Dendroecological reconstruction of 1980s mountain pine beetle outbreak in lodgepole pine forests in northwestern Colorado

The mountain pine beetle (MPB) infested 1.6 million ha of forest in Colorado and southern Wyoming from 1996 to 2010, causing extensive tree mortality, especially in lodgepole pine forests. Identifying the extent to which MPB outbreaks have occurred in the past will further our understanding of the current outbreak's causes and consequences. We explore the use of dendroecological methods to reconstruct a prior MPB outbreak event, which occurred in northwestern Colorado in the early 1980s. We used coarse-scale maps of MPB and GIS layers of suitable MPB habitat based on stand attributes to identify 15 stands of probable MPB activity in the 1980s. At 9 sites where field observations indicated probable past MPB activity, we collected tree cores from canopy host trees and subcanopy non-host (Engelmann spruce and subalpine fir) and host trees. The relatively synchronous dates of death of host trees determined by crossdating against live or recently killed trees (i.e., after 1996) confirmed that host trees had died in the 1980s period of reported MPB activity. Tree cores from subcanopy trees of both host and non-host species were used to detect accelerated radial growth (i.e., growth releases) of surviving trees following the death of canopy trees. Over 90% of subcanopy host and non-host trees sampled showed increased radial growth following the 1980s outbreak when evaluated 1) through visual inspection of ring-width series and 2) using a mathematical kernel to identify a period of >= 150% growth increase maintained for at least 5 y. Over half of the canopy-size lodgepole pines that survived the 1980s outbreak also accelerated their growth rates following the 1980s outbreak, although less sharply and for a shorter duration than the subcanopy trees. These results demonstrate the feasibility of using dendroecological methods to detect previous MPB outbreaks in lodgepole pine forests in Colorado over the past several decades and also identify limitations for extending reconstruction efforts back in time beyond the documentary record.