It doesn't take an expert to understand that logging and violent storms cause massive damage to forests. What is less obvious, however, is the devastating effect that the removal of trees and vegetation can have on streams and lakes.
Nitrate concentrations in waterways can soar by as much as 400 percent when the nitrogen cycle is disrupted by logging and the nitrate once tied up in organic matter washes into streams. Algal blooms and fish die-offs almost always follow this sort of nitrogen fertilization event.
Scientists studying the mountain pine beetle epidemic in the West, which now stretches from Mexico to Canada, have worried for a long time that all of those dead trees would mean serious trouble for local water quality as well.
But a team of researchers led by William Lewis, a professor of ecology and evolutionary biology at the University of Colorado at Boulder, have found that despite the ravaged appearance of forests in Colorado, where 80 to 90 percent of the canopy in many watersheds is gone, streams and lakes are remarkably healthy.
The small trees and understory vegetation that the pine beetles mostly leave alone appear to be compensating for the loss of mature trees by drastically increasing their uptake of nitrate. The results were published online this week in the Proceedings of the National Academy of Sciences.
Pine beetles prefer to infest the largest trees in the forest, seeking out a bigger living space and also greater insulation during the winter, when their population can plummet with the temperature.
So, "in an intact forest, smaller trees aren't taking up nitrate nearly as fast as they can," Dr. Lewis explained. "They have to compete with the larger trees for this nutrient."
"But when the big trees are gone, the little trees are sort of released from certain growth restraints imposed by the older trees," he said. "With so much nitrate available in the forest they start growing faster. What's shocking is that the understory can completely compensate, at least in Colorado, for the loss of the big trees.
"We never expected that pine beetles would be so innocuous when it comes to watershed chemistry," Dr. Lewis added.
He and his team are still in the process of analyzing data for organic carbon, phosphorus and major dissolved solids like calcium and magnesium.
Dr. Lewis suggests that their research carries lessons for tree harvesting and forestry management. "It shows just how important it is to protect the understory," he said. "Even a heavily logged area can serve a really vital ecological function if about half of the understory vegetation can be left intact."
The pine beetle infestation is still officially in progress, although scientists are hopeful that beetle populations will naturally die back as they essentially run out of new trees to invade. Spring will tell.
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