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The forces and mechanics related to the wind firmness of 30-year-old loblolly pine (Pinustaeda L.) trees were tested by toppling them with a winch and cable system. The ability of trees to resist toppling, expressed as the critical turning moment, was determined by measuring the force exerted by the winch and the height on the tree where the center of force was exerted. Critical turning moments were closely and positively related to stem taper (R2 = 0.91) and various measures of tree size, including tree weight (R2 = 0.96), stem volume (R2 = 0.94), and cubic diameter at breast height (R2 = 0.93). The flexibility of tree stems, measured by the angle of stem deflection during tree pulling, was negatively related to tree size. Measures of center of gravity, crown:stem ratio, and stem moisture content were not significantly related to critical turning moment. Soil moisture content was only weakly significant and negatively related to critical turning moment. With few exceptions, trees subjected to simulated wind stress treatment resembled southern pines subjected to natural acute wind stress. In both cases, root systems were rarely damaged and stem failure occurred instead of uprooting. However, pulled trees tended to break lower on the stem than wind-broken trees.

A simulation model using empirical data was developed to determine the effect of crown shedding and streamlining from stem and branch bending on the survival of mature loblolly pine (Pinustaeda L.) trees exposed to acute wind. Data collected from tree winching experiments were used to derive the forces necessary to cause stem failure. These data, combined with data on the vertical distribution of biomass and frontal surface area of sampled trees and hypothetical within-stand wind profiles, were used to calibrate models of static wind effect. Data collected on crown biomass, stem deflection, and branch bending were employed in the model to conduct a sensitivity analysis of the relative contribution of various crown adjustment mechanisms in preventing stem breakage or windthrow. Compared with the base-line model, which included no crown adjustment mechanisms, crown loss of 25% or greater provided the greatest protection against stem breakage or windthrow from forces generated by a hurricane winds of average strength (165 km/h), followed (in order of greatest to least protection) by stem bending, branch streamlining, and crown loss of 10%. However, at the highest wind velocities modeled (249 km/h), stem bending afforded greater protection against wind-generated mortality than 25% crown loss or branch streamlining, but not more than 50% crown loss. Generally, study trees most susceptible to windthrow were tall with a high center of gravity, large crown weight, and small stem taper.

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