Edinburgh Napier University

  Background and Aims: Although density-specific stiffness, E/, (where E is Young's modulus and is wood density) is often assumed constant by the elastic similarity model, and in determination of critical buckling height (Hcrit), few studies have tested this assumption within species. Here this assumption is tested for Pinus radiata growing across an environmental gradient, and theory is combined with data to develop a model of Young's modulus.

Methods Analyses: use an extensive series of environmental plots covering the range of climatic and edaphic conditions over which P. radiata is grown in New Zealand. Reduced major axis regression was used to determine scaling exponents between log-log plots of Hcrit vs. groundline diameter (D), and E/ vs. D. Path analysis was used to identify significant direct and indirect (through stem slenderness) edaphic and climatic influences on E.

Key Results: Density-specific stiffness exhibited 3-fold variation. As E/ scaled positively with D, the exponent of 0·95 between Hcrit and D exceeded the assumed value of 0·67 under constant E/. The final path analysis model included mean air temperature in early autumn (Taut) and slenderness as significant (P < 0·05) positive direct influences on E. Tree leaf area index and Taut were indirectly associated with E through their significant (P < 0·05) positive direct relationship with stem slenderness. Young's modulus was most sensitive to Taut, followed by stem slenderness then leaf area index, and the final model explained 76 % of the variance in E.

Conclusions: The findings suggest that within species E/ variation may influence Hcrit and the scaling exponent between D and Hcrit so important in assumptions regarding allometric relationships. The model presented may provide a useful means of determining variation in E, E/ and Hcrit across environmental gradients.