<i>Effects of irrigation and artificial drought on the growth and health of</i>Pinus radiata<i>near Canberra, A.C.T</i>

Abstract

Summary A series of plots comparing 1) irrigation at 12.5 mm per week 2) normal rainfall and 3) artificial drought was established in a 9-year old stand of Pinus radiata D. Don. near Canberra, A.C.T. The degree of drought-induced dieback was inversely proportional to water supply. Mortality was at first greatest on the draughted plots but later these plots became more resistant to mortality caused by natural drought than plots which received normal rainfall. After 12 years of treatment total basal area production was 64, 46, and 33 m2 ha-1 on the irrigated, normal and draughted plots respectively. Self-thinning maintained live basal area at about 35 m2 ha′ on the normal plots and 25 m2 ha-1 on the draughted plots. Although there was some mortality on the irrigated plots there was no indication of an upper limit for carrying capacity for live basal area. The cessation of irrigation was followed by substantial deaths which reduced live basal area to a level comparable with the normal plots. Examination of seasonal growth patterns revealed that stem shrinkage occurred in all but the wettest summers. Although the treatments affected growth rates there were no clear effects on the pattern of seasonal growth. Total annual water input (rainfall + irrigation) was an important determinant for annual volume and basal area increments. Production functions based on the outputs from a process-based model of P. radiata growth (BIOMASS) were of comparable accuracy to empirical multiple regression models.