But we also saw a third pattern, and this is what we call reorganization, that’s where ecosystems are different from human beings. If we go through an illness, we’re not going to turn into a different kind of organism, right? But ecosystems do that all the time.

In the face of the stress of climate and disturbance (as from fire), ecosystems will shuffle what species are in their community. This is not new, nor is it particularly a problem. Reorganization of ecosystems is a natural process.

But the climate that we’ve created by our own stupidity is completely unnatural. So what we’re seeing is the extinction of species, the loss of forest, the reduction in available water, the die-out of major groups. And that is not something that we as humans should be proud of.

What does all this mean for the future of Arizona’s forests and the people who live and recreate there, in terms of both short-term and long-term impacts?

We’ve been seeing short-term impacts for the last 20 years with these gigantic wildfires, including the largest fire ever in New Mexico that is burning right now. They’re affecting our economy, the quality of our air, the availability of water and the integrity of our soil. People are losing their houses, and their jobs and their livelihoods. Their health has been compromised. These events are not good for people, no matter how you look at it them.

In the longer run, we have pretty good reason to believe that species are going to rearrange where they’re living in the Southwest. Some will migrate to higher elevations or farther north. There are lots of species in Mexico, grasses and trees, that are well adapted to the climate we’re creating in the future, that will come in and take their place.

So, if you were to be Rip Van Winkle, go to sleep for 100 years, what would you see when you woke up in 2122? I would say that you would see a lot of areas that used to be forest that look now like grassy savannas that are heat tolerant, because it’s going to be considerably hotter and rainfall is going to be more erratic.

Is there anything Arizonans can do to help resist the loss or conversion of our forests?

The first thing we can do is to support the people who manage our public lands. They’re trying their best to keep our forests and grasslands resilient. How they do that is by forest thinning, prescribed burning and a variety of treatments for grasslands to keep them in healthy condition.

We can also make sure that we don’t have bad impacts on ecosystems ourselves. That means maybe not driving off-road and causing a huge scar that might take decades to recover. We’re past the point where we can tolerate the damage happening with ecosystems as it is.

Obviously, being very careful with fire is as critical as it ever was. And being very careful how we use water, because the wasteful use of water creates an additional stress on many ecosystems. We are just sucking water out of our ecosystems at an incredible rate. And a lot of it, honestly, is used more for human vanity than for any essential purpose. Conspicuous consumption of water in the desert is really unforgivable.

But beyond that, if you love forests, if you love streams and you love wildlife, then do everything in your power to reduce your carbon footprint. Don’t be a contributor to what’s making the problem worse. That involves some hard choices: not flying somewhere just for the weekend, maybe living in a smaller house, not driving a big gas-guzzler a quarter mile to the store for a quart of milk. There are a lot of changes we can make individually.

Then, of course, we need to make sure that we have people in Congress who understand the importance of climate change and the extreme urgency of addressing it. We can support ecosystems by supporting the movement to try not to make climate change worse. Every little bit we can do to reduce our emissions, it adds up, and it makes us more conscious citizens of the planet.

Joan Meiners is the Climate News and Storytelling Reporter at The Arizona Republic and azcentral. Before becoming a journalist, she completed a Ph.D. in Ecology. Follow Joan on Twitter at @beecycles or email her at joan.meiners@arizonarepublic.com.

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