Snowpack in the West is facing a historic drought, and new research from the University of Washington shows that forest thinning with modern tools cannot only reduce wildfire risk, but it can also increase the snowpack in winter by up to 30%. The researchers suggest this could help recover lost water and safeguard future water supply.
Forest thinning involves shredding and mulching small trees, shrubs and brush — vegetation that is least resistant to fire. Creating more gaps between trees opens up sections of the forest floor that are shaded by the remaining forest. Snow that’s on shaded ground faces less sun exposure, preserving snowpack more efficiently than when the snow is caught by trees.
Cassie Lumbrazo, a research scientist at the University of Washington and the University of Alaska Southeast, joins us to explain the wide range of benefits these thinning methods can have on forest ecosystems and the impact these findings could have during a record-low year for snowpack in our region.
Note: The following transcript was transcribed digitally and validated for accuracy, readability and formatting by an OPB volunteer.
Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. The American West has seen record low snowpack this winter. Sadly, this could be a taste of what’s to come. Snowpack is expected to decline because of climate change, but a new study offers at least a glimmer of hope. Scientists at the University of Washington found that forest thinning can actually preserve snowpack and protect water supply. Cassie Lumbrazo is now a researcher at the University of Alaska Southeast. She joins us now. Welcome back to Think Out Loud.
Cassie Lumbrazo: Hi, thanks so much for having me.
Miller: I want to start with what’s at stake here in the big picture. Why is snowpack so crucial in the West?
Lumbrazo: Some people are aware, but really the American West depends on seasonal snowpack for water resources, for drinking water, for agriculture, and even recreational purposes. And our seasonal snowpack basically stores all of our water that we’re gonna need through the summer and releases it in a timely manner for us out in the West. So when it doesn’t rain between June and maybe all the way until October, the seasonal snowpack is slowly melting, releasing water into the rivers for wildlife and protecting a lot of the landscape for us. So it’s a pretty unique resource.
Miller: A kind of aquatic battery when there’s not precipitation. So what are the projections for what’s going to happen with snowpack in parts of the West, including in Eastern Oregon, in the coming years?
Lumbrazo: Yeah, this study was focused on in the Eastern Cascades of Washington, but it’s very similar in the eastern part of Oregon as well, in the eastern side of the West. In the coming years it is projected that the seasonal snowpack will start to decrease in the amount. It’s not just the amount though, it could decrease in the amount of days that snow is on the ground in total. So we could have the snowpack actually melting out sooner, and this could mean that later when we need water, during the fire season in September, October, then maybe that water that should have melted out later is not there.
Miller: What led you to study the connection between forestry practices, how much and where, chopping down trees happens, and snowpack?
Lumbrazo: This is really interesting. I know there are so many groups that are interested in managing the forest for a lot of different reasons, and one of the main reasons for managing the forest is for wildfires. There’s groups that are managing it to reduce the amount of fuels in the forest, and people manage for wildlife as well. They want to make sure there’s habitat for wildlife, and we also want to manage them for water resources to protect the snowpack and make sure that we can keep snow on the ground as long as possible.
So this conversation with the forest management, with wildfires, is that these forests are already being managed for wildfires. There’s a lot of resources allocated to do some forest management practices to reduce the risk of wildfires. And what we wanted to do was see if these forest management practices that are already happening, see how they impact the snowpack on the ground and see if it’s a positive or negative impact, basically looking at this directional shift for the climate.
Miller: How different were the tree cutting regimens on the different patches of land that you looked at? I guess I’m just wondering how you went about doing this research.
Lumbrazo: Yeah, I can’t take credit for all of that. There was a huge, huge effort for many years, there’s a forest collective in the Yakima River Basin that worked on this, and the Nature Conservancy actually prepared these experimental forest treatments in this region of the Eastern Cascades as a way to represent kind of the full distribution of forest treatments that could happen in the Eastern Cascades.
And they kind of curated this experimental forest for us. They’re using it for other purposes as well, but they let us come in and study the snow on the ground, so there really was this wide distribution of all of the possible forest management practices that would happen in Eastern Washington. Everything from thinning at different levels to doing different controlled burns. This was a huge effort and a lot of actually individually contracted loggers out there doing the management. So yeah, a complicated experimental forest.
Miller: And then I imagine a lot of different variables. So in the big picture, what can you tell us about what you learned and how big a difference forestry practices make in the eventual snowpack?
Lumbrazo: Kind of the best news of all of this is that we really found that these forest management practices, over the full distribution of them, they had a positive impact on snow storage. And really we were kind of worried that this was gonna be a huge negative impact on snow storage, but actually the eastern side of the Cascades in this region historically, there hasn’t been as much mitigation as there usually has been in the past, and the forests have really regrown very dense.
So the forest is kind of being restored to a more natural condition through these thinning practices, and we found that overall there was a positive impact on snow storage, and then you can get into the weeds and really see, we dove in to see which level of thinning would have the largest or smallest impact on snow storage. And really what we found is that opening gaps in the forest canopy in this area, so not just thinning in like this uniform way over an area, but really opening larger gaps between the forest is the best way to retain the snowpack for this region.
Miller: Just to be clear, does forest management have any bearing on how much precipitation there is or just how much of that precipitation ends up as snow on the ground?
Lumbrazo: Yeah, a lot of the forest management, primarily this type of forest management, was focused on wildfire, reducing the risk of wildfire and reducing fuel availability. So they weren’t originally thinking about the snowpack on the ground. And that’s kind of where we want to start bridging the gap between the hydrologists and the forest managers and the wildfire managers and kind of have this conversation going of, if we’re managing the forest for just wildfires, can we also manage it to maintain the snowpack?
Miller: But what are the reasons that chopping down some number of trees, it opens up a little bit more canopy in some places? Why would that lead to more snowpack?
Lumbrazo: Yeah, that’s a great question. Sometimes it’s a bit counterintuitive, but the first thing you can think about is that the tree canopy is very widespread and when it’s snowing, it can actually capture the snow that’s falling and it prevents that snow from reaching the ground. And so that’s one process, called snow interception. But there’s other processes that happen, like the trees can actually provide shade to snow on the ground during very high, sunny days. So another thing is that the trees actually can heat up in the sun and it can melt the snow around them.
So you have multiple physical processes happening at once, and the point is that for different regions and different climates, there might be one physical process that’s dominating. And that’s kind of where the study fits into this local area of the eastern side of the Cascades, is us finding which one of these processes is dominating and what we found is that there’s a lot of snow interception, so we need to open some space. We need to open some space so the snow can hit the ground, but we don’t want to open too big of spaces, because we know the eastern side gets some sunny days in the spring, and we need the trees to actually shade those gaps where the snow fell, and so we need to make sure that the gaps don’t get too big that the trees don’t shade them anymore.
Miller: So it’s the kind of Goldilocks of tree management. Enough of a gap in the canopy that snow can fall, but not so much that too much sun hits it, making it melt too quickly.
Lumbrazo: Yeah, exactly, that’s it.
Miller: But what’s the problem of having snowfall on top of the canopy? I mean, might that just melt like snowpack on the ground and then lead to, I don’t know, kind of an aerial snowpack? What’s the problem with that?
Lumbrazo: There’s a lot of things happening. When the snow hits the canopy, the one option is that, if you’re ever walking the forest and you just feel like all of the snow melting and dripping on you from the canopy, so really this can happen is that the water can drip onto the ground below and actually be absorbed by the snowpack, and that water is still gained by the snowpack below.
But another option is that it can go through a process of sublimation where the solid water and the snow on the canopy actually goes directly back to the atmosphere. And that means that that water is lost to the watershed going back to the atmosphere. So really that’s one large possibility, especially in very dry areas like the eastern side of the Cascades.
Miller: One of the things that I’ve taken from what you’re talking about is just how complicated this is. It can vary by topography, by whether it’s a west or east or a south facing slope versus a northern one. Given just the complexities of this, do you think that you and other teams will be able to give really concrete advice to forestry managers that they can use wherever they are?
Lumbrazo: Yeah, this is a great point, and it’s a really important part of the conversation. I think that comes next, is that there are a lot of these physical processes that are very regional specific and the best we can do is say that we have this information at this elevation, in this climate. And something we know is that as the climate is warming, we have climate change, a lot of our snowpack is going to be moving towards higher elevations, and so this lower elevation snowpack is gonna be more representative of higher elevations.
And we can extrapolate a bit to other areas and provide some suggestions, but another important piece of the study is just showing that if other places are managing for wildfires, they should also take a look at what that’s doing to the snowpack in their region. So I think that we can provide some suggestions immediately, like this fall, for forest managers in the eastern side of the Cascades based on this study, and then we can also say in other regions where you’re managing, hey, replicate some piece of this and see what’s happening in your region.
Miller: The research that eventually was crunched and turned into this study, it was done a number of years ago at this point, but only released now at a time of record low snowpack. I’m just curious what that timing has meant to you.
Lumbrazo: Yeah, it actually means a lot. That’s such a good point. This research was done during my PhD a few years ago, and it took a while to really come to this final story and get this work published and it’s more important than ever, it feels. I think the conversation about water resources in the West is something that’s been happening ever since I lived out west, but for over 10 years. But it’s particularly important this year. People are looking at the snowpack out west and thinking, oh my gosh, people trying to ski or just seeing how low reservoirs are. So it’s kind of this nice moment where we can look back on this work and say it’s still important or more important than ever.
Miller: Cassie, thanks very much.
Lumbrazo: Yeah, thank you so much for having me.
Miller: Cassie Lumbrazo is a research scientist at the University of Alaska Southeast. She joined us to talk about the connections between forest thinning or forest management and snowpack.
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