The effects of compaction and residue management on soil properties and growth of Eucalyptus grandis at two sites in KwaZulu-Natal, South Africa.
Rietz, Diana Nicolle.
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Concerns have been raised over the long-term site productivity (LTSP) of short rotation plantation forests, such as those of Eucalyptus, in South Africa. This is because diminished productivity of long rotation plantations overseas has been found to be generally due to decreases in soil porosity and organic matter. Since soil porosity and organic matter in plantations are mainly affected by soil compaction by harvesting machinery and residue management, the more frequent harvesting of short rotation plantations are of particular concern. Therefore the effects of soil compaction and residue management on soil properties at two sites, one a low organic carbon, sandy soil (Rattray), the other a high organic carbon, clay soil (Shafton) were investigated. The potential of early E. grandis productivity as an indicator of changes in soil properties at these sites was also evaluated. Three different levels of compaction (low, moderate and high) were applied to the sites by three methods of timber extraction, i.e. manual, logger and forwarder loaded by a logger, respectively. Three types of residue management, i.e. broadcast, windrow and residue removal were also applied. A factorial treatment design was used to ensure a resource-efficient study that allowed separation of main and interaction effects. Various soil physical and chemical properties were measured at intervals from before treatment implementation, until approximately 44, and 38 months after treatment implementation at Rattray and Shafton, respectively. Trees were planted at a commercial espacement at both trials, and their growth monitored over the same time period. In addition, to accelerate early growth, negate silvicultural variation, and determine changes in stand productivity with treatments, a portion of the treatment plots were planted at a very high density and harvested when these trees reached canopy closure at about six months of age. Moderate and high compaction treatments at both sites resulted in significant increases in penetrometer soil strength, and often in bulk density. Increasing residue retention decreased the compaction effects of machinery and, generally, increased the total quantity of nutrients contained in residues and soil. Changes in soil bulk density and organic matter as a result of the treatments in turn affected soil water characteristics, generally decreasing plant available water capacity with increasing compaction intensity and residue removal. Tree growth measurements showed that at both sites, tree productivity was negatively affected at some point by increasing compaction. In contrast, residue management only significantly affected tree growth at Shafton, initially increasing and later decreasing growth with residue removal. These variations in tree growth over time in response to treatments are most likely a result of changes in tree characteristics that occurred with age. In addition, trees did not always reflect changes in soil properties that may affect LTSP, most likely because these soil properties had not yet reached levels that would affect tree growth. It was therefore concluded that early tree growth is not always a good indicator of changes in LTSP, and that soil properties are a more reliable indicator. Plantation management practices that lead to soil compaction and residue removals will negatively impact LTSP in South Africa. However, variable responses of the two soils indicate that soils vary in their sensitivity to compaction and residue management. This therefore needs to be quantified across a range of major soil types in the South African forestry industry.