Two sources of variation, one in the pattern of growth, the other in the pattern of rainfall, were reduced by utilizing vertical sequences rather than horizontal sequences of ring width, and utilizing calculated water deficits rather than rainfall per se. Results from two 21-year-old plantations of Pinus resinosa Ait. on a sand soil in Lower Michigan showed a high correlation between ring width over a 10-year period and moisture variables. Moisture deficits alone for the current growing season had a simple correlation of -.83 with ring width, while rainfall alone for the current year had a correlation of .69. Multiple regression analysis accounted for over 80% of the annual variation in ring width (R=.91) by water deficits and rainfall together, for both the previous and the current growing seasons. Fourteen per cent of the variation was associated with moisture conditions of the previous season (July through September), and 68% was associated with moisture conditions of the current season (May through September). It is concluded that these correlation coefficients of .80 to .90 are approaching the upper limit for relating single environmental factors, such as moisture, to tree growth.