Ecological Archives E092-124-A2

Rosemary L. Sherriff, Edward E. Berg, and Amy E. Miller. 2011. Climate variability and spruce beetle (Dendroctonus rufipennis) outbreaks in south-central and southwest Alaska. Ecology 92:1459–1470.

Appendix B. A description and the analysis of Superposed Epoch Analysis (SEA) used to evaluate climate conditions during and prior to release events from 1761–2003.

For comparison purposes at the interannual scale (0–5 year), Superposed Epoch Analysis (SEA; Grissino-Mayer 1995) was also used to compare climate conditions during release and non-release years, analogous to standard approaches used to evaluate fire-climate relationships (e.g., Schoennagel et al. 2007; Grissino-Mayer 1995). Each SEA assessed whether the mean value of each climate record (listed in Methods: Climate data sets) was significantly different during (year 0) and prior to (lag years 1–5) release event years (1761–2003) for corresponding time periods. We tested for a significant difference in climate values between release and non-release years by a bootstrap method that determines confidence intervals (95% and 99% CI) derived from 1000 Monte Carlo simulations, in which random years were selected and the expected mean values of climate conditions were calculated over the entire record (Mooney and Duval 1993; Grissino-Mayer 1995).

revised figure B1

FIG. B1. Interannual-scale analysis of climate conditions during and prior to growth release events attributed to spruce beetle outbreaks. Superposed epoch analysis (SEA) of the departure from mean tree-ring indices correlated with a) August precipitation (POn index), b) April–July temperature (MT index), c) Pacific Decadal Oscillation (PDO) and d) El Niño – Southern Oscillation (ENSO) during (year 0), preceding (lag years -1 -5; x axis) and one year following (year 1) the initiation years (year 0) of significant release periods from 1761–2003. Dotted (dashed) lines indicate statistically significant departures from the mean at the 95% (99%) confidence level.

LITERATURE CITED

Grissino-Mayer, H.D. 1995. Tree-ring reconstructions of climate and fire history at El Malpaís National Monument, New Mexico. Ph.D. Dissertation. University of Arizona, Tucson, AZ.

Mooney, C. Z., and R. D. Duval. 1993. Bootstrapping: A nonparametric approach to statistical inference. Sage University Paper Series on Quantitative Applications to Social Sciences, 07-095. Newbury Park, CA.

Schoennagel, T. L., T. T. Veblen, D. Kulakowski, and A. Holz. 2007. Multidecadal climate variability and climate interactions affect subalpine fire occurrence, western Colorado (USA). Ecology 88: 2891–2902.


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