Idaho forest growth response to post-thinning energy biomass removal and complementary soil amendments

Abstract

Utilization of woody biomass for biofuel can help meet the need for renewable energy production. However, there is a concern biomass removal will deplete soil nutrients, having short- and long-term effects on tree growth. This study aimed to develop short-term indicators to assess the impacts of the first three years after small-diameter woody biomass removal on forest productivity to establish optimal biomass retention levels for mixed-conifer forests in the Inland Northwest region, and to evaluate the ability of soil amendments to compensate for potential adverse effects from biomass removal. We examined impacts of four biomass retention-level treatments at two study locations: full biomass removal (0x), full biomass retention (1x), double biomass retention (2x), and unthinned control. We combined biomass retention with four soil amendment treatments: biochar (B), fertilizer (F), fertilizer and biochar combined (FB), and an untreated control (C). We considered treatment effects on basal area and total stem volume growth for all trees per plot (plot trees) and for the six largest trees per plot (crop trees). Biomass removal had no effect on plot (P > 0.40) or crop tree growth (P > 0.65) compared to normal biomass retention. High biomass retention (2x) decreased plot tree growth as compared to normal biomass retention (1x) levels (P < 0.05) after three years. This growth difference was not explained by soil moisture, temperature, or nutrient uptake. While there were strong tree growth differences between study locations, patterns of biomass and amendment treatment responses did not differ. Fertilizer increased basal area growth and total volume growth (P < 0.10) as expected, because nitrogen is limiting in the region. Biochar had no effect on tree growth (P > 0.47). Initial findings after three years suggest removing small-diameter biomass for biofuel feedstocks is feasible in the Inland Northwest without negative impacts on tree growth.