Big trees play major roles in old-growth forests, from providing fire resistance to producing genetically strong offspring. A new study led by Utah State University’s Department of Wildland Resources has found that these trees also protect melting snowpacks in water-stressed environments.
Having a sufficient supply of water helps trees endure hot summer temperatures, survive wildfires, and fend off invasions of bark beetles. Yet, during hot summers, there is not much precipitation. Thus, maintaining a good, thick snowpack for as long as possible during the spring or even summer months is extremely helpful to the forests.
The longer the water is released into the soil and available to “thirsty” trees, the better these forests fare. Moreover, melting snow is also part of the runoff filling the West’s reservoirs and benefiting human communities.
Ironically, one of the biggest hindrances for building snowpacks in a forest are the trees themselves. Their branches catch snow before it hits the ground and give it back to the atmosphere through evaporation or sublimation. Moreover, large trunks emit a significant amount of heat, or longwave energy, that contributes to melting. For these reasons, the ground beneath large trees tends to build only a shallow snowpack.
However, researchers found that, in the case of big trees, the same wide branches that prevent the snow from reaching the ground directly under a tree also provide a cooling stretch of shade which blocks direct sunlight from melting the snow under a wide radius surrounding the trees. These savings can outweigh the detriment of canopy cover and longwave energy.
The scientists performed calculations to determine what a forest looked like that maximized the benefits of shade and minimized the negative impact of canopy and longwave energy. They discovered that big trees were surrounded by doughnut-shaped zones where snowpack could best endure – away from the longwave energy produced by the trunk, but still within reach of the cooling shade. The best forests for long-lasting snowpacks had trees spaced in such a way that these doughnut-shaped zones bumped up against each other, but did not overlap. However, in order for this equation to work though, the spaced-out trees need to be both big and healthy.
“Snow is a key resource for fresh water supply and ecosystem function. Our study highlights that conserving big trees – the very trees that often survive forest fires – in forest ecosystems where fire is part of the ecological cycle can help facilitate both,” concluded lead author Michaela Teich, a senior researcher at Utah State University.
The study is published in the journal Ecohydrology.