THE EFFECT OF MOISTURE AVAILABILITY ON WOOD DENSITY AND VESSEL CHARACTERISTICS OF EUCALYPTUS GRANDIS IN THE WARM TEMPERATE REGION OF SOUTH AFRICA

Productivity of forest plantations in South Africa is highly dependent on soil moisture availability. Soil moisture availability is often limited, and evaporative demand is high. Eucalyptus grandis, planted extensively in South Africa, is highly intolerant of adverse conditions, and performs poorly when planted on shallow soils and/or on dry sites. A study was conducted to assess the effect of moisture availability on the wood density and vessel characteristics of E. grandis grown in the warm temperate regions of South Africa. Gamma-ray densitometry and image analysis techniques were used. This study compared cores taken at breast height from compartments representing varying levels of moisture availability. Different levels of moisture availability were achieved using combinations of mean annual precipitation (MAP) and estimated soil water storage (SWS). Preliminary results indicated a strong relationship between MAP and mean vessel percentage. Significant negative correlations were found between site index (SI) and other properties: mean density, mean vessel frequency, mean vessel percentage. Mean wood density, in some instances, showed a trend of decline with increasing water availability. Vessel frequency declined significantly with MAP at medium and high SWS levels. Relationships between water availability and wood properties are expected to become clearer and more robust as the study progresses.