INTRODUCTION

The pathogen Phellinus weirii (Murr.) Gilb. causes root rot that has reduced growth and caused mortality in timber stands regenerated with Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in the Pacific Northwest(Thies and Sturrock 1995). Planting Douglas-fir after clear-cutting is a widely used silvicultural practice in the Pacific Northwest. Phellinus can survive as a saprophyte in roots and harvested stumps, then spread by root contact to susceptible planted seedlings (Bloomberg and Reynolds 1982; Thies 1983). Traditional management practices to mitigate damage from P. weirii are planting resistant tree species, or removing the harvested stumps and roots, and thus the pathogen, after harvest. Removing stumps is a costly and invasive treatment, but it has been shown to be effective in reducing pathogen levels (Thies 1984; Morrison et al. 1988). This study focuses on the long term effects on soil bulk density and soil nitrogen levels caused by stump removal to control Phellinus in managed timber stands.

Forest harvest rotations take place over many years, but are generally more frequent and severe distubances than the ecosystem normally experiences. To maintain forest productivity, the long-term effects of intensive forest practices such as stumping must be considered as well as short-term effects and gains. Concerns about the damage done to the soils and thus productivity on stumped sites has led to a number of studies to assess the effects of these treatments (Smith and Wass 1994; Wass and Smith 1997; Page-Dumroese et al. 1998; Quesnel and Curran 2000). This study is a continuing look at effects on forest soil bulk density and soil nitrogen in regenerated clear cuts 20+ years after stumping operations took place at five sites in Washington and Oregon (Thies and Nelson 1988; Thies et al. 1994).

It is hypothesised that the use of heavy equipment will cause increases of bulk density that may reduce tree growth in the regenerating stand. New data were analyzed from an ongoing study by Walt Thies of the USFS of five P. weirii infected sites that were clear-cut and planted with the susceptible species Douglas-fir. Research plots were established on the sites with and without stump removal. Earlier studies had shown decreases in P. weirii survival due to nitrogen applications (Nelson 1975), so four different nitrogen application rates were tested on the plots in a factorial design. For this study, measures of bulk density were obtained using a neutron probe. To determine long-term effects of nitrogen applications, soil samples were taken from each plot as well as forest floor samples and measures of forest floor depth. The soil and forest floor samples were subjected to CHN analysis to determine total nitrogen and carbon concentrations.

The design of study provided mapped locations for all the plots. This enabled the location of sample points in maps created with Geographic Information System (GIS) software. Analysis and mapping of the data using ArcGis (ESRI 2001) is presented in the third chapter of this thesis project.