This paper describes an interactive computer program which simulates daily cell growth and differentiation in a single radial file of tree cells. The growth processes are controlled by 22 model parameters, half of which are constants, the remainder time-dependent. The program user specifies the constants and the form of the time variations desired. The program computes daily values for the time-dependent parameters, and applies these values to the calculation of cell diameters, cell division, cell wall thickness, and ring width for each day of the growing season. Output is in tabular and graphical form. The tabular listing consists of the cell diameter at each position in the radial file, and for the xylem it also prints cell wall thickness and a relative density for each cell. The graphical output plots cell diameter, wall thickness, and relative density vs. file position. The program was designed primarily as an instructional tool and has been used for this purpose with good results. Because of its flexibility it has potential for research, and some possibilities for such use are discussed.

A widely spaced grid of tree-ring chronologies most suitable for dendroclimatic analysis of western North America is selected objectively on the basis of 1) numbers in the sample, length in years, and site locations, 2) statistical characteristics of the chronologies, and 3) correlation of chronologies with those on neighboring sites. The chronology statistics are then analyzed to characterize the quality of the selected set. The procedures used in this study are recommended for future climatic analysis to assure objectivity in the selection of quality tree-ring data and to allow comparisons of the statistics for new chronologies to the established data sets.

A new technique is developed for examining non-climatic variations in widths of annual tree rings. For each tree core, the technique involves making an adjustment for regional climate as inferred from a regional chronology based on surrounding sites. The technique is applied to two stands in Gila County, Arizona, where air pollution is potentially a limiting factor on tree-ring growth. For the stand closer to the pollution sources, a marked decrease in tree-ring widths minus climate is evident during the period 1908 to 1920. Although this decrease coincides with a period when two smelters were operating nearby, air pollution cannot be definitively identified as the cause of the decrease in ring widths.