Snowpack is the accumulation of snow that has fallen on the ground and remains stored for months. It insulates the ground beneath and prolongs the supply of water through the landscape into the summer months.
Through this insulation, snowpack becomes essentially water-in-the-bank, slowly providing water overtime rather than all at once, feeding reservoirs and benefiting forests and communities.
In fact, over 80 percent of the Wasatch Front’s main water source comes from snowmelt. However, snowpack from forests in the mountainous regions of the West have been declining significantly in recent decades.
To better understand the interactions between snowmelt and trees, a recent study conducted at the Yosemite Forest Dynamics Plot has shown how large trees can influence the retention of snowpack.
Due to the Rim Fire of 2013, only large healthy trees in this old-growth forest plot survived. Michaela Teich who currently works in Austria at the Institute for Natural Hazards explains how this provided an opportunity to better understand the acclamation of snow.
“Big trees were still standing while the ground vegetation and smaller trees were almost gone", said Lutz. "We were wondering how these big trees after the fire may affect the snowpack in terms of snow depth, because trees hold this falling snow back so that it can’t reach the ground. So you have less snow in forests, ” says Teich.
Jim Lutz, co-author of the study and Forestry professor at USU spent two years at the Yosemite plot with Teich and a research team gathering data on snowpack.
“So the present study on snow is in this post fire forest, which is very complicated, because we have large living trees, we still have small living trees, but we have all the post fire processes. Because we were looking at this forest, you know, 5, 6, and 7 years after the fire, we're able to investigate even more processes, about how trees and snow interact,” says Lutz.
In addition to these factors, Teich’s knowledge about snowpack helps us understand how trees in the forest can in some ways hinder the ability for snowpack to form. Their branches, or what some call crowns catch snow, preventing it from ever reaching the ground. Also, because trees are dark in contrast to the surrounding snow, they absorb the sun's energy and emit heat and radiation known as longwave radiation.
“When the sun shines on the snowpack, it melts faster and what you often see and fire district landscapes where most of the forest cover is gone, that you have a deeper snowpack because you don't have this interception effect of the tree crowns”, says Teich.
For these post-fire landscapes, there is an additional key variable that Teich mentions was assessed in their study.
“One very important point also after fire is that you have a release of little woody debris pieces. These pieces lower the snow albedo, meaning the snowpack can take more energy in and also melts faster," says Teich.
This site in Yosemite is the largest permanent monitoring plot in the National Park system. Every tree greater than one centimeter in diameter is measured and mapped on an annual basis by Jim Lutz, visiting over 34,000 trees each year!
“The question is what's the balance of all of these processes, the ways trees negatively affect snowpack and the way they positively affect snowpack," Lutz says.
In addition to the tree data, Teich says how she was able to capture temperatures of different places. "There are also some temperature loggers installed on a regular grid throughout the plot. And from these temperature loggers, you can then calculate the date when the snow at this specific point is gone —So we knew when the snow was gone, we knew the trees surrounding it — and then kind of put that together in our conclusions how the interactions between those trees influence the snow melt.”
Being able to capture all these factors including tree mapping, solar radiation, and many other variables, Teich used statistical modeling to find when snow retention was maximized.
“So in this forest, in California, where trees do get to be very large in terms of diameter, and in terms of height, we found that a distribution of large trees more than two feet in diameter, helped retain snow,” says Lutz.
When these large trees are more than 10 meters apart from each other, they maximize snow retention by providing lots of shade, which blocks significant amounts of solar energy - keeping surrounding snow drifts intact longer into the summer months.
The study’s results were clear, widely distributed large trees keep snow on the ground longer. Lutz explains why these things are so important for those who live out west.
“So that's important to everyone in the West because we get a lot of our water from snowpack, and anything we can do to manage our forests so that we retain more water, is going to help us in the field especially if the future is drier," says Lutz.
Understanding the roles these big trees have on snow duration can help increase water yield in increasingly drier landscapes.
Lutz says the next step is to apply this approach to other old-growth forests around the world.
“The sweet spot is going to be different in the mountains of Utah than it is in California, because it's colder and the trees don't reach such heights. So the principles we were able to prove, but the specifics about how we would retain the most snow in Utah, they're going to be different. And so we need to look at this in different kinds of forests,” says Lutz.
