Forest Biomass Harvesting and Site Productivity: Is Policy Ahead of Biogeochemical Science?

Emerging bioproduct and energy markets provide incentives for harvesting greater quantities of biomass at shorter intervals and have raised environmental concerns, including effects on the productive capacity of forested sites. In response to these concerns, governments and non-governmental organizations have developed biomass harvesting guidelines (BHGs) with provisions for retaining specific proportions or quantities of biomass on site and restricting harvests on sites deemed sensitive. These guidelines are largely voluntary but may be incorporated in some form into forest practice mandates and certification systems. BHGs are well intentioned and based on a reasoned, conceptual understanding of the role of harvest residues in sustaining soil organic matter, nutrient availability, and future site productivity. Management restrictions come with economic and environmental costs, however, and the science supporting them deserves greater scrutiny. Field experiments show that forest responses to biomass harvesting vary widely and are often counterintuitive. With site-specific data lacking, BHGs tend to rely on default assumptions supported by best professional judgment. These include (i) the natural or unmanaged state is an ideal frame of reference, (ii) conventional harvesting retains and distributes most residues across the site, (iii) biomass harvesting removes virtually all residues, (iv) decomposing residues always enhance soil C and site productivity, (v) biomass harvesting is conducted without operational practices that alleviate site deficiencies and sustain productivity, and (vi) changes in forest state are equivalent to changes in forest function. While harvesting-induced nutrient deficiencies can be prevented or corrected with fertilizers or other soil amendments, soil disturbance and exposure may warrant greater attention. Effective BHGs are science-based, operationally feasible, and protect values of interest while allowing managers the flexibility to prevent or mitigate potential impacts within constraints imposed by existing forest practice rules, best management practices, and forest certification provisions.