Tall trees stimulate water in forests by protecting the snowpack

Tall trees play an outsized role in old-growth forests – from providing fire resistance to producing strong genetic offspring, tall trees provide forests with multiple ecological benefits. New research gives managers another reason to honor behemoths — tall trees protect slush in water-stressed environments. Research by Michaela Teich, Kendall Becker and Jim Lutz of Utah State University’s Department of Wildland Resources and her colleague Mark Raleigh of Oregon State University, details the ecological puzzle of how tall trees interact with snow of the forest.

A good water supply allows trees to withstand hot summer temperatures, survive wildfires and fight off attacks from beetle infestations. But during the hot western summers, rainfall tends to be sparse. A good deep snowpack is water in the bank as far as forests are concerned; the longer the winter snowpack lasts into the spring and summer months, the longer water is released into the ground and available to thirsty trees. This snowmelt is also part of the runoff that fills western reservoirs and benefits communities. The goal of the research was to find ways for managers to help this essential water source persist longer in the spring and summer.

An obstacle to the formation of a snowpack in a forest is, ironically, the trees themselves. Tree branches capture snow before it hits the ground and return it to the atmosphere by evaporation or sublimation. The ground directly under tall trees, cut off from the sky by branches, tends to form only a shallow snowpack, while forest floors farther away benefit from deeper accumulations. Add long wave radiation to this equation – any tree with good solid weight on its trunks emits invisible energy that humans cannot see (as light) or feel (as heat). But the subtle and constant bombardment inflicted on the adjacent snowpack adds just enough energy to pull the snow away from the freeze threshold and to limit how long it can last into the warm season.

But tall trees aren’t all bad news for snowpacks. The same wide branches that keep snow from reaching the ground directly under a tree also provide a cooling expanse of shade that blocks direct sunlight from melting snow over a fairly wide radius surrounding a tree. And the savings are significant; they can offset both the detriment of canopy cover and longwave energy.

With the balance of these factors in mind, the researchers performed calculations to determine what a forest looked like that maximized the benefits of shade and minimized the negative factors of the canopy and long-wave energy. . What they found was a donut…. at least a donut-shaped area surrounding tall trees where the snowpack could best support, away from the long-wave energy produced by the trunk and still within reach of cooling shade. The best forests for a long-lasting snowpack had trees spaced in such a way that these donut-shaped areas bumped into each other, but did not overlap.

“Snow is a key resource for freshwater supply and ecosystem function. Our study underscores that conserving large trees – the very trees that often survive wildfires – in forest ecosystems where fire part of the ecological cycle can help facilitate both,” says Teich.

But the spaced trees need to be both healthy and tall for the equation to work, the researchers say. Healthy, because thick treetops cast the most shade. And tall, because these trees are tall and cast shade farther – plus they’re more likely to withstand wildfires that occur in the West.

“This work, at a major Smithsonian-affiliated research site, exposes the fundamental issues with retaining snow on the ground,” says Lutz. “What we need now is further investigation into the specific tree species, sizes, and densities that optimize snow retention in different forest types in the American West.”

Source of the story:

Material provided by SJ & Jessie E. Quinney College of Natural Resources, Utah State University. Original written by Lael Gilbert. Note: Content may be edited for style and length.

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