Tree Growth, Soil Quality and Biodiversity Response to Forest Biomass Removal in the Inland Northwest.

The amount of biomass utilization during harvesting can be important for sustaining forest productivity.  There is concern that excessive or frequent biomass removal will detrimentally affect future forest productivity and belowground processes; however, there are not adequate guidelines for acceptable biomass removal and associated tree growth responses.  Inadequate guidelines results from the time (decades) needed to achieve meaningful tree growth response following harvest treatments.  Other response variables besides growth may also indicate tolerable amounts of biomass utilization. 

To consider potential indicators, we are monitoring impacts of biomass removal and retention in several ongoing long-term research projects.  Some projects include post-harvest amendments such as herbicides for competition control, fertilizer for nutrient replacement, or biochar for carbon replacement.  We also monitor seedling growth following clearcutting and residual tree growth following thinning.  Monitoring of indicator processes include soil moisture and temperature, nutrient fluxes, soil microbial activity, and diversity of invertebrates and soil microbes. 

Soil moisture is typically higher and soil temperature is typically cooler under residual slash treatments, which corresponds with decreased seedling growth and increased post-thinning tree growth following thinning.  We attribute few within location trends in soil CO₂ efflux or soil microbial responses to high spatial variability within study plots.  In contrast, significant within-location responses of insect taxa to amendment treatments show that invertebrates are early indicators of biomass utilization impacts.  None of the amendment treatments substitutes for all of the characteristics that result from retained biomass.  Herbicide applications result in dramatic improvements to seedling growth corresponding with increases in both soil moisture and temperature.  Fertilizer response exceeds that of biomass retention in thinned stands while applications of char have little effect relative to biomass removal treatments.  We will continue monitoring to see if short-term responses continue and if they translate into long-term effects on stand growth and development.