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

Synopsis The effect ofsoil compaction on the growth of Eucalyptusgrandis and two clonal hybrids (Eucalyptus grandis x urophylla (E. gxu) and Eucalyptus grandis x camaldulensis (E. gxc)) was evaluated on harvesting extraction roads at three sites in the Zululand region ofKwaZulu-Natal. Significantlylower initial survival was observed in the extraction road compared to the uncompacted area for E. gxu and E. grandis, the effect being site dependentwhereas no significant differences were observed for E. gxc at any site. Soil compaction resulted in significantlylower tree growth on the extractionroad at8 years on a Constantia soil (4toll%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 compartmentwas 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 growthlosses since growthvariesin a cyclical mannerwith distance from the extraction road. The lackofsubstantial growthlosses onold extractionroads 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 shownthat, provided controlled traffic is practised rotation after rotation, the effects ofharvestingextractionroads onthe productivity of Eucalyptus plantations on sandy soils in Zululand are not pronounced.

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