Decoupling Tree-Ring Signatures of Climate Variation, Fire, and Insect Outbreaks in Central Oregon

Abstract

Dendroecological methods play a critical role in developing our understanding of forest processes by contributing historical evidence of climate variability and the temporal characteristics of disturbance. We seek to contribute to these methods by developing a research protocol for decoupling radial-growth signatures related to climate, fire, and insect outbreaks in central Oregon. Our methods are based on three independent, crossdated tree-ring data sets: 1) a 545-year tree-ring climate reconstruction, 2) a 550-year fire history, and 3) a 250-year pandora moth outbreak history derived from host (Pinus ponderosa) and non-host (Abies grandis-Abies concolor) tree-ring chronologies. Based on these data, we use visual criteria (marker and signature rings), statistical comparisons, and Superposed Epoch Analysis (SEA) to identify the timing of growth anomalies and establish the temporal relationships between drought, climate variation (ENSO and PDO), fire events, and pandora moth (Coloradia pandora) outbreaks. Our results show pandora moth outbreaks generally coincide with periods of below-average moisture, whereas fire in central Oregon often follows a period of wetter than average conditions. Fire events in central Oregon appear to be related to shifts in hemispheric climate variability but the relationship between fire and pandora moth outbreaks remains unclear.

Simulated western spruce budworm defoliation reduces torching and crowning potential: a sensitivity analysis using a physics-based fire model

“…Insect outbreaks, including those of western spruce budworm, have been thought to increase potential forest fire severity or probability of occurrence, or both, by increasing dead fuel loads (Schowalter 1986;Stocks 1987;McCullough et al 1998;Swetnam and Betancourt 1998;Hummel and Agee 2003;Ryerson et al 2003;Parker et al 2006;Pohl et al 2006) However, most studies have focussed on insects that cause high tree mortality, like mountain pine beetle (Dendroctonus ponderosae Hopkins) and eastern spruce budworm (Choristoneura fumiferana (Clemens); Stocks 1987;Fleming et al 2002;Jasinski and Payette 2005;Page and Jenkins 2007;Hoffman et al 2012;Jolly et al 2012a). In contrast, western spruce budworm causes relatively little tree mortality (Fellin and Dewey 1982;Alfaro and Maclauchlan 1992), outbreaks are not as widespread (Powell 1994;Swetnam et al 1995), and attacked trees retain few dead needles.…”

Are outbreaks of cambium‐feeding beetles generated by nutritionally enhanced phloem of drought‐stressed trees?

“…The pandora moth, Colorado pandora Blake, occurs in outbreak numbers on ponderosa and other pines attacked by these beetles and has coincided for over 600 years with periods of drought. The larvae of this moth feed preferentially on old needles, avoiding new foliage and exhibit the prolonged life cycle typical of insects feeding on senescing tissues (Speer et al 2001;Pohl et al 2006;White, 2015).…”

Northern range limit mountain pine beetle (Dendroctonus ponderosae) outbreak dynamics and climate interactions in mixed sub-boreal pine forests of British Columbia.

“…SEA is a compositing technique that calculates mean conditions in a response variable before, during, and after a set of events which have been superposed onto the variable such that all events occur at time zero and the annual variation surrounding each event can be summarized across the entire time series. It is widely used to study fire-climate relationships (Baisan and Swetnam 1990, Swetnam 1993, Grissino-Mayer and Swetnam 2000, Hessl et al 2004), but has also been used to analyze climatic and tree growth variation associated with insect outbreaks (Nola et al 2006, Pohl et al 2006. I used an 8-year window surrounding each MPB event including the 5 years preceding and 2 years following the onset of the outbreak which allowed us to examine climate relationships that may be more important in the years building up to or immediately following a MPB population eruption than in the event year itself, such as a lagged response to winter temperature associated with slow population growth.…”