, Volume 20, Issue 2, pp 380–392 | Cite as

A Decade of Streamwater Nitrogen and Forest Dynamics after a Mountain Pine Beetle Outbreak at the Fraser Experimental Forest, Colorado

  • Charles C. RhoadesEmail author
  • Robert M. Hubbard
  • Kelly Elder


Forests of western North America are currently experiencing extensive tree mortality from a variety of bark beetle species, and insect outbreaks are projected to increase under warmer, drier climates. Unlike the abrupt biogeochemical changes typical after wildfire and timber harvesting, the outcomes of insect outbreaks are poorly understood. The mountain pine bark beetle (Dendroctonus ponderosae) began to attack lodgepole pine (Pinus contorta) at the Fraser Experimental Forest in 2002 and spread throughout the research area by 2007. We compared streamwater nitrogen (N) from 2003 through 2012 with data from the previous two decades in four watersheds with distinct forest management histories, stand structures, and responses to the beetle outbreak. Watersheds dominated by old-growth had larger trees and lost 85% of overstory pine and 44% of total basal area to bark beetles. In contrast, managed watersheds containing a mixture of second-growth (30–60 year old) and old-growth (250- to 350-year old) had higher density of subcanopy trees, smaller mean tree diameter, and lower bark beetle-induced mortality (~26% of total basal area). Streamwater nitrate concentrations were significantly higher in old-growth watersheds during the outbreak than pre-outbreak levels during snowmelt and base flow seasons. In mixed-age stands, streamwater nitrate concentrations were unaffected by the outbreak. Beetle outbreak elevated inorganic N export 43 and 74% in two old-growth watersheds though the amounts of N released in streamwater were low (0.04 and 0.15 kg N ha−1) relative to atmospheric inputs (<2% of annual N deposition). Increased height, diameter, and foliar N of measured in residual live trees augmented demand for N, far in excess of the change in N export during the outbreak. Reallocation of soil resources released after pine mortality to overstory and understory vegetation helps explain high nutrient retention in watersheds affected by bark beetle outbreaks.


watershed biogeochemistry forest disturbance nitrate Rocky Mountains 



We thank Manual Martinez, Mark Dixon, and Banning Starr for streamwater sampling, Louise O’Deen and Derek Pierson for laboratory analyses and Byron Collins, Lucas Herman, and Jarrett Tishmack for forest inventory measurements. Support for Fraser’s long-term watershed monitoring came from the USDA Forest Service and the US Geological Survey. Project support was provided by the US Forest Service, Rocky Mountain Research Station, the US Forest Service Chief’s Emergency Fund, and the Colorado Water Conservation Board. Derek Pierson created the Fraser base map. We gratefully acknowledge Robert Stottlemyer for establishing and supporting Fraser’s streamwater chemistry sampling and analysis. Comments from Christian Giardina, Susan Miller, Dan Binkley, Monica Turner, Stephen Hart, and two anonymous reviewers greatly improved the manuscript.


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Copyright information

© Springer Science+Business Media New York (outside the USA) 2016

Authors and Affiliations

  • Charles C. Rhoades
    • 1
    Email author
  • Robert M. Hubbard
    • 1
  • Kelly Elder
    • 1
  1. 1.U.S. Forest Service, Rocky Mountain Research StationFort CollinsUSA

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