Does soil compaction on harvesting extraction roads affect long-term productivity of <i>Eucalyptus</i> plantations in Zululand, South Africa?: scientific paper

Abstract

The effect of soil compaction on the growth of Eucalyptus grandis and two clonal hybrids (Eucalyptus grandisx urophylla (E. gxu) and Eucalyptus grandis x camaldulensis (E. gxc)) was evaluated on harvesting extraction roads at three sites in the Zululand region of KwaZulu-Natal. Significantly lower initial survival was observed in the extraction road compared to the uncompacted area for E. gxu and E. grandis, the effect being site dependent whereas no significant differences were observed for E. gxc at any site. Soil compaction resulted in significantly lower tree growth on the extraction road at 8 years on a Constantia soil (4 to11% clay content) for all species/clonal hybrids (8 to 26% decrease) but there was no significant effect on tree growth at either of the other sites (8 and 5 year old stands) where the soils had a coarser texture. Even when a significant growth effect was observed in the extraction road the net effect on the compartment was reduced with increasing width between the extraction roads. Thus a 26% growth loss on an extraction road corresponded to a 3, 7% decrease in volume in the compartment in a 7th row extraction system. The paper emphasises the difficulty in quantifying growth losses since growth varies in a cyclical manner with distance from the extraction road. The lack of substantial growth losses on old extraction roads is attributed to relatively low levels of soil strength even when compacted, increasing available water capacity with increasing bulk density on very sandy soils, non-critical aeration levels when compacted and the effects of old tree roots providing access through compacted zones. This work has shown that, provided controlled traffic is practised rotation after rotation, the effects of harvesting extraction roads on the productivity of Eucalyptus plantations on sandy soils in Zululand are not pronounced.


Keywords: Harvesting impacts, Soil compaction, Soil damage, Zululand, Long-term site productivity


Southern African Forestry Journal No.199 2003: 41-54