Changes in the canopy and growth of Pinus radiata in response to nitrogen supply

The response of Pinus radiata trees to nitrogen nutrition was followed as the stand grew from canopy closure to first thinning stage to study the nature and the longevity of the growth response to applied nitrogen. The effect of nitrogen application on needle growth, foliar nitrogen, chlorophyll and carbohydrate levels were examined as well as the effects on canopy development and stem growth. The size of needles that were elongating when the nitrogen was applied increased with an increase in foliar nitrogen, but fully expanded needles did not increase in size. Nitrogen and chlorophyll concentrations increased in all age classes of foliage on the tree within three months after treatment and there was a significant correlation between nitrogen and chlorophyll concentrations. Concentrations of carbohydrates (sugar and starch) were not significantly affected by nitrogen application. Soil mineral nitrogen returned to pre-application levels within a year after application with little chance for further uptake from the soil. However, elevated foliar nitrogen concentrations and greater increments in stem growth continued for a further two years, leading to an increase in basal area over the five year interval from nitrogen application of up to 45% above the control. The longevity of the growth response was promoted by retranslocation of nitrogen within the foliage to support an increase in the leaf area index as well as foliar nitrogen concentrations in needles formed later. At the highest level of nitrogen addition, the estimated amount of nitrogen retranslocated within foliage attached to the tree was 260 kg N ha(-1) in comparison to 58 kg N ha(-1) which fell in litter over the same period. Application of nitrogen increased stem growth rates, but did not extend the duration of growth during the growing season. Nitrogen application increased productivity initially by increasing the photosynthetic efficiency of foliage, and later through increased foliage production. The increase in productivity with improved nitrogen nutrition is also discussed in relation to evidence from this site which showed that improved nitrogen nutrition lowered summer water stress in trees and this contributed to increased productivity. (C) 1997 Elsevier Science B.V.