SITE WATER-BALANCE AND TREE WATER STATUS IN IRRIGATED AND FERTILIZED STANDS OF PINUS-RADIATA

Soil water content and pre-dawn needle water potential were measured at 2-week intervals for 4 years in a field study of the interaction of water and nutrients in controlling growth of 10- to 14-year-old Pinus radiata near Canberra, Australia. Growth on the shallow low-fertility duplex soil was limited by both nutrient and water deficiencies. The 40 cm deep A horizon, which contained about 85% of the fine root system, had a plant-available water (PAW) holding capacity of about 60-70 mm or about 8 days supply at peak summer transpiration rates. About 33% of total annual precipitation was unavailable to the trees because of canopy and litter interception losses (28%) and run-off (5%) resulting from dissimilarity in seasonal distribution of precipitation and potential water use. Net canopy interception was 20% of annual rainfall and litter/understorey interception was 8%. Net interception was strongly affected by size of rainfall event during individual storms, ranging from more than 55% of a 5 mm event to less than 10% of a 40 mm event. However, stand basal area (as it reflects mass of foliage) was the main source of variation in annual net canopy interception, varying from 15% of total precipitation at a basal area of 20 m2 ha-1 to 25% at 34 m2 ha-1. Trees appeared to extract soil water below the conventionally accepted wilting point (-1.5 MPa) and progressively dried out the profile to at least 2 m depth, resulting in growth being restricted by water deficiency. Water availability was a more important determinant of water use than was canopy size, but in non-water-limited stands annual transpiration varied directly with foliage mass and ranged from 0.88 to 1.19 times pan evaporation. Seventy percent of annual water use occurred in the 6 months from October to March inclusive. Differences in water use between stands were largest in summer because of maximum differences in water availability and maximum foliage mass at that time. At soil water contents above 40% PAW, pre-dawn needle water potential was independent of soil water content, but it increased with increasing soil temperature and foliar nitrogen concentration. Below 40% PAW, pre-dawn potential decreased linearly with declining soil water content.