1. Using Tree Rings to Study the Dynamics of Forest Insect Outbreaks
Dendroecology Pre-Session Course, 2007
Laboratory of Tree-Ring Research
The University of Arizona
Tucson, Arizona

2. 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 years length
Spatial coverage over landscapes & regions
Both climate & insect outbreak proxies

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

5. 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.

6. WSBW outbreaks in the late 1970s and early 1980s caused widespread mortality in the west, often killing >60% trees within affected stands. Nevertheless, some host trees survived.

7. The effects of past defoliation events are often visually obvious on increment core samples, sometimes revealing multiple past outbreaks within individual trees.

8. 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

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

10. 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.

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

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

15. Aspen tent caterpillar is a very widespread defoliator in North America.

16. Aspen tent caterpillar defoliation event in Sangre de Cristo Mountains, New Mexico, 2000.

17. Defoliation is usually total, leaving only leaf petioles
Defoliation is usually total, leaving only leaf petioles. Sometimes, the aspen will re-foliate within the same growing season.

18. Growth suppressions (1942-1946 and 1950-1956)
possibly associated with apical death and crooked
growth form in this aspen stand at Camp May. {
Stem crook and
dead leader
Evidence of past defoliation events are sometimes visible in old aspens as crooked stems and old spike tops.

19. “White Rings” in aspen are recognized as a fairly good indicator of a past defoliation event.

20. Ring-width chronologies from seven aspen stands in northern New Mexico show synchronous low growth periods (and white ring events) indicative of multiple past aspen tent caterpillar outbreaks.

21. Host vs. non-host ring-width comparisons can be quite effective in identifying outbreaks, and helping to assure that climatic episodes) e.g., droughts) are not confounded.

22. 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

23. 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:

24. 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.

25. 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

26. What is the role of climate variation in triggering, ending, and synchronizing forest insect outbreaks in time and space?

27. A network of host and non-host trees in Northern New Mexico were used to reconstruct regional WSBW history (Swetnam and Lynch 1993).

28. Drought reconstructions from tree-rings were used to compare with the budworm histories.

30. Century-Scale Patterns of Western Spruce Budworm and Douglas-fir Tussock Moth Outbreaks in Northeastern Oregon.
Swetnam and Wickman, unpublished.

32. Ryerson, Swetnam, and Lynch. 2003. Canadian Journal of Forest Research

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

34. Precipitation is weakly periodic, whereas budworms are strongly periodic. The period lengths are similar and “in phase”.
Autocorrelation function:
Univariate power spectra:
Crosscorrelation function:
Bivariate power spectra:
Swetnam & Lynch 1993 Ecological Monographs

38. The most recent outbreak absence is the longest in the entire record, and it coincides with the warmest decades in 1200 years.

39. The 622-year Pandora moth 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):

40. Gray, Fastie, Betancourt and Jackson, 2003. Geophys. Res. Lett.
Spectral analyses of drought reconstructions show significant, but changing periodicities at decadal scales.
Ring-Width Index
Period (yrs)
Gray, Fastie, Betancourt and Jackson, Geophys. Res. Lett.

43. and his “sympathetic” pendulum clocks, 1662
Christian Huygens
and his “sympathetic” pendulum clocks, 1662
PAP Moran The statistical analysis of the Canadian lynx cycle. II. Synchronization and meteorology. - Australian Journal of Zoology.

44. Summary
Distinctive ring features can be used to reconstruct episodic forest insect outbreaks. These characteristics are specific to both the host tree species and the attacking insect.
Network approaches are useful for evaluating landscape to regional scale outbreak patterns, and associated land use and climate effects on outbreak dynamics.
Western spruce budworm and pandora moth have complex “quasi-cyclical” dynamics. Strong budworm quasi-cycles are associated with weak drought/wet quasi-cycles.