THE ROLE OF ZINC IN FORESTRY .2. ZINC-DEFICIENCY AND FOREST MANAGEMENT - EFFECT ON YIELD AND SILVICULTURE OF PINUS-RADIATA PLANTATIONS IN SOUTH-AUSTRALIA

Widespread, patchy occurrence of the disease called 'dieback' of radiata pine was identified with natural zinc deficiency. The disease became evident in the 1890s, less than 15 years after afforestation with radiata pine began. Trees 2-4 years old became affected, but many of the trees recovered naturally on the more fertile sites after about 8 years although with malformed trunks. A strategy was adopted which minimised the effect of the disorder by avoiding the soils more likely to produce symptoms but at the expense of a reduced planting programme. Other constraints on practice were perceived All plantations were assessed for productivity in 1935 and dieback-affected stands were distinguished from normal, both territorially and in management plans. The cause was identified in 1936, confirmed locally in 1939 and remedial Zn sprays applied thereafter. The effect on yield, discounted value of returns and net present value has been estimated from the data collected and yield-tables produced then, and from yield-tables now current. Current costs and returns (1987) have been used. Productivity was reduced most, by 20%, in the lowest site-quality classes (S.Q.) associated with low fertility soils. The opportunity cost (O.C.) of zinc deficiency was from A$470 ha-1 (S.Q. VII) to A$4710 ha-1 (S.Q.III) at 5% C.I. rate. The value of returns was reduced by about 70% as a result of constraints on management options. Much-intensified silviculture has been applied since 1969; certain research treatments identified that Zn deficiency could be introduced with enhanced growth rates Estimation of Zn removal in harvesting and application of appropriate fertilizers have prevented dieback in current practice. Intensive practice has increased the O.C. of Zn deficiency by an additional A$2000 ha-1. © 1990.