Public and market-driven concerns over the potential environmental impacts of clearcut harvesting led to the introduction of alternative silvicultural systems, or variable-retention (VR) harvesting, in British Columbia in the 1990s. These harvesting techniques are now used, or are being considered for use, in most ecosystems throughout the province. Concomitant with the introduction of operational VR harvesting, a number of field trials were established in different biogeoclimatic (BEC) zones throughout BC to verify the effects of these new harvesting techniques on ecosystem processes. Some of these trials have now been monitored for almost 10 years, and the results of many individual studies have been published in the scientific literature. However, a critical task remains: synthesizing the findings from the diverse studies carried out in these different ecosystems. The productivity of most temperate and boreal forests is primarily limited by low soil nitrogen (N) availability. Early work in Scandinavia and the eastern US suggested that clearcut harvesting could lead to increased loss of N through leaching from sites. This leaching loss is thought to arise from harvest-induced changes to a number of key N-cycling processes, including increased rates of litter and forest floor decomposition, and increased rates of N mineralization and nitrification. Concerns were therefore raised about the potential for clearcutting to result in decreased site productivity because of leaching losses, and in environmental and health problems caused by increased nitrate in aquatic ecosystems and sources of drinking water. In BC, it is usually assumed (in the absence of data) that loss of N following clearcutting would be mitigated by using variable-retention harvesting. We therefore synthesized long-term results from silvicultural systems trials to determine, at a provincial level, the effect of different harvesting methods on nitrogen cycling processes. We determined: ? the extent to which VR harvesting retains the biological legacies of mature stands, using N cycling processes as indicators; ? which method or intensity of VR harvesting is the most appropriate for mitigating negative impacts; ? differences in responses of forests to harvesting in a range of ecosystems, and evaluate whether responses are universal or specific to individual ecosystems; ? whether elevated levels of available N after harvesting in BC are universally caused by microclimatic and/or litter input changes such that mitigation measures could be recommended.
Cindy Prescott and Lucie Jerabkova.
Prescott, Cindy E., Jerabkova, Lucie. 2006. Synthesis and extension of research on the nutritional sustainability of variable retention harvesting. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report