Effects of establishment and management of a Douglas fir plantation on high country soil fertility

The objective of this research project was to determine the effect of afforestation with Douglas fir (Pseudotsuga menziesii) on the concentrations, amounts and distribution of organic matter and selected nutrients in soil previously under extensively grazed tussock grassland in the Mackenzie basin of the South Island. The study site was an existing Douglas fir plantation trial established in 1982 by the New Zealand Forest Research Institute at Ribbonwood Station in the Ahuriri catchment near Omarama. The main objective was to investigate the effect of stand density on timber production and the trial consists of 59 plots. The treatments selected for evaluation in the present study were three replicate plots each of unthinned forest (c. 1 000 stems per hectare), moderately thinned forest (thinned to 500 stems per hectare and pruned to 4m crown in 1993), and severely thinned forest (thinned to 250 stems per hectare and pruned to 4m crown in 1993). In addition, two adjacent areas under the original pasture vegetation were included for comparison. In 1997 soil samples were taken to 80 cm depth under forest and pasture, and the weights of soil in the various depth increments were recorded. A high level of inherent soil variability was evident within the Ribbonwood trial area which made effective comparison between the different treatments very difficult. This variability was probably due to a combination of factors including parent material, topography and past forest vegetation. Nonetheless, results showed that concentrations of organic carbon, total nitrogen and exchangeable cations (calcium, magnesium, potassium) were generally lower in soil under unthinned forest compared with thinned forest and adjacent pasture. These findings indicated that the practice of thinning and pruning reduced the impact of afforestation on soil organic matter and nutrient reserves during the first 15 years of the rotation, which in turn was attributed to a combination of reduced nutrient uptake, the return of significant quantities of organic matter and nutrients in thinning and pruning slash, and the persistence of a pasture understorey. The overall effects of the different forest treatments on soil properties at the Ribbonwood site are likely to change in the future with continued stand development. It is clear that the land use change from pasture to plantation forestry can have an impact on the chemical properties of soil, and further work is required to clarify and quantify these effects and assess the likely long-term implications for soil fertility and sustainable land management in New Zealand