EFFECT OF NITROGEN ON GROWTH AND WATER RELATIONS OF RADIATA PINE FAMILIES

The effect of nitrogen fertilisation on growth, foliar nutrients and water relations of four families of radiata pine (Pinus radiata, D. Don) currently in the Australian breeding program was examined from age six to 11 years. At this stage, the stand was ready for commercial thinning. The annual rainfall at the site varied from 563 to 733 mm. Application of nitrogen fertiliser resulted in stem wood volume at age nine years of 178 m(3) ha(-1) in the controls, compared with 228 m(3) ha(-1) in plots treated with 600 kg N ha(-1). Pre-dawn needle water potential (Psi) measured in three consecutive summers (when rainfall ranged from 53 to 106 mm) were consistently higher (less water stress) in nitrogen fertilised than in control trees. Similarly, the water stress integral (S Psi) decreased consistently with increasing levels of nitrogen, although total water use in fertilised trees would have been substantially higher because fertiliser application increased the leaf area index. The relationship between S Psi and basal area was strong and paralleled that of foliar nitrogen concentration and basal area growth. Therefore, nitrogen application increased growth rates of trees by improving the nutrient status of trees and lowering the water stress on trees in summer. Families showed markedly different responses of basal area growth to nitrogen, ranging from an increase of 9.4% over three years for the least responsive family to 99.0% for the most responsive. There was no nitrogen x family interaction on Psi or S Psi suggesting that the large genetic variation in the growth response to nitrogen is mediated by factors other than water relations. These results have implications for managing highly productive plantations grown in an environment where rainfall is low compared to potential evapotranspiration.