Nutrient Limitations on Intensive Biomass Production in PNW Douglas-Fir Plantations.

Nutrient Limitations on Intensive Biomass Production in PNW Douglas-fir Plantations. Rob Harrison, Kim Littke, Austin Himes, Erika Knight, Jason James, Christiana Dietzen, Stephani Michelsen-Correa, Marcella Menegale, Irae Guerrini, Eric Turnblom, Marcia Ciol, Scott Holub, Greg Ettl and Tom Terry. Nutritional management for bioenergy in the Pacific Northwest includes a wide variety of approaches for managing nutrients in forests that may have additional biomass and associated nutrients removed compared to traditional harvest. The first step in nutritional management is identifying where nutrients are in the forest ecosytem, including their physical, chemical and biological state (nutrient status), and whether or not they are changing over time. The mere presence of a particular nutrient within a forest does not mean it is available for the use of the growing trees, and transformations of nutrients from an unavailable to an available state during stand development is a determining factor in productivity more than the total nutrient in the ecosystem, but knowing the total amounts and locations of nutrients is an important first step. In the coastal North American Pacific Northwest, we estimated the risk of depletion of nitrogen (N), the regionŐs most growth-limiting nutrient, for 68 intensively managed Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco) plantations varying widely in productivity. We projected stands to rotation age using the individual-tree growth model ORGANON and then calculated a stability ratio for each stand, defined as the ratio of N removed during harvest to total site N store (soil and forest floor). We assigned a risk rating to each site based on its stability ratio under whole-tree and stem-only harvest scenarios. Under whole-tree harvest, 49% of sites were classified as potentially at risk of long-term N depletion, whereas under stem-only harvest, only 24% of sites were at risk. Six percent and 1% of sites were classified as under high risk of N while under whole-tree and stem-only harvest, respectively. The simulation suggested that sites with <9.0 and <4.0 Mg ha_1 site N store are potentially at risk for long-term N depletion and productivity loss under repeated whole-tree and stem-only harvest, respectively. Sites with <2.2 and <0.9 Mg ha_1 site N store are at high risk of N depletion under whole-tree and stem-only harvest, respectively. The areas with the highest concentrations of at-risk sites were those with young, glacially derived soils on Vancouver Island, Canada, and in the Puget Sound region of Washington.