DYNAMICS OF STEM GROWTH OF PINUS-RADIATA AS AFFECTED BY WATER AND NITROGEN SUPPLY

Changes in tree stem diameter were measured every 2 weeks on sample trees, and at intervals of 3 months on all trees, as part of a major field study investigating the impact of irrigation and fertilisation on growth of 10- to 14-year-old Pinus radiata stands near Canberra, in the Australian Capital Territory. Annual basal area growth rates varied from 2.3 m2 ha-1 (untreated stands in dry years) to 7.2 m2 ha-1 (irrigated and fertilised stands). The effect on patterns of growth and total increment of a range of environmental and stand parameters, including water and nutrient availability, temperature and canopy characteristics, were examined. Water and N availability interacted positively to increase growth. In the absence of fertilisation, irrigation increased growth by about 60%. The response to N fertilisation varied from zero during drought years to 200-300% when water was plentiful. Indices of either water or N availability, when used alone, were not well correlated with growth increments. However, the water stress integral (S(PSI), a temporal integration of the effects of both water and N availability on pre-dawn needle water potentials), was well correlated (r2 = 0.91, P>0.001) With annual basal area increments, as were combinations of indices of water and N availability. Growth in winter was low but stands with adequate water and nutrients were able to maintain slow growth through most winters. On the other hand, stands suffering either water or nutrient stress failed to grow at all in winter. Available water extended the period during which growth was possible whereas improved N availability affected the magnitude of growth but only when water was available.