CONCLUSIONS

The heavy equipment necessary to remove infected stumps to mitigate growth loss and mortality from root rot in regenerated timber stands obviously impacts the soil. The major concern for forest managers has been the suspected increase of bulk density caused by heavy equipment, which may lead to growth reduction in succeeding timber stands. This study compared plots treated with stump removal with untreated plots on five sites in the Pacific Northwest. Measurements were taken 23 to 27 years after treatments were applied. Some long-term effects on soil caused by stump removal operations on these sites can be seen from this study.

Bulk density of the soil at the sites studied were increased by the stump removal operation at all sites except Gates, but this increase is slight, averaging 3% twenty-plus years after treatments, and is not likely to have a significant impact on the growth of the succeeding stand. Bulk densities measured at Sweethome 11 and 22 years after stump removal showed a decline of bulk density of 7% in the plots with stumps removed in the 11 years between measurements. At Sweethome, average bulk densities in stump removal and no stump removal plots are now similar, indicating a recovery from the stumping procedure in slightly over 20 years. The increases in bulk density resulting from stump removal was statistically significant after 20+ years only in the finer-grained soils at Apiary and LaGrande. If the soils of a site are not highly susceptible to compaction, and are dry at the time of stump removal, increases in bulk density do not appear to have an impact that will significantly reduce productivity.

Soil and forest floor carbon and nitrogen concentrations were measured in these treated and untreated plots. These measurements were taken to assess the effects of a one-time nitrogen application designed to reduce Phellinus weirii survival and increase seedling survival made immediately after the treatments. The results from these nitrogen measurements showed lower mineral soil nitrogen concentrations in the stump removal plots at all five sites, a reduction averaging 20%, and a statistically significant difference caused by the stump removal at all sites except Hoodsport. The mineral soil carbon concentrations were reduced in the stump removal plots at all sites, a reduction averaging 22%, this was a significant difference due to stump removal at all sites except Hoodsport. Forest floor depth was measured, and the average was lower in the stump removal plots at all sites, averaging 24% lower. These three measures indicate a loss of soil organic matter resulting from the stump removal operation. Stump removal operations on low productivity sites are likely to be impacted by large reductions in soil nitrogen and organic matter, and managers should look closely at trade-offs between losses of site productivity and the gains from mitigating root rot. The long recovery time needed to restore soil organic matter and associated soil health and nutrient cycling must be considered.

The five different soils at these sites showed similar responses to stump removal, with small increases in bulk density at all sites except Gates, and large reductions in mineral soil nitrogen, mineral soil carbon and forest floor depth at all sites. A GIS analysis of the data suggests that inherent site variability may affect bulk density at Gates, and nitrogen at Hoodsport and Sweethome, but this does not alter the similarity in results from the different soil types.

The reduction in mineral soil nitrogen and carbon, and forest floor depth increases from unharvested controls, to clear-cut only, to clear-cut with stump removal, at all sites except Hoodsport, indicating increasing losses of soil nitrogen and organic matter with increasing disturbance.

The practical results of the stumping study are straightforward. The bulk densities are not elevated enough to cause growth reduction. The lowered soil nitrogen and organic matter may be important to future site productivity. Managers should consider the productivity of a site when deciding whether to remove infected stumps or plant resistant species. Managers utilizing stump removal should consider factors infuencing the levels of disturbance such as stump size, slope, equipment and operator skill. Minimizing the area and amount of disturbance of soil and forest floor caused by the stumping operation is the key management quality criteria.