Juvenile tree growth on some volcanic ash soils disturbed by prior forest harvest.

Abstract

The effects of mechanical disturbance from traditional ground-based logging and site preparation on volcanic ash soil and associated tree growth were investigated by using two study approaches in a retrospective study. This research was conducted on volcanic ash soils within previously harvested units in the Blue Mountains of northeast Oregon and southwest Washington. We assessed soil and tree attributes and their association with higher and lower levels of soil disturbance. The two approaches were independent efforts that yielded very different results.In the first study approach, we used larger measurement plots linked to a portion of some randomly established monitoring transects in 10 harvest units. Sets of higher and lower disturbance plots were chosen and positioned in each unit based on predetermined soil bulk densities along the midlines of the plots. Crop trees in the plots were then measured. Although our two sets had significantly different bulk densities, we found no associated differences between sets of tree attributes.The second approach involved soil disturbance in five harvest units. Therein we mainly used visual cues to identify portions having higher and lower levels of soil disturbance. Smaller plots were purposely assigned and positioned within some of those conditions. Plot sampling of soils and trees followed, and the results reflected significant differences in soil disturbance, i.e., 24 percent greater bulk density (from compaction) and 38 percent less ash thickness (from displacement and compaction) between disturbance levels. Soil disturbance differences were associated with 17 to 29 percent less juvenile tree size and growth. Longer term effects are unknown.