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ABSTRACT? Righting of two tilted white pine (Pinus strobus L.) stem leaders by compression wood formation was followed for 16 weeks. The natural curves and three deflection curves under added end loads were determined from weekly field photographs. Data for self-loading and cross sectional diameters were interpolated from original estimated and final measurements. A mechanical-mathematical model was developed to predict curves under zero gravity for each stem each week. The model estimated stiffness of the leaders independently for each week, and the stiffnesses were consistent throughout the experiment. A second model was developed to simulate the deflection curves assumed when the zero gravity curves were subjected to different end loads. These predicted curves were nearly identical to the observed curves from the photographs, thus verifying the assumptions in the first model. Data from this study will be used to investigate the mechanical aspects of compression wood induction and action as the stem is bent upward toward the vertical.The induction of compression wood formation appears to be a response to disorientation in a gravitational field leading to various mechanical effects. The action of compression wood results in the mechanical bending of a tilted stem. A great deal of work has been done on the more biological aspects of compression wood over the years, but not very much on the mechanical aspects (9, 10), in part because the analysis of the nonlinear bending of tapered rods has only recently been practical using modern high speed computers. We have chosen to analyze a relatively simple example of compression wood induction and action, the erection of tilted, young pine stems. Westing has performed a similar experiment to investigate a different aspect of the compression wood response (8).The analysis of this simple experiment to obtain the stress distribution and changes in slope during righting requires periodic data on the shape, stiffness, cross sectional dimensions (including distribution of compression wood), and orientation to the vertical of the stem as it is erected during the course of a season of compression wood formation. This paper reports the mathematical models and computer techniques developed to analyze the shapes and stiffness of the leaders from simple, nondestructive, field photos and data taken during the experiment, combined with cross sectional data taken after the experiment was over. Another paper will report on the correlation among orientation to vertical, stress distribution, and compression wood formation and on the mechanical strains developed by compression wood as it erects the tilted stems.MATERIALS AND METHODS Data Collection. On May 16, 1969, two fast growing, 3.5 m tall, white pine trees (Pinus strobus L.) in a field near Amherst, Massachusetts, were bent over and attached to stakes just below the 1968 whorl ( Fig. 1) so that the 1968 leaders (67 and 77 cm long) were tilted at, respectively, 30°and 900 from the vertical. Four side view photographs were taken of eac...