Bark beetle epidemic changing water quality as well as forest health,
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Ghosts of a forest past: bark beetle-killed trees overlook Grand Lake, Colo.
Credit: Lindsay Bearup |
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Bark beetle-felled trees fall, landing on a river and likely affecting the health of its waters.
Credit: Lindsay Bearup |
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Scientists take soil and microbial samples beneath a beetle-killed tree.
Credit: Thomas Cooper, Lightbox Images |
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Beetle-killed trees line a river; researchers have found water quality changes.
Credit: Lindsay Bearup |
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Beetles burrowing into pines create open areas in the trees' bark.
Credit: Lindsay Bearup |
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Researchers conduct experiments on the influence of chemicals in pine needles on soil.
Credit: Thomas Cooper, Lightbox Images |
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The
following is part eleven in a series on the National Science
Foundation's Science, Engineering and Education for Sustainability
(SEES) investment. Visit parts one, two, three, four, five, six, seven,
eight,
nine and ten in this series.
In
mountains across the Western United States, scientists are racing
against time--against a tiny beetle--to save the last lodgepole pines.
Forests are bleeding out from the effects of the beetles, their conifers' needles turning crimson before the trees die.
Now, researchers are also hurrying to preserve the region's water quality, affected by the deaths of the pines.
"When
these trees die," says hydrologist Reed Maxwell of the Colorado School
of Mines, "the loss of the forest canopy affects hydrology and the
cycling of essential nutrients."
Maxwell and other scientists recently published results of their study in the journal Biogeochemistry.
Co-authors,
in addition to Maxwell, are Kristin Mikkelson, Lindsay Bearup, John
McCray and Jonathan Sharp of the Colorado School of Mines, and John
Stednick of Colorado State University. Mikkelson is the paper's first
author.
Bark beetle numbers: heating up
"The mountain pine beetle outbreak in Western states has reached epidemic proportions," says Maxwell.
Bark
beetles, as they're known, are native to the United States. They're
so-named as the beetles reproduce in the inner bark of trees. Some
species, such as the mountain pine beetle, attack and kill live trees.
Others live in dead, weakened or dying hosts.
Massive outbreaks of
mountain pine beetles in western North America since the mid-2000s have
felled millions of acres of forests from New Mexico to British
Columbia, threatening increases in mudslides and wildfires.
Climate
change could be to blame. The beetles' numbers were once kept in check
by cold winter temperatures and trees that had plenty of water to use
as a defense.
But winters have become warmer, and droughts have
left trees water-stressed and less able to withstand an onslaught of
winged invaders.
"A small change in temperature leads to a large
change in the number of beetles--and now to a large change in water
quality," says Tom Torgersen, director of the National Science
Foundation's (NSF) Water, Sustainability and Climate (WSC) Program,
which funded the research.
WSC is part of NSF's Science, Engineering and Education portfolio of investments.
"Bark beetles have killed 95 percent of mature lodgepole pines," says Maxwell.
Death of a lodgepole pine
But the trees don't die immediately.
When
beetles invade, a blue fungus spreads inside a tree's trunk, choking
off transpiration and killing the tree in about two years.
The trees turn blood-red, then the ashen gray of death, dropping their needles to the forest floor.
"Some
of the most important effects of bark beetles may be changes in the
hydrologic cycle," says Maxwell, "via snow accumulation under trees and
water transpiration from trees and other plants."
Biogeochemical changes may be even more important, he says, with carbon and nitrogen cycles interrupted.
"We're
studying these hydrologic and geochemical processes through a
combination of field work, lab research and computer modeling," says
Maxwell.
Whither the beetles, so the trees, forests...and waters
Changes in tree canopies affect snowpack development and snowmelt.
For
example, a lack of needles on branches lets more snow fall through the
canopy--snow that would otherwise be caught on branches. A tree without
needles also has less shade beneath it.
The result is a shallower snowpack, earlier snowmelt and less water in spring.
"The real question," Maxwell says, "is how these processes translate from individual trees to hillslopes to large watersheds."
Dead
trees don't transpire water. Once a forest has died, this important
flow of moisture from the ground to the atmosphere ceases.
That
can mean a loss of as much as 60 percent of the water budget, although
increases in ground evaporation or transpiration from understory shrubs
and bushes may compensate for some of the lack.
"Combined with
what's happening to snowpack depth," says Maxwell, "it becomes a
complicated relationship that can change the timing and magnitude of
spring runoff from snowmelt--and an entire year's water resources."
Tree mortality also appears to affect forest carbon and nitrogen cycles through increases in dissolved organic carbon.
"We've
seen changes in drinking water quality in beetle-affected watersheds
that are almost certainly related to high dissolved organic carbon
levels," says Maxwell.
As Maxwell, Mikkelson, Bearup and
colleagues discovered, there's a lag time between beetle infestation and
water quality declines, "so tree and forest water transport processes
are very likely involved," says Maxwell.
All watersheds great and small
The
observations prompted the researchers to study processes at the
individual tree and hillslope scale to better understand what's
happening in watersheds large and small.
"Watersheds are complex, interrelated systems," says Maxwell, "which makes understanding them more challenging.
"We're
developing complex, numerical models of bark beetle-infested watersheds
that include our best understanding of how and where water flows. The
models are allowing us to isolate individual processes by turning them
on and off in 'what-if' scenarios."
Along with on-the-ground
observations, he says, "they're showing us more of the complex story of
pine beetle effects on Western watersheds.
"We now know that healthy watersheds ultimately depend on healthy forests."
Western
streams and rivers soon may be part of dead and dying forests,
surrounded only by the ghosts of lodgepole pines past.
Related WebsitesNSF Science, Engineering and Education for Sustainability (SEES) Programs:
NASA
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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