Snow Lasts Longer in Beetle-killed Forests

Forests of dead pines left standing after a bark beetle infestation have been discovered influencing snow and weather in some ways like cleared patches and other ways like live forests. The outcome is a snowpack under dead trees that takes longer to melt in spring than does snow sitting in clearings or within healthy forests.

In the heart of British Columbia's mountain pine beetle epidemic, south of Vanderhoof, research found that a forest of recently-infested lodgepole pine trees, many no longer with needles, accumulated 47 percent the amount of snow on the ground as did a nearby logged site. Another close-by patch of live, mature pine trees built up only 27 percent of the clearing's snowpack.

Snow accumulation in this case is calculated as snow water equivalent (SWE) when it peaked at the end of winter in March just before spring melt. This particular study occurred during a winter of relatively light snowfall.

The green tree canopy trapped most of the snow that fell over winter. Although branches and stems in the dead forest intercepted some snow, without foliage they let more snow reach the ground.

Despite the clearing amassing the greatest amount of snow, it melted faster there than in the two forests. Snow in the clearing was exposed to substantially more sun and wind than snow amid trees. Radiation in the opening averaged nearly five times higher than in both forests where trees shaded the ground. Winds blew three times faster across the clearing compared with the forests, whose vegetation blocked air flow. Even though dead, the stems and branches left standing in the beetle-killed patch still served to shade the ground and hinder wind much like a live forest.

While the dead stand had deep snow somewhat similar to the clearing, its meteorological conditions were more like those of the live forest. So the daily release of water during spring melt in the dead stand was close to that in the live stand, and half of the rate that was in the clearing. The combinations of snow accumulation and melting rate resulted in the snowpack taking 15 days after the winter peak to entirely disappear in the live forest, 20 days in the clearing and 22 days in the dead forest.

For snow hydrology, the dead pine forest sits in transition between a mature green forest and a clearing. It can take 10 to 15 years for snags to lose their needles and later branches before completely toppling. Then as understory trees grow, a forest previously visited by pine beetles could be in a transition state for quite some time.