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HAROLD C. FRITTS

Abstract

The relationships between climatic factors and fluctuations in dated tree-ring widths are statistically evaluated. A wide ring indicates that the year's climate was moist and cool, and a narrow ring dry and warm. In general, ring width relates to a 14-month period from June through July but most tree-ring chronologies exhibit a closer relationship with autumn, winter, and spring moisture than with summer moisture. The climatic relationships for evergreen trees are attributed largely to the influence of environmental factors on photosynthesis and the accumulation of food reserves. Under abnormally dry and warm conditions, especially during the autumn, winter, and spring, little food is accumulated, new cells are formed more slowly during the growing period, and the resulting ring is narrow.

Relative 10-yr. departures are calculated for the entire length of 26 tree-ring chronologies from western North America. Those portions after 1500 are used to map areas of high and low moisture. Periods of widespread drought are noted in 1576–1590, 1626–1635, 1776–1785, 1841–1850, 1871–1880, 1931–1940. Periods of widespread and above average moisture occurred during 1611–1625, 1641–1650, 1741–1755, 1826–1840, 1906–1920. The moist periods of 1611–1625, and 1906–1920 were most widespread and markedly above average.

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William C. Ashby
and
Harold C. Fritts

A multivariate technique was employed to assess tree-ring growth and climatic relationships of white oak in northern Illinois-Indiana, including LaPorte, Ind. Climate accounted for 59% of the variance in ring-width chronology, and prior growth accounted for 2%. Since the reported LaPorte precipitation anomaly is considered to be largely a phenomenon of summer convective storms (Changnon, 1968), the importance of the late spring and summer precipitation on ring width suggests that white oak rings from the LaPorte stands versus the control stands would be likely to show differences in growth after 1930 if a precipitation anomaly had occurred.

Our particular tree-ring analysis neither proves nor disproves the precipitation anomaly at LaPorte. It does suggest that some factor in the LaPorte area became increasingly more limiting to tree growth than climate during the decade of 1940, and caused a gradual reduction of growth. Our best inference is that this growth reduction may have been a direct result of toxic effects from severe air pollution correlated with high levels of smoke-haze in Chicago during the decade of the 1940s. This would suggest that rings from trees in other urban areas should be studied, as they may contain a record of past air pollution or, in the absence of pollution and other effects resulting from disturbance by man, may be suitable for documentation of anomalies in climate.

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Harold C. Fritts
,
Terence J. Blasing
,
Bruce P. Hayden
, and
John E. Kutzbach

Abstract

Ring widths from trees on certain sites reflect climatic variation. Therefore, long time series derived from replicated and precisely dated ring-width chronologies may be utilized to extend climatic records into prehistoric times. Multivariate analyses of tree-ring chronologies from western North America are used to derive response functions from which one can ascertain what climatic information each ring-width chronology contains. In addition, multivariate analyses are utilized to calibrate a large number of ring-width chronologies of diverse response functions and from widely dispersed sites with a large number of regional climatic variables. A series of transfer functions is derived which allows estimates of anomalous climatic variation from tree-ring records. Reconstructions of anomalous variations in atmospheric circulation for portions of the Northern Hemisphere back to 1700 A.D. are obtained by applying the transfer functions to tree-ring data for time periods when ring data are available but climatic data are not.

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