Globally, concern over rising carbon emissions is driving intensive woody biomass harvesting for use as feedstock in the bioenergy sector. The rationale behind this trend is simple: using wood residue as a fuel source can provide energy products while dramatically reducing carbon emissions, potentially mitigating the effects of climate change. Scandinavian countries are already intensively harvesting woody residues for bioenergy production; Finland saw a 22-fold increase in woody residue harvesting from 1995 – 2003 (Walmsley et al., 2009), and stump harvesting is becoming increasingly common. Bioenery production in Canada is largely within the forestry sector, accounting for 6% of Canada’s energy needs (Bradley, 2006). With a 16.3 million ha mountain pine beetle outbreak, British Columbia is poised to drastically increase its woody residue harvesting (Ministry of Forests and Range, 2010). However, reduced carbon emissions may be negated if removal of forest residue reduces long-term carbon gains in aboveground biomass via loss of soil fertility associated with residue removal. Striking a balance that guarantees a reliable feedstock without compromising future forest productivity presents a challenge to foresters and ecologists – just how much of the dead stuff can we take?
The effects on site fertility of logging residue removal have been studied extensively since the 1970s. A common fear present throughout the literature is that harvesting of logging residue exports nutrients and organic matter off site, resulting in long-term loss of soil productivity. This research has largely concluded that the short-term effects of residue removal vary widely depending on site conditions; however, one observation remains fairly consistent: the nitrogen content of woody reside left on site increases over time. Woody residue can be likened to a nutrient sponge; it somehow soaks up nitrogen from the forest floor though the decay process.
Wood decay fungi have largely been hypothesized as responsible for the ‘nutrient sponge’ capacity of woody residue. While an excellent source of carbon, the primary requirement for growth of decomposing fungi, woody residue is a poor source of nitrogen. Without access to a more abundant nutrient source, fungal growth on wood is often limited. Wood decay fungi have overcome this limitation through use of their extensive networks of hyphae, which explore through the soil for nutrients. Once these nutrients are found, they are then transferred back through the hyphal network to the woody residue and used to decompose the abundant carbon resource, hence the nutrient sponge.
Recent research using radioisotopes has estimated that a common wood decay fungus, the sulphur tuft (Hypholoma fasciculare), can increase the nitrogen content of woody residue by more than 200%, suggesting that wood decay fungi can act as a strong sink for nitrogen (Philpott, 2012). How much, when, and in what form the fungal-scavenged nitrogen is released from woody residue remain important research questions. In northern temperate forests, nitrogen is the one of the most limiting nutrients for tree growth, and silvicultural prescriptions often aim to reduce post-harvest nitrogen losses via leaching. Given that fungi growing on woody residue are capable of accumulating nitrogen from the surrounding forest floor, promoting conditions to maximize this fungal-mediated sponge phenomenon may help retain nitrogen on site. This would involve ensuring woody residue is distributed homogenously throughout a cut-block and is in contact with the forest floor to promote fungal colonization.
Current regulations regarding woody residue management in British Columbia require retention of a minimum of 4 logs per hectare (Forest and Range Practices Act, 2010). While a recent report by British Columbia’s former Chief Forester provides guidance for woody debris management, binding regulations are limited to the 4-log rule (Snetsinger, 2010). Management guidelines for slash and smaller woody residue are non-existent and this material is often piled, and burned or left to decompose during site preparation. As it stands, this resource is largely wasted, but as markets for bioenergy products open in British Columbia, much of this logging residue may eventually be more thoroughly utilized.
If one of the goals of biomass harvesting is to lessen the impact of climate change by reducing the use of carbon-intensive energy products, then sound science should guide residue management that balances demand for feedstock with potential losses of site productivity through residue removal. The science has demonstrated that fungi growing on woody residue accumulate nitrogen through the decay process and this mechanism may represent an important nitrogen sink after harvest. The merits of biomass harvesting far outweigh other energy alternatives, but there are also risks to future forest productivity. As always, policy should strive to strike a balance between forest health and human demands for forest products.
Tim Philpott recently completed his MSc at UBC (Forest Sciences) and is currently a graduate intern at the the UBC Malcolm Knapp Research Forest.