Variation in Loblolly

The effect of physiographic region on microfibril angle (MFA) in loblolly pine (Pinus taeda L.) ia 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 mixadeffects 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 pf interest differ significantly by regioq. It was found that MFA differs significantly between the South Atlantic, Gulf, and Hilly regions, compar#l 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'fcund to be significantly smaller in the North Atlantic and Piedmont regions. A test of the lower bound o f i M F ~ indicates that the Piedmont region bas a signiflmtty larger low& asymptote. These results combidd indicate thi[ overall, MFA values are larger in the North Atlantic and Piedmont regions. ! Keywordr: Clustered At4 IobloUy pine, repeated measures. random-effects, wood p r o m e s .