A tree-ring reconstruction of western spruce budworm outbreaks in the San Juan Mountains, Colorado, U.S.A.

Abstract

Tree-ring records were used to reconstruct spatial and temporal patterns of western spruce budworm (Choristoneura occidentalis Freeman) outbreaks in mixed conifer forests of southern Colorado. Reconstructions in 11 host stands showed a regionally synchronous pattern of at least 14 outbreaks during the past 350 years. Intervals between outbreaks were highly variable within stands, but at the regional scale outbreak intervals were more consistent. Spectral analyses of regional outbreak time series confirmed periodicities at about 25, 37, and 83 years. Comparison with an independent drought reconstruction indicated that outbreaks typically corresponded to increased moisture, while relatively little budworm activity occurred during dry periods. In contrast to other published reconstructions in Colorado and New Mexico, reconstructions from this study area did not exhibit significant 20th-century changes in the frequency of outbreak occurrence or magnitude of growth reduction. Sharply reduced growth during outbreaks was not clearly visible on the increment core samples, and budworm-induced reductions in tree-ring growth were usually detectable only after comparison with nonhost tree-ring series. This finding emphasizes that defoliation effects on ring growth can be highly relativistic. Hence, caution should be exercised in reconstructing insect outbreak histories based only on visual detection approaches, or without comparison with nonhost or nondefoliated tree-ring control series.

The relationship between climate and outbreak dynamics of Dothistroma needle blight in northwest British Columbia, Canada.

“…Dendrochronological techniques have been used to reconstruct disturbance histories and environmental change over broad temporal and spatial scales , Speer et a l 2001, Ryerson et al 2003. Tree-ring Crossdating is the most important principle of dendrochronology .…”

“…Absolute values of tree-ring widths are functions of multiple factors, including tree age, climate variations, endogenous and exogenous disturbances, and genetics (Ryerson et al 2003). This is why dendrochronologists standardize tree-ring widths prior to quantitative analyses.…”

“…This is why dendrochronologists standardize tree-ring widths prior to quantitative analyses. For example, it is recognized that the relative effects of age-related impacts on growth must be removed and transformed into a stationary index prior to interpretation (Fritts and Swetnam 1989); otherwise the relative changes in other factors can be masked (Ryerson et al 2003). This procedure reduces the variance among cores and transforms ring widths into dimensionless index values.…”

“…Similarity of climate response in the host and non-host species is necessary for identifying insect outbreaks in host trees (Ryerson et al 2003). Preliminary comparison analyses included mean sensitivity and autocorrelation statistics, graphical plots, correlation coefficients, and response function analyses.…”

“…where I/n is the host index at year t, SDh is the standard deviation of the individual host tree series for the period common to both series, SDn is the standard deviation of the non host composite chronology for the period common to both series, Int is the non-host index at year t, and 7n is the mean of the non-host index for the period common to both series (Swetnam et al 1985, Ryerson et al 2003. The software program OUTBREAK automates the correction procedure (Holmes and Swetnam 1996).…”