Researchers at the Colorado School of Mines, US, found no evidence that outbreaks of the mountain pine beetle have affected peak or average daily streamflows in seven western US states. Instead, the authors believe that changes in streamflow patterns are caused by climatic variability.

"When we did not find a consistent pattern of change in the watersheds impacted [by the mountain pine beetle], we decided to dig a little deeper to see if we could explain why some watersheds were significantly impacted while others weren't," said Kimberly Slinski. "This study doesn't provide conclusive proof that rainfall and temperature patterns are driving the streamflow patterns, but the data clearly show that [they] are related."

Watersheds in the western US are undoubtedly suffering from unprecedented losses in trees, thanks to insect outbreaks as well as wildfire and drought. Currently, an epidemic of the mountain pine beetle extends over five-million hectares of conifer forest. The beetle introduces blue-stain fungi, which chokes the trees' water uptake, leading to needle loss and, ultimately, death.

Researchers know that widespread loss of trees through human deforestation tends to boost streamflow – at least until the trees grow back – because less water is intercepted and transpired, and more sunlight is able to shine through canopies and melt snow. The effect of beetle outbreaks has been less clear, however. Small, isolated studies have given conflicting results, and the situation is thought to be more complex because the trees die off sporadically, leaving the ecosystem to respond in unknown ways.

Slinski and colleagues took a different approach, instead asking whether there is any widespread statistical evidence that outbreaks of mountain pine and spruce beetle have significantly affected streamflow. They took data from stream gauges supplied by the US Geological Society for 33 watersheds in the western US most strongly affected by bark beetles, and calculated various statistics for each gauge, including minimum, maximum, mean and median daily streamflow. They compared the measurements taken before the beetle outbreaks with those collected afterwards.

Finding no statistically significant evidence that the beetles had affected the streamflow, the researchers explored other factors such as watershed size, tree type and climate variability. It turned out that precipitation and temperature did exhibit significant links with streamflow. Watersheds with higher streamflows during beetle outbreaks experienced higher than average precipitation, and vice versa; what's more, greater peak flows during outbreaks could be linked with increased spring precipitation, while earlier spring snow-melts could be linked with higher spring temperatures.

But Slinksi warns that this may not be the last word on the effect of beetle outbreaks on water supplies. "Hydrologic data are always more sparse than we would like to make definite conclusions, and some impacted watersheds across the west had very limited data," she said.

The researchers now plan to investigate another potential effect on the water supply from bark beetles – water quality.

The study is published in Environmental Research Letters (ERL).

Related links

Related stories