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Study: Thinning Arizona’s forests will save trees in a warming climate

A pile of cut trees lie in a ponderosa pine forest
Melissa Sevigny
Tree thinning underway in a ponderosa pine forest south of Flagstaff

A new study from The Nature Conservancy says thinning ponderosa pines can save trees in the long run. The research modeled what will happen to northern Arizona forests as the climate grows hotter and drier. KNAU’s Melissa Sevigny spoke with lead author Lisa McCauley about how the results relate to the nation’s largest restoration project, the Four Forest Restoration Initiative (4FRI).

So this seems counterintuitive, that cutting some trees down now will save some trees in the future, can you tell me more about what’s going on there?

Yeah, it’s kind of a tradeoff. What we would do with the prescriptions that would be implemented, are thinning, mechanical thinning, taking out the trees, follow those up prescribed burns. What you’re doing is taking out the small trees on the landscape and leaving some of the bigger trees on the landscape. But if we don’t do that, everything is likely to burn up in a catastrophic wildfire, and we’ve shown could also die from future droughts. … One of the biggest factors of when tree die is when there’s lots of other trees on the landscape, there’s a lot of competition for water…. We found that restoration projects like 4FRI can reduce the densities of those trees and make it more likely that the competition allows for more survival of trees—fewer trees are dying.

Tell me about your results, how significant is this effect?

We found that, in general, 4FRI would have a 25% reduction in drought mortality into the future. Those benefits are higher midcentury, 2050-ish, in the next twenty or thirty years. Those benefits a little bit lower when it gets hotter and drier toward the end of century. But overall it equates to about a 25% reduction in tree mortality with climate change because of the prescriptions that 4FRI is implementing.

In other words, as we look at possible climate futures, there could be futures where it’s so hot and so dry we’re definitely going to lose some ponderosa pines, but thinning does help reduce that loss.

It does, and that benefit increases the more trees we thin.

Given all of these positive benefits, can you talk a bit about some of the barriers to doing more of these thinning and restoration projects?

There’s not a lot of economic incentive. These trees that we’re taking out of here are small, they’re not worth a lot. It’s not like thinning or clearcutting large forests that you used to see back in the eighties, you’re not getting a lot of money out of the trees that are on here…. We need to train workforces, and incentivize private industry to buy some of the equipment. … We’re hopeful that some of the money that’s coming from the Infrastructure Act, which being used to implement fire reduction strategies, can help with some of the barriers of incenting private industry and things like that.

What’s the main thing you want people to know about this research?

We’re starting to get the idea across that forest restoration is beneficial to help reduce fires. You see fires every summer all over the news, big fires, fires getting bigger and bigger, burning more areas. I think we’re starting to get the picture across that forest restoration can help with those benefits. I think what we’re trying to do is make the case that there are other co-benefits we can get from forest restoration, that we’re not just reducing the risk of wildfire. These forests will be healthier, we’re going to sequester more carbon in these forests despite the fact we’re thinning out a bunch of trees. We’re going to reduce the risk that more of these trees will die from drought. It’s helping us make the case that forest restoration is important and that it’s something that we need to be doing into the future.

Lisa McCauley, thanks so much for speaking with me.

Thank you.

See The Nature Conservancy's research on forest thinning here.

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.