EFFECTS OF FOREST SOIL DESICCATION ON THE GROWTH OF EUCALYPTUS-REGNANS F-MUELL SEEDLINGS

The growth rate of Eucalyptus regnans seedlings in their first year ca n be much increased if the soil is first dried and then rewetted. The ratio of growth on predried soil to growth on undried soil (the Growth Ratio or GR) reaches a maximum at air-dryness (pF 6.0 - 6.4). In E. r egnans forest soil, GR is greatest in humus-rich topsoil and declines with depth. The effect of air-drying persists for several months after rewetting when soil is stored under glasshouse conditions. It is larg ely unaffected by repeated drying and wetting, by the rate of drying o r by the season of collection. The mixing of dried and undried soil or the placement of a layer of dried soil above undried soil produces an enhancement of growth proportional to the amount of dried soil added. Firing of a litter layer above soil at wilting point increases subseq uent seedling growth to that in air-dried soil. The addition of ash fr om a litter fire to undried soil produces an increase in growth approx imately equal to that caused by air-drying The drying effect is most p ronounced in soils from mature E. regnans forest and nearby brackenlan d and is less in dense younger forest, frost-hollow grasslands and old grassy gaps in the mature forest. The effect is restored by the inocu lation of E. regnans mycorrhizal roots from both dried and undried soi l. The effect varies along an gradient from 500 to 1500 m a.s.l. and i s a maximum in the wet E. regnans climatic zone and a minimum in zones or local aspects where forests are normally subject to frequent dryin g. The stimulatory effect on seedling growth in soils of the E. regnan s zone may have an effect on the outcome of competition during regener ation in large gaps. Part of the growth responses previously ascribed to the 'ash-bed' effect may be due to the desiccation effect in these soils.