Solutions to the receding Great Salt Lakehave been at the forefront of conversation in recent years. One proposed solution is strategically removing trees, in a process called “tree-thinning,” in order to allow more water runoff to get to the lake. While some studies provide evidence that landscape disturbances like tree-thinning can increase runoff, other studies suggest that tree-thinning has no effect, or may even be detrimental to water runoff. Recent discussion has led many to believe that this strategy would not be effective here in Utah and may end up costing the state astronomical amounts of money.
Sara Goeking works for the US Forest Service, monitoring forests across the nation. She received her Ph.D. in Watershed Sciences from Utah State University last year, where she focused on the responses of water to tree-thinning and other forest disturbances in the western United States.
Goeking’s recent paper on the impacts of forest disturbances on streamflow and snowpack came out in 2022. The study compared data from 159 watershed systems across the western U.S.
“I found that there were certain circumstances where snowpack and total streamflow were likely to decrease instead of increase after a forest disturbance,” said Goeking.
Goeking found certain characteristics associated with decreased streamflow, particularly as it relates to the concept of evapotranspiration. Evapotranspiration is the process of water loss through evaporation, and water loss to plants as they uptake water through their roots and release it into the air through their leaves.
Decreased streamflow was most likely to happen when not all canopy or vegetation was removed during thinning, and leftover vegetation soaked up more water than the trees that previously inhabited the location. Another characteristic that leads to decreases in streamflow is high solar radiation and aridity, where removal of trees allows for increased winds and sunlight, leading to more evaporation of moisture from the landscape.
For Utah, that second characteristic is particularly important as our climate is incredibly arid.
“In Utah, the evaporative demand is very high just about everywhere in the state. Some places it's higher than others. So, basically, wherever a forest canopy is lost, it's not necessarily true that what would have been transpired doesn't just evaporate from the soil or the snowpack anyway,” Goeking explained.
The reality of Goeking’s study was that tree-thinning’s effectiveness varies dramatically, given different conditions and ecosystems. Taking into account the climate and ecosystem is essential to using tree-thinning as a strategy for water management. The arid climate here in Utah makes for particularly high amounts of evapotranspiration, and that is something to consider in our water issues.
“We're constrained first and foremost by how much precipitation falls from the sky. We lose some of that to evapotranspiration. And what is left is our water supply, both for ecosystems for a Great Salt Lake and for our water supplies as people as well. So the evapotranspiration response to any forest change is what's going to determine how that forests change affects our water supply,” said Goeking.
While tree thinning can massively impact watershed systems, as a management process it is most commonly associated with wildfire management, and most research on the subject has been done in relation to wildfire. Furthermore, the research on thinning and wildfire management has mostly centered around ponderosa pine.
Larissa Yocom studies fire ecology as an assistant professor of wildland resources at Utah State University.
“Forest management and thinning for fire risk reduction has a really long and well-supported batch of science for particular forest types which we don't have here in Utah,” said Yocom.
Ponderosa pine have thick bark and drop their lower branches, leaving the trunk mostly bare. This allows surface fires to move along the ground without killing the trees. Here in Utah, most of our forests consist of mixed conifers. This includes different species, many of which do not drop their lower branches and don’t play that same role in wildfire management.
Not only is tree-thinning not necessarily a solution backed by scientific evidence, but the logistics are expensive and complicated given Utah's landscape, and that is important to consider.
“Logistics in terms of just what thinning would look like in some of our watersheds, and that's a huge challenge, because our watersheds are very steep in Utah. And so, forest management in that type of place is extremely expensive. You can't take heavy equipment at most of these slopes,” Yocom said.
Essentially, research has shown over the past ten years that there are both decreases and increases in stream flow, given reduction in canopy. The reality of tree-thinning as a strategy to increase water flow to the Great Salt Lake is that the evidence does not fully take into account the climate and ecosystem here in Utah, and there is not the research backing to invest in such efforts.
Goeking’s full article, “Variable streamflow response to forest disturbance in the western US: A large-sample hydrology approach” can be found here and her paper, “Forests and water yield: A synthesis of disturbance effects on streamflow and snowpack in western coniferous forests” can be found here and gives further illumination on the subject.
