SUPPRESSION OF CLIMATE SIGNALS IN RED SPRUCE (PICEA RUBENS) BY SPRUCE BUDWORM OUTBREAKS, NORTH-CENTRAL MAINE

As geoscientists continue to use tree-rings to better understand the recent geological past, it is important that they are acquainted with external, biological factors that may control growth. For example, spruce budworm (Choristoneura fumiferana, Clemens), an indigenous defoliator in eastern North American fir-spruce stands, has been a major factor in their growth in the past few hundred years. In this study, two stands of red spruce (Picea rubens) were sampled in north-central Maine, approximately 50 kilometers due north of Mt. Katahdin in the high hills of the St. John Uplands ecoregion. 50 tree cores were collected, skeleton plotted and digitized from 26 trees resulting in a master chronology (1879-2013), which was confirmed with COFECHA and detrended with ARSTAN. This chronology was tested for climate controls on growth using PRISM climate data using the DENDROCLIM2002 program. Correlation-function results from DENDROCLIM reveal that red spruce in the North Maine Woods grow well in years with a higher dew point in April, lower precipitation in May, and warmer January and April months; suggesting that root elongation is the most sensitive stage of growth. These correlations, however, were not temporally continuous. To test for potential spruce budworm outbreaks, the master chronology was run through the OUTBREAK program, which is used to quantify spruce budworm outbreaks by comparing a host chronology against a non-host chronology. Results showed a correlation between tree-ring widths and both the 1914-1926 (visually obvious in our chronology) and the 1976-1986 (cryptic) spruce budworm outbreaks. DENDROCLIM analysis of temporal stability, however, shows all climate correlations as insignificant during the latter outbreak period. In many cases, these climate correlations do not return, or, if they do, continue in a less significant manner following the outbreak. These results suggest a budworm-outbreak suppression or masking of climate signals recorded in these red spruce. In future reconstructions using dendrochronology, geoscientists must take care to prevent misperception of biological attacks with climate signals.