1. Using Tree Rings to Reconstruct Multi-Century Insect Dynamics and
Climatic Linkages
Thomas W. Swetnam
Laboratory of Tree-Ring Research
The University of Arizona
Tucson, Arizona

2. Websters:
“interlude… 1. A short, humorous play formerly presented between the parts of a miracle play or morality play…”

4. Forest Insect Outbreaks
Ecologically and economically important
Long periods between outbreaks
Forestry and insect population records are short (often <50 years), and usually only for small areas
Tree Rings
“Proxy” records of environmental variations
Can provide records 300 to 700+ years length
Spatial coverage over landscapes & regions
Both climate & insect outbreak proxies

5. Tree-Ring/Forest Insect Outbreak Examples:
Western Spruce Budworm
Pandora Moth
Tent Caterpillar
Regional Synchrony
Climatic Entrainment?

7. The range of western spruce budworm extends from British Columbia to the Southwest. Outbreaks encompassing millions of hectares have occurred several times in the 20th century.

10. An example of a growth suppression in white fir defoliated by spruce budworm

12. Dead tops and many old dead branches are common signs of past defoliation events.

13. Comparisons of host and non-host tree-rings series enables us to separate climate effects from defoliation effects.
Douglas-fir versus ponderosa pine:
Grand fir versus ponderosa pine:
Host minus Non-Host series:

14. Dated Host Tree Increment Cores Dated Nonhost Tree Increment Cores
Measurement of cores
Measurement of cores
Raw ring width measurements
Raw ring width measurements
Check dating and measurements (COFECHA)
Check dating and measurements (COFECHA)
Standardization (ARSTAN)
Standardization (ARSTAN)
Standardized Host Tree Summaries
Standardized Control Chronology
Remove climatic variation from host tree summaries (OUTBREAK)
This is a schematic diagram of the way in which the climate signal is removed and how we arrive at defoliation indices.
The host and non-host chronologies are treated in a similar manner at the outset in terms of crossdating and standardization.
The growth of control or non-host species is then subtracted from the host chronologies, using a program called OUTBREAK, but its not quite that simple.
Corrected Indices
Outbreak definition rules
(duration and threshold)
Outbreak Periods
Verification with Historical documents
From Ryerson, 1999

15. Counts of trees or sites recording outbreaks in tree-ring series can be aggregated over regional scales – providing time series that are many times longer than 20th century insect population time series.

16. The tree-ring records are tested against 20th century defoliation and insect population time series. Matches are usually fair to good -- but it is evident that tree-ring series are only “noisy approximations” of insect population numbers.

17. Outbreak synchrony over large regions is evident in the tree-ring reconstructions and 20th century records. What are the causes of synchrony over such large areas? Dispersal? Climatic entrainment?
Swetnam and Wickman, in review, Forest Science

18. Drought reconstructions from tree-rings are based on massively replicated and broadly dispersed tree-ring data that have been statistically calibrated and verified with modern instrumental meteorological data.

21. Century-Scale Patterns of Western Spruce Budworm and Douglas-fir Tussock Moth Outbreaks in Northeastern Oregon
Swetnam and Wickman, in review, Forest Science

24. Synchrony in endemic/outbreak and drought/wet periods in the San Juan Mountains, southwestern Colorado.
Ryerson, Swetnam, and Lynch, in review, Canadian Journal of Forest Research

25. The 622-year reconstruction shows outbreaks in “quasi-cycles” of about and 37 to 41 years. Amplitude pairs from singular spectrum analyses illustrate the changing strength of quasi-cycles through time.
Speer, Swetnam, Wickman and Youngblood, Ecology 82(3):

27. The Pacific Decadal Oscillation (PDO)
The Pacific Decadal Oscillation (PDO). Typical wintertime Sea Surface Temperature (colors), Sea Level Pressure (contours) and surface windstress (arrows) anomaly patterns during warm and cool phases of PDO.

28. Drought reconstructions from tree rings show synchronized “mega- droughts” over regions to continents.
a. Yellowstone
b. Bighorn Basin
c. Colorado Plateau
d. SE Rocky Mountains
e. SW Rocky Mountains
1250
1350
1450
1550
1650
1750
1850
1950 a b c d e
RWI
YEAR AD
Gray, Fastie, Betancourt and Jackson, in review.

29. Gray, Fastie, Betancourt and Jackson, in review.
Spectral analyses of drought reconstructions show significant, but changing periodicities at decadal scales.
Ring-Width Index
Period (yrs)
Gray, Fastie, Betancourt and Jackson, in review.

30. Key Questions:
What are the causes of regional synchrony in forest insect outbreaks?
What role does climatic variability play in controlling forest insect outbreaks?
What experiments can be designed to address these questions?
What modeling approaches might be used to address these questions?

31. Porcupine Feeding (Spencer, 1964)

32. Autocorrelation function:
Univariate power spectra:
Crosscorrelation function:
Bivariate power spectra:

33. Larvae in tent

38. Pandora moth leaves a distinctive ring “signature” of very narrow latewood bands and reduced overall ring width.

41. Results Totals 249 146 Site Name(Site Code) Radii trees years
Canada Bonita (BON) 30 17
Pajarito Mtn. North(PMN) 37 20
Camp May West (CMW) 57 31
Pajarito Mtn. West (PMW)
Camp May North (CMN) 35
Camp May (CMY) 24 19
Baca Ranch (BAC) 29
Totals
249
146

42. Regional droughts tend to correspond with low budworm activity, and wet periods tend to correspond with high budworm activity.

43. Adults

46. Using outbreak definition rules applied to the corrected ring-width indices, we can sum the number of trees or sites recording outbreaks.

47. Dead leader from budworm defoliation
In addition to the tree-ring evidence, and comparisons with documentary records, other indicators confirm long histories of outbreaks within sampled stands.
Dead leader from budworm defoliation

49. Synchrony of suppressions

51. Outbreak synchrony over large regions is evident in the tree-ring reconstructions and 20th century records. What are the causes of synchrony over such large areas? Dispersal? Climatic entrainment?
Swetnam and Lynch, Forest Science 35(4): , 1989