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How Hotter Weather, Drier Monsoon Seasons Affect Forests

By Mark Brodie
Published: Wednesday, November 25, 2020 - 1:14pm

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Bighorn Fire
Inciweb
The Bighorn Fire.

STEVE GOLDSTEIN: Wildfires have burned more than 950,000 acres across Arizona so far this year, according to the State Department of Forestry and Fire Management. That includes the Bush Fire and the Bighorn Fire, which both cracked the top 10 list of biggest wildfires in state history. Arizona has also seen lower than normal monsoon activity over the last two seasons, which our next guest says is a problem for those parts of Arizona's forests that need to recover from wildfire. Don Falk is a professor in the School of Natural Resources and the Environment at the UofA. He spoke with my co-host, Mark Brodie, who started the conversation by asking what it does to the forests when there's a monsoon season like we've had for the past two years, which was not much of a factor.

DON FALK: The North American monsoon really regulates the fire season here in our part of Arizona and New Mexico and northern Mexico. And the way it works is that when the monsoon moisture and rains arrive, that really shuts down the active fire season — the season during which fires can really spread and cover large landscapes. So as long as the monsoon arrives, our fire season is pretty much over by the early part of July. But the monsoon is not the same every year. And we certainly have had years, this year and last year in particular, where the monsoon was weak or delayed. And the result of that is that fires can burn longer into the summer, and we end up with much larger areas burning in those weak monsoon years.

MARK BRODIE: So I want to ask you about what happens after wildfires, because, you know, the thinking is that the trees burn and other brush and plants burn and then the forest is regenerated. But I'm wondering if the monsoon doesn't come, if there's no rain, if there's no water coming, do the forests still regenerate like that?

don falk
Don Falk
Don Falk is a professor in the School of Natural Resources and the Environment at the University of Arizona.

FALK: I'm a forest ecologist, and so I'm really interested in what happens after the fire, what happens when all the TV crews have gone home. That's when the forest starts to recover. And we're accustomed now, I think, in media presentation of big wildfires, just with the language that's used — they'll talk about areas being scorched and devastated. And that is not necessarily the legacy of even a large wildfire like the ones we had this year, the Bighorn Fire in the Catalinas. These leave a very complex mosaic of some heavily burned patches, but some areas that are inside the perimeter of the fire that really didn't see anything at all. You would walk out there the day after and think, "Was there actually a fire? I don't see it." And that is really the key to post-fire recovery, because those areas that were severely burned, they're going to get their seeds and the other material they need to recover from the adjacent areas that didn't burn so severely. That's the natural process of recovery. And provided that the severely burned areas aren't too huge, then that material is going to find its way and the process of recovery will begin.

BRODIE: Given what we've been hearing about and what scientists have been warning about in terms of climate change and increased fires and what we've been talking about in terms of a lack of precipitation to help tamp down some of these fires, when the forests start to regrow, are you seeing that the same types of trees and plants are growing, or are you seeing different types of maybe more differently adapted types of trees and plants starting to grow in some of those burned areas?

FALK: There's no question that our changing climate, which we are driving by our own carbon emissions, alters not only how big the fires get and how long they can burn, but also the process of recovery. For example, this year we had a big fire in the Catalina Mountains outside of Tucson, but we had no monsoon. And I've been up on the mountain several times looking for seedlings, which ordinarily would be present in the summer. They'd be running on monsoon moisture. But there was no monsoon, and there is essentially no seedling crops. So there is really no recovery happening. Now, that's not entirely bad because no rain also means no big post-fire flooding, which is a really serious problem. But it's definitely true that as we go into this increasingly hot, dry period of climate change, that that is a very challenging climate for tree seedlings, especially. If you're a seedling and you have just germinated in the spring, you need that monsoon moisture to survive. And if it doesn't come, that seedling is going to die, and it will have to wait till next year. So a succession of years with a missing monsoon could easily tip the balance away from the kinds of trees that we're accustomed to seeing and towards more drought tolerant shrubs and grasses which may actually come in and take the place of what used to be forest.

BRODIE: Is there a danger if a seedling has to wait a year? Like, does that affect how it grows or how well a tree might grow if it doesn't start to regrow right away?

FALK: For most first-year tree seedlings, if they don't get monsoon rain, they'll all be dead by the fall. And then that has to start — that process has to start all over again with new seeds the following spring. It's only when small trees and saplings get to be maybe five to 10 years old that they could possibly tough it out during a bad monsoon.

Bighorn Fire scar
InciWeb
Scar from the Bighorn Fire along Meadow Trail on June 29, 2020.

BRODIE: So do you think that it's possible that at some point, maybe several generations down the road, that the forests of the Southwest will not be comprised of, for example, pine trees, but something more desert-adapted or more drought-tolerant?

FALK: It's highly likely that the vegetation of our part of Arizona is going to change, but I don't think it's going to take so many generations. In some areas, it can happen very, very abruptly. And this is where climate and fires have a way of interacting. You can have kind of the, the tightening ratchet of climate just making things more and more difficult for trees. And they get drought-stressed and they may be attacked by insects. And that causes a change that's expressed over many, many decades. But then a large wildfire comes into the picture and changes things overnight. So when you have climate stress plus a big wildfire, that's when you come back the next year and you may see that you've already passed a tipping point away from forest and toward other kinds of vegetation. Up in the forested areas, there are more drought-tolerant species that may be moving in. And so what we may find is that we'll see a turnover in what's living in our mountains away from the species that we used to enjoy and appreciate, like the conifers, pines and Douglas fir and those kinds of species toward native shrubs and grasses. Are they still native? Yes. But is it a forest? No. And is it the forest that we grew up and knew and appreciated before all this happened? No, it'll be a different place. And that may be a very difficult pill for us all to swallow.

BRODIE: All right, that is Don Falk. He's a professor in the School of Natural Resources and the Environment at the University of Arizona. Don, thanks for your time. I appreciate it.

FALK: Happy to be here. 

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