The effect of physiographic region on microfibril angle (MFA) in loblolly pine (Pinus taeda L.) in the southern United States was evaluated. MFA was determined at 1.4, 4.6, 7.6, 10.7, and 13.7 m up the stem of 59 trees, representing five physiographic regions. A nonlinear mixed-effects model was developed to test for regional differences in the initial value of MFA, the rate at which MFA changes with ring number from pith, and the lower bound of MFA achieved. Our results suggest that the parameters of interest differ significantly by region. It was found that MFA differs significantly between the South Atlantic, Gulf, and Hilly regions, compared to the North Atlantic and Piedmont regions. The initial value of MFA was found to be smaller in the Piedmont compared to all other regions. Similarly, the rate at which MFA changes with ring number was found to be significantly smaller in the North Atlantic and Piedmont regions. A test of the lower bound of MFA indicates that the Piedmont region has a significantly larger lower asymptote. These results combined indicate that overall, MFA values are larger in the North Atlantic and Piedmont regions
