CHAPTER I
LONG-TERM EFFECTS OF STUMP REMOVAL FOR ROOT DISEASE CONTROL ON FOREST SOIL BULK DENSITY AND NITROGEN

INTRODUCTION

Phellinus weiiri is a native pathogen in the forests of the northwestern United States causing laminated root rot and mortality in Douglas-fir and other susceptible tree species. Timber losses caused by P. weirii on the west side of the Cascade Mountains have been estimated at 4.4 million cubic meters per year (Nelson et al.1981). This facultative saprophyte is a natural part of the ecosystem, present in most Douglas-fir forests of the Pacific Northwest (Thies and Sturrock 1995). P. weirii is a basidiomycete whose classification is still being defined. It is generally agreed that two closely related species can cause butt rot in western red cedar or mortality in Douglas-fir. The original name of Poria weirii (Murill 1914) was changed to Phellinus weirii (Gilbertson 1979). Recent studies suggest Phellinus sulphurascens as a proper name for the Douglas-fir type, and P. weirii for the cedar type. Although further studies may result in both species being classified in the genus Inonotus (Banik et al. 1993; Larsen et al. 1994), current protocol is to use the name Phellinus weirii for the Douglas-fir form until agreement on nomenclature is widely recognized by forest pathologists.

In a natural stand, the tree mortality caused by P. weirii creates openings that will be occupied by resistant or early successional plant species, and creates habitat changes that increase overall species diversity in the forest. Reproduction from spores is not known to be an important factor in spread of P. weirii in a stand. Spread of the fungus is usually vegetative by root contact. In a managed forest, P. weirii can exist as a saprophyte in the stumps and roots of the harvested trees. When seedling roots come in contact with this remaining P. weirii the parasitic stage resumes and can cause growth reduction, mortality and economic loss, especially in harvested stands that are replanted for Douglas-fir timber production. (Bloomberg and Wallis 1979; Thies 1983; Bloomberg and Reynolds 1985). Spread of the fungus by root contact can create infestations many hectates in size (Thies and Sturrock 1995). Clear-cutting followed by planting Douglas-fir has been a widely used silvicultural prescription in the Pacific Northwest, especially west of the Cascade Mountains. Controlling the spread of P. weirii in these managed stands has been a concern for forest managers, and removal of P. weirii infected stumps and roots before replanting has been shown to reduce the Phellinus inoculum presence and thus the mortality in the succeeding stand (Morrison et al. 1988; Thies 1994). Despite this success, the impact on long and short term site productivity and physical soil properties, especially bulk density, by heavy equipment needed to remove the stumps has been a concern in utilizing this method (Smith and Wass 1994; Wass and Smith 1997; Page-Dumroese et al. 1998; Quesnel and Curran 2000). Addition of nitrogen fertilizer has been suggested to accelerate the displacement of P. weirii by Trichoderma spp, the main fungal competitor invading the P. weirii infected root fragments left after the stump removal (Nelson and Thies 1985). Studies have shown that application of urea reduced P. weirii survival in buried wood cubes (Nelson 1975).

A long-term study has examined combined effects of stump removal and fertilization on root rot infections and tree growth and survival in the succeeding stand. Since 1978, replicated treatments with and without stumping and with four different nitrogen fertilizer treatment levels have been underway at three sites in western Oregon, one in western Washington, and one in eastern Oregon (Thies and Nelson 1988; Thies et al 1994). Growth and soil parameters were measured in harvested stands now stocked with 20-year-old Douglas-fir.

To assess the continuing long-term effects of these treatments on forest soils, soil bulk density, mineral soil total nitrogen and forest floor were measured at all sites. This report summarizes the findings from these 2003 observations. There has been concern that the removal of stumps with a bulldozer will increase the soil bulk density enough to reduce growth in the succeeding stand. Soil bulk density measurements were taken in treated and control areas. Soil displacement and disturbance has been shown to reduce nitrogen levels and site productivity (Vitousek and Matson 1985a, Dyck and Beets 1987). These concerns, along with the long term effects of the nitrogen fertilization treatments, are addressed with measures of total nitrogen taken from the mineral soil and forest floor. The forest floor was sampled for depth and mass. This report focuses on the soil aspects of the stumping treatments. These soil measures will be related to the growth and survival of the planted trees, site productivity, and the presence and pathogenic capacity of P. weirii as the study continues.