1964 Annual Report to the United States Weather Bureau, Department of Commerce, Washington 25, D.C. / Project: Dendroclimatic History of the United States. OC 406. Grant Cwb-10798 / December 31, 1964 / Introduction: The details of the climatic history of the United States during recent centuries are not known. In this period, as in more ancient times, there is much indirect evidence of significant changes of climate. Dendroclimatic analysis represents an especially promising source of information on the chronology and character of such climatic changes, especially those in the semiarid regions of western North America. It is the purpose of this report to present: (1) some recent analyses of the climatic factors influencing ring growth; (2) a brief discussion of the current theory concerning the model of tree growth and climate and (3) a first approximation of synoptic dendroclimatological patterns from 1500 A.D. to 1940 A.D. using 26 selected tree-ring chronologies from western North America. This material is being circulated to professionals in related fields in hopes that they may compare these results with their own findings and make appropriate criticisms. The authors welcome any suggestions, especially those pertaining to correlation or lack of correlation of the maps with other lines of evidence. The paper is to be presented at the VII International Congress of the International Association for Quaternary Research which meets at Boulder, Colorado, during August of 1965. The analyses of the tree growth relationships were sponsored in part by the National Geographic Society and the U. S. Department of Interior, National Park Service, through the Wetherill Mesa Archeological Project. The authors are indebted to past and present staff at the Laboratory of Tree-Ring Research for the development of the regional tree-ring chronologies, and to the Numerical Analysis Laboratory, The University of Arizona, for free computing time and services. They are also indebted to James A. Erdman, Maurice E. Cooley, Nicholas Matelas, and Julie McMahan, who assisted in various phases of the project.

Reprinted from: Monthly Weather Review, Vo. 93, No. 7, Pp. 421 to 443, 1965 / 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.