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operative and controlling tree growth during the 20th century were the same ones that operated prior to that period. Clearly, this assumption can be ques- tioned and the poor fit between "observed" and "estimated" climatic fluctuations discussed above may well be the result of such factors not remaining constant over time. Further work on the problem of dendroclimatology has been carried out in cooperation with the Dendro- chronology Group (see Krebs, this vol. p. 69). An attempt was made to account for the variance of the first master tree chronology (ponderosa pine) for the San Juan area in terms of climatic parameters for different months at Durango and Pagosa Springs. The former was chosen simply because it has the nearest long-period record and the Weather Bureau station nearest to where the cores were taken (23 miles to the southeast). The technique of stepwise multiple regression analysis was used. The dependent variable in each computed regression was mean annual tree growth increment; the independent variables are shown in Table 6. In the first analysis, using data from Durango (1895-1965), it is evident that the station is too remote from the "tree growth" area to account These variables are precipitation for the preceding December, June mean maximum temperature, August mean maximum temperature, and April mean temperature. Clearly the fact that this station is so close to the study site has greatly increased the correlation. It is interesting that precipitation for the December prior to the tree growth year is a principal variable in both regressions. In both cases the partial correlation coefficient is positive, indi- cating a relationship between increased December precipitation and increased tree growth in the following year. This is presumably due to a re- charge of ground water supply early in the spring. Fritts (1972) has also indicated that winter pre- cipitation is important to growth of ponderosa pine during the following growing season. If the relationship continues to show up in subsequent more detailed analyses, then the implications of cloud seeding project for increasing wood growth should be considered. In addition to pointing to those climatic factors important for tree growth in the San Juans, it was anticipated that stepwise multiple regression would help to evaluate the effect of cloud seeding on tree growth increments, i.e. given a number of climatic parameters which have been shown from historical Table 6. Independent variables entered in stepwise multiple regression Durango Pagosa Springs April May Mean monthly T max T min and June July August September T mean December preceding the increment year January Monthly total precipitation May June July & August preceding the increment year April May Mean monthly T max June T min and July August September December preceding the increment year January March April May June July August September T mean Monthly total precipitation adequately for annual tree growth variance. The two main variables, June mean maximum temperature and precipitation for the preceding December, together accounted for only 18 percent of the variance of tree growth increments. In the second analysis, 33 independent variables were entered from Pagosa Springs for a 26 year period (1940-1965). Four main variables account for 71 per- cent of the total tree growth increment variance. climatic records to be closely related to tree growth, then it may be possible to predict in any given year what growth increment could be expected. If the annual increment in a seeded year is sig- nificantly different from the predicted value then cloud seeding may be the reason. However, there are many problems involved in this method of analysis. 58