American Journal of Climate Change, 2018, 7, 253-270 http://www.scirp.org/journal/ajcc ISSN Online: 2167-9509 ISSN Print: 2167-9495 Early 20 th Century Climate-Driven Shift in the Dynamics of Forest Tent Caterpillar Outbreaks Barry J. Cooke 1 , Jens Roland 2 1 Natural Resources Canada, Canadian Forestry Service, Great Lakes Forestry Centre, Sault Ste. Marie, Canada 2 University of Alberta, Edmonton, Canada Abstract Using tree-ring analysis, we show that the dynamics of forest tent caterpillar [Malacosoma disstria (Hbn.)] outbreaks in Alberta, Canada shifted at the turn of the 20 th century from cyclic, synchronous behaviour 1850-1910 to complex, asynchronous behavior 1910-1993. This shift in dynamics coincided with the emergence in 1910 of a latitudinal gradient in outbreak stability and periodic- ity reflecting a similar gradient in the periodicity of winter temperatures. We postulate that the synchronizing strength of winter temperatures has dimi- nished as a result of climate warming, and that any synchronizing strength due to inter-population migration has been superseded by regionalized pat- terns of periodic forcing caused by weak low-frequency variability in winter temperatures. We speculate that a decrease in polar vorticity at the start of the 20 th century led to increased meridional jet stream flow and more frequent arctic weather anomalies, resulting in a loss of synchronous decadal periodic- ity in outbreak occurrence. These changes in insect disturbance probabilities, including rising uncertainty, have profound consequences for forest distur- bance risk management. Keywords Dendroentomology, Forest Insects, Outbreak Cycles, Synchronization, Disturbance Ecology, Climate Change 1. Introduction Highlights: • The dynamics of forest tent caterpillar outbreaks in Alberta shifted at the turn of the 20 th century from simple, cyclic, synchronous behavior to com- plex, multi-frequential, asynchronous behavior. This shift coincided with the emergence of a latitudinal gradient in outbreak stability and periodicity re- How to cite this paper: Cooke, B.J. and Roland, J. (2018) Early 20 th Century Cli- mate-Driven Shift in the Dynamics of For- est Tent Caterpillar Outbreaks. American Journal of Climate Change, 7, 253-270. https://doi.org/10.4236/ajcc.2018.72017 Received: December 14, 2017 Accepted: June 11, 2018 Published: June 14, 2018 Copyright © 2018 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access DOI: 10.4236/ajcc.2018.72017 Jun. 14, 2018 253 American Journal of Climate Change