Small bug, big impactWritten by Christy Martinez
Although the culprit is only the size of a grain of rice, it’s said that, if you were to walk through an affected forest in mid-June, you could hear the crunching of bark beetles feeding on the trees, at up to 10,000 larvae per tree.
Even though they’re small, bark beetles have a big impact, and they’ve left many wondering about their affects on snowpack, runoff and water resources.
Although bark beetles are the main cause of death in many trees across the West, UW Department of Botany Associate Professor Brent Ewers says what actually kills the trees is blue stain fungi, which hitches a ride on the beetles.
“In the head of the beetles there are pockets where the fungus is carried, and it blocks up the sapwood so the tree goes into massive drought stress and can’t move water anymore,” explains Ewers. “That’s important from a water standpoint, because the way the tree dies changes the way the water is partitioned within the forest, and will also change the way the trees compete with each other as they die from these bark beetle epidemics.”
The current beetle epidemic started in British Columbia, Canada in the late 1990s and early 2000s and progressed south to New Mexico. Ewers confirms that current populations are on the decline, because they’re running out of food.
“The climate, water and nutrient impacts of this outbreak will be regional,” he says. “The carbon release from the British Columbia outbreak was greater than all the boreal fires circumpolar, or around the globe. These bark beetles have a massive impact on regional water budgets, energy change and nutrient cycling.”
One approach to investigate the beetles’ impact on water sources is through 120-foot towers equipped with instruments to measure the total exchange of water vapor from a forest, as well as carbon dioxide levels. One such site in the Snowy Range, at 9,200 feet, has collected data since 1999. Another site in a high-elevation spruce fir forest sits at 10,500 feet.
“We’ve generated hypotheses about what we think might happen because of the beetle,” says Ewers, noting that includes measuring how much leaf area is in a forest, as that is the primary driver of hydrologic models.
“We have the total amount of water used by the trees, the interception of precipitation by the leaf area and the total productivity of the forest. We think all of those will be related to each other,” he notes. “If we have a bark beetle attack, we expect to see a decline in all of them in the first few decades after the attack, until we get succession, no matter if it’s sagebrush, grasses or different types of trees.”
In addition to how much water the forest uses, Ewers says researchers also anticipate affects on water quality.
“We expect dissolved organic matter to increase as the number of living trees goes down, and we expect soil moisture and temperature to increase, which could contribute to streamflow, and we also expect nitrogen availability to increase,” he explains. “Trees are really effective at removing nitrogen from soils, and when they’re gone that scrubbing doesn’t occur.”
Research finds that trees hit by beetles use 50 percent less water one month after they’ve been hit.
“The mechanism for death is a rapid decline in water use, and the main mechanism is the loss of the ability to transport water, so in a very short time after beetles hit the trees we’ll have a change in the partitioning of water in the forests,” says Ewers.
On measuring how much water comes off the forests, Ewers says researchers take into account the response of evapotranspiration (ET), and how much energy is available to convert the water into vapor.
“We get more ET going on before than after the bark beetle outbreak, supporting the idea that, even in the first year, because of fungus, the total ET coming off these forests is going down,” he says.
The amount of carbon exchanged by the forests is also impacted.
“From 2005 to 2007, before the beetle outbreak, the amount of carbon exchange by the forests is much higher than after, and this gives even more biological confirmation that tree death changes ET, and we can also see that in carbon exchange,” says Ewers.
Of the total impact of bark beetles in a water year, Ewers says, “Looking at the three years before and after the outbreak, ET over the summer is reduced by 25 percent, but in the entire year it’s reduced only 15 percent, because bark beetles don’t have as much of an impact on sublimation in the winter as they do on ET in the summer.”
Sublimation refers to the transition of a substance from a solid phase to a gas phase without passing through an intermediate liquid phase.
“Even though we’ve had some of our wettest years in the last three years, we see lower ET because of the outbreak. There’s more precipitation coming in, but less ET going out, and that’s got to mean we’ve got a change in how water is being used by these forests,” he says.
As far as the beetles’ effect on snowpack, Ewers says, “The take-home message is that, at the moment, we can’t tell. The years where bark beetles were hitting the forests were also the highest snowpack years.”
Following massive snow surveys with several thousand points characterizing areas dead, dying and living, Ewers says research shows an impact on snowpack probably less than 10 percent.
“Currently we can’t see a bark beetle impact on snowpack, just because of how variable it is in the forest,” he adds, noting that wind is where the impacts are in Wyoming related to snowpack distribution. “As the trees die, if we don’t get trees back in the same places, that will dramatically impact snowpack, just from wind redistribution,” he states.
However, deep soil moisture – at 50 to 100 centimeters – has increased noticeably.
“Even right after snowmelt we have significantly higher soil moisture in stands hit by beetles than in stands not impacted, and this is consistent over the entire growing season, and two different growing seasons,” says Ewers.
Although the research on individual stands shows a decrease in water use and an increase in soil moisture, Ewers says studies in Colorado with a watershed approach find that there is no extra streamflow in a bark beetle outbreak, and no change in the runoff ratio.
“We’re scratching our heads, because our studies indicate we should get more water in our streams, but it’s not matching up very well,” says Ewers. “Post-outbreak, there is no statistically enhanced water yield off these forests.”
He says possible reasons are higher temperatures, a snowpack that melts out earlier, more sublimation or more ET from the living trees left behind.
“We don’t necessarily know what’s going on at the watershed scale from this bark beetle outbreak,” says Ewers. “At the stand level, we have a large reduction in ET, and the amount of water used by the forest declines dramatically, but there’s a surprising lack of increased streamflow in Colorado watersheds, and that suggests we may be missing some scaling mechanism between a stand and the watershed.”
Ewers says future research will analyze how long it will take before regenerating trees and vegetation will use the excess water in the stands, and will also look into the dramatic changes in the nitrogen and carbon cycle, coupling them with the water cycle.
Finally, Ewers says, “We need to do a better job of scaling between individual stand measurements and what’s going on with the watershed, so we’re starting that with remote sensing.”