The effect of land preparation at re-establishment on the productivity of fast growing hardwoods in South Africa

Abstract

Appropriate land preparation at re-establishment is of major importance to forest managers to ensure maximum survival and vigorous early growth of South African plantation forests. This work revisits a series of trials carried out over the last two decades in an attempt to evaluate the long-term effects of land preparation at re-establishment on the productivity of fast—growing hardwoods (Acacia mearnsii and Eucalyptus spp.). A wide range of land preparation techniques was tested: ploughing, ripping, subsoiling, ridging and complete site preparation (de-stumping, ripping and discing) inter alia. Of the eleven trials originally planted, five reached rotation age (between 9 and 11 years) and, apart from one which was destroyed by a storm at 5 years, the rest, at time of writing, were close to clearfelling. With the exception of one trial where significant growth responses were recorded, and apart from a few significant lower-order interactions, final survival, basal area and volume were not affected by land preparation when compared to a standard pitting operation. Overall, growth responses in terms of basal area and volume to various forms of surface cultivation, ripping, subsoiling and terracing ranged between +21% and—23% for all species tested. Intensive site preparation (e.g. de-stumping, ripping/subsoiling and discing) has generally not resulted in improved growth of E. grandis or A. mearnsii across a range of sites and in some cases even resulted in a reduction in site productivity. Erratic growth responses of Eucalyptus spp. and A. mearnsii to ripping were recorded. Forest management should focus efforts on less intensive and more cost-effective methods of re-establishment such as pitting, shallow ripping to mark a planting line, or surface ploughing unless a known growth limitation exists (such as a root-limiting compacted B horizon overlying a horizon with available water). It is conjectured that the lack of major growth responses to intensive site preparation is due to a combination of the good physical condition of forest soils in the summer-rainfall growing region of South Africa, and beneficial perforation effects of tree roots from previous rotations. The results imply that topsoil disturbance and mixing of soil layers caused by intensive site preparation may pose a threat to long-term site productivity.