Recurrent monitoring of permanently marked tree populations provides long-term data that are useful for evaluating the effects of climatic variation on tree mortality.  In this study we addressed the following questions:   1) What are the temporal patterns of tree mortality in Colorado’s Front Range subalpine forest? 2) How are these patterns associated to climatic variation?  To answer these questions ten permanent forest plots initially established in the early to late-1980s in the subalpine zone of Colorado’s Front Range were re-measured for mortality, recruitment, and radial growth during the summer of 2007.   Each plot ranging in size from 324 to 2916 squared meters contained an average of over 400 permanently tagged and mapped trees (> 4 cm dbh).  Six plots represent a topographic-moisture gradient of old (i.e. > 300 years) stands at xeric sites dominated by limber pine or lodgepole pine to mesic stands of Engelmann spruce and subalpine fir.  Four plots are relatively even-aged (c. 100 to 120 years old) post-fire stands of mainly lodgepole pine.  To supplement the 1982-2007 data on tree mortality from direct measurements, all standing dead and fallen but preserved dead trees were cored to determine death dates using dendrochronological methods. 

Results/Conclusions Observed tree mortality rates were highly variable at annual and decadal time scales, and increased mortality was associated with episodes of warmer and drier conditions.  Periods of increased mortality reconstructed from tree rings also corresponded to warmer and drier periods of the climate record.  Bivariate event analysis (Ripley’s K in one-dimension) of cross-dated death years showed tree mortality occurred at annual to decadal lag times following high cumulative seasonal and annual water deficits.  These results imply future increases in tree mortality in the context of the trend towards a warmer and drier climate during the early 21st century.