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Study: Thinning and burning help a forest survive drought

Melissa Sevigny
Firefighters monitor a 2018 prescribed burn on the Gus Pearson Natural Area north of Flagstaff.

A new study from Northern Arizona University shows thinning and regular prescribed burns can help ponderosa pine forests survive both drought and wildfire. The work took place at a thirty-year-old research site on the Fort Valley Experimental Forest north of Flagstaff. KNAU’s Melissa Sevigny spoke with ecologist Andrew Sánchez Meador about the findings.

Okay, so this experimental area has places that have been thinned, places that have been thinned and routinely burned, and then some places that have been left alone.

Yeah. And in this study, the initial thinning was followed by repeated burning every four years. In fact, we’re going to burn it this fall. That repeated burning, that frequent fire interval, is key to the function of this ecosystem.

What did you find out?

We actually found that both young and old trees responded to the thinning and grew really fast. That’s not a surprise with young trees. When you open up and make resources available, young trees can tend to thrive and jump on that. But what we found was trees 500 years old had dramatic increases in growth rates, suggesting that if we treat, both young and old trees can increase their vigor, they can be more resilient to drought, insect and disease, and what we also found was that if we introduce fire into this system, we don’t increase the mortality in that.

So the trees in the treated area are more likely to be resilient to the drought?

Yes. And to increased fire or being exposed to fire. Which is an equally important thing. Because you see a lot of messaging now advocating for putting fire back into these systems. People are scared of fire, rightfully so. But what we do know is, putting fire back into the system, seems to have only positive benefits in a frequent fire, naturally functioning system.

How do you hope the information from this study will help inform forest management in this region?

First and foremost, as we work to translate this science, I hope it makes the public feel better about seeing reduction in forest density. These forests we all live in here in Arizona, they are the only forests we’ve ever seen, so when we come here and someone tells us that these forests are unhealthy and too dense, that’s hard for us to understand. But these type of studies repeatedly show these forests are degraded and if we open them up we can increase vigor and resilience and make them healthy…And I think the other piece of this is that we know a lot about this. I often get the question about: we’re experiencing increasing severity of fire, and frequency of fire. The burn year is getting longer and longer and longer. What we can do to counteract it? Well, we know that thinning followed by the reintroduction of prescribed burning, that can counteract it. It took us 150 years to get into this mess, it’s going to take us a long time to get out of it, but we know that thinning and burning is a way out of it, we just need to be able to do it at a landscape scale in order to get ahead of these wildfires that we’re experiencing.

We’re on two decades of drought and we know that our ponderosa pines are struggling. After doing this research are you feeling more optimistic about the future of ponderosas, or are you still worried?

That’s a great question. Because I often say that if I look forward to the future climate… We know that we’re getting warmer, and the availability of precipitation is becoming less for plants and trees. If you were to manufacture or engineer a species of tree that is well suited for that, ponderosa pine would probably be at the top of the list of the thing that you created. It’s well suited for the future climate. What it is not well suited for is the current density and the way wildfires behave, and insects and disease behave. I’m very optimistic that if we can do more restoration at the landscape scale, if we can create more continuous parts of the landscape that are restored, we’re setting our ponderosa pines forest up to have the best chance they can have.

Andrew, thank you so much for speaking with me.

Thank you for your time and your interest.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.