New research suggests B.C’s beetle-infested forests will see a rise in snowpack as their canopies grow thin.
But in some areas, changes in snowpack are showing up later than expected.
The six-year study compared snow levels under beetle-infested pine trees, green trees of other species, and a clearcut stand on the Bonaparte Plateau north of Kamloops.
As expected, after the beetle-killed trees shed most of their needles, their thinner canopy allowed more snow to fall directly onto the forest floor.
But Rita Winkler, a forest hydrologist and a lead author of the report, said the research team was surprised how long it took to see significant changes.
Four years after it was first attacked in 2005, the infected stand had turned from green to bright red crowns, and they were losing needles quickly.
But it was another two years and a lot more fallen needles before the snowpack under the infected pine trees finally shifted from the levels that researchers measured in the healthy mixed stand to what they saw in the clearcut.
“What we’re thinking is that you have to go to almost totally defoliated trees before you will see an effect on the snowpack,” said Winkler. By the time they saw a change, the pine canopy had been reduced to almost no needles and a few coarse grey stems.
One reason for the longer-than-expected effect is that forest canopy is just one among a complex and changing web of inputs that determine how fast a snowpack builds or evaporates.
As extra needles fall in an infested stand, for example, they darken the snow surface and absorb more heat from the sun until they are buried again by new snow. That albedo effect offsets the needle loss, Winkler said. But another, much more important factor is the weather.
“I tell the public that if they live in valley bottoms, just because you didn’t see any change in watershed response last year or this year, that’s largely a function of the fact that the snowmelt seasons have been so benign.”
“We haven’t had major flooding for a number of years now,” she said.
Since 1994, more than 17.5 million hectares of lodgepole pine have already been infected by mountain pine beetle across B.C. Another 6 million hectares of Alberta pine and several U.S. stands are also in danger.
“In this particular place, it’s taken much longer than we expected,” Winkler said. “I don’t think that’s necessarily the case all over the province.”
Data from the snowpack study will be used in another experiment at Upper Penticton Creek, which has been ongoing since 1984. That study aims to see how the rate of cutting a stand affects streamflow.
