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

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

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

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

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.

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

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

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:

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 Corrected Indices Outbreak definition rules (duration and threshold) Outbreak Periods Verification with Historical documents From Ryerson, 1999

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.

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.

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

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.

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

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

The 622-year reconstruction shows outbreaks in “quasi-cycles” of about 18-24 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): 679-697. 2001

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. http://tao.atmos.washington.edu/pdo/

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.

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.

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?

Porcupine Feeding (Spencer, 1964)

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

Larvae in tent

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

Results Totals 249 146 Site Name(Site Code) Radii trees years Canada Bonita (BON) 30 17 1877-1992 Pajarito Mtn. North(PMN) 37 20 1886-1992 Camp May West (CMW) 57 31 1863-1992 Pajarito Mtn. West (PMW) 1851-1992 Camp May North (CMN) 35 Camp May (CMY) 24 19 1793-1999 Baca Ranch (BAC) 29 1743-1999 Totals 249 146

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

Adults

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

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

Synchrony of suppressions

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):962-986, 1989