The relationship between tree mortality from a pine beetle epidemic and increased dissolved copper levels in the upper Big Thompson River, Colorado
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Bark beetle outbreaks in the Rocky Mountains caused substantial tree mortality starting in the late 1990s, and continued into the 2000s, with the most severe mortality occurring from 2002 to 2012. Over the same time period, concentrations of dissolved copper in the Big Thompson River (BTR), Colorado, USA, increased significantly and are high enough to negatively affect aquatic life. We examined correlations between dissolved copper and tree mortality in the BTR. Two sites, one consisting of water from the western side of the continental divide and one consisting of water from the eastern side, demonstrated a positive relationship between percentage tree mortality and dissolved copper. The relationships were similar except that the best relationship occurred with a 3-year lag between tree mortality and subsequent dissolved copper levels at the eastern site and with a 5-year lag at the western site. The differential time lag is potentially the result of different levels of carbon in the soil in the watersheds associated with each site because carbon can affect copper mobility. Our results suggest that bark beetle-induced tree mortality may contribute significantly to dissolved copper levels in the BTR.
KeywordsBark beetle Water quality Dissolved copper Tree mortality
We thank Judy Billica for posing the question addressed. Al Paquet, Richard Thorp, Keith Stagg, and Tim Bohling provided helpful comments on earlier drafts of this manuscript. The Big Thompson Watershed Forum would not exist and be able to provide analyses such as these without the sponsorship of all of our donors. In particular, we thank the City of Fort Collins, the City of Loveland, the City of Greeley and the Northern Water Conservation district. We thank the Big Thompson Watershed Science Committee and Board of Directors who reviewed earlier versions of this manuscript. Finally, we thank the United States Geological Survey staff for collecting and analyzing the water quality samples included in this manuscript. In particular, we thank Greg Smith and Sue Hartley for their dedication to water quality data collection and dissemination.
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