Site quality impacts tree form, heartwood content and veneer production of plantation-grown Pinus patula

Abstract Plantation forest species were introduced into South Africa due to limited availability of native forests for wood-derived products. Currently, the Mexican pine species, Pinus patula, is the most widely planted softwood species in the country. To study the effect of growth environment on wood and processing properties for the species, sample plots were established in a 20-year rotation covering a wide range of soil geologies and altitudes in Mpumalanga, South Africa. Temperature and seasonal rainfall were also determined for the sample plots. Randomly selected sample trees were harvested from the plots and processed at a plywood plant to determine veneer recovery and quality. Trees grown on sites composed of granite soils, with higher annual maximum temperatures and less rainfall, found in the Highveld region, displayed superior tree size, slenderness, and volume growth, compared to trees grown on dolomite and shale soils common to the Lowveld region. Veneer derived from Lowveld trees had more splits which were largely related to defects. Larger trees also had a greater percentage volumetric heartwood and a smaller live crown, compared to smaller trees. Highveld trees had greater net veneer recovery and produced better quality veneer than trees grown on the Lowveld. In the Mpumalanga forestry region, strong co-relatedness exists between soil geology, altitude, and climate. Although tree form and wood properties were found to differ with varying soil geology and altitude, these differences were primarily related to climate rather than soil properties. These findings highlight the pitfalls associated with neglecting either climate or soil properties when analysing site-specific growing conditions on tree growth and form.