Relationships among growth, δ13C, foliar nitrogen concentration, foliar nitrogen content and intercepted radiation at different cultural intensities, planting densities and site indices reveal the importance of water use efficiency in mid-rotation loblol

The results indicated that CAI of stem volume, stem wood biomass and total above-ground biomass were affected by SI or the interaction between SI and management intensity. The significant relationships between SI and CAI growth indices were all positive indicating greater growth at greater site indices. A negative correlation was found between δ13C, a surrogate for water use efficiency, and SI, meaning stands had lower water use efficiencies at sites with higher SI values. Foliar N concentration and content were both affected by silvicultural intensity, while IPAR was only affected by planting density. Among the physiological parameters, δ13C was the only parameter significantly correlated with the CAI indices. Significant negative correlations were found between CAI of stem volume, stem wood biomass, total above-ground biomass and δ13C. These results indicated that moisture availability affected growth of the highly fertilized mature stands in this study. The findings reported here, when compared to similar studies in younger stands, suggest that growth limitations shifted from light availability to soil moisture availability in older stands when nitrogen was not limiting.