Risk to Long-term Site Productivity Due to Whole-tree Harvesting in the Coastal Pacific Northwest

1. Risk to Long-term Site Productivity Due to Whole-tree Harvesting in the Coastal Pacific Northwest Austin Himes1,2, Rob Harrison1, Darlene Zabowski1, Eric Turnblom1, David Briggs1, Warren Devine3, Kimberly Hanft1 (1) School of Environmental and Forest Sciences, University of Washington, Seattle, WA; (2) Greenwood Resources, Portland, OR; (3) Engineering and Environment, Joint Base Lewis-McChord, WA Results and Discussion Introduction There has been worldwide concern regarding the rate of nutrient removal inherent to whole-tree harvesting, but in many regions, relationships between site characteristics and risk of nutrient depletion are not well understood. In the coastal Pacific Northwest region of North America, depletion risk of the most growth-limiting nutrient, N, was predicted for 68 intensively managed Douglas-fir plantations varying widely in productivity (see Figure 1). Under whole-tree harvesting, 49 percent of the sites in the study were classified as potentially at risk of long-term N depletion (i.e., stability ratio ≥10), whereas under stem-only harvesting, only 24 percent of sites were at risk. Six percent and one percent of sites were classified as under high risk of N depletion (i.e., stability ratio ≥30) under whole-tree and stem-only harvesting, respectively (see Figure 2). The simulation suggested that, given plantations of similar planting density and age, sites with less than approximately 9.0 and 4.0 Mg ha-1 of total site N are at elevated risk for long-term N depletion and productivity loss under repeated whole-tree and stem-only harvests, respectively (see Figure 3). The highest concentration of at-risk sites were those with young soils derived from glacial materials on Vancouver Island, Canada and in the Puget Sound region of Washington. Management for conserving N, and possibly fertilization with N, may be necessary for sustained productivity. Figure 2.Estimated N removal under WTH and SOH for 68 Douglas-fir plantations, by site N store (total soil N to 1.0-m depth plus forest floor) and parent material. The solid line represents a stability ratio of 0.3 and the dashed line represents a stability ratio of 0.1. Methods To estimate harvest removals under whole-tree harvesting (WTH) and stem only harvesting (SOH) systems, the growth of each of the 68 stands was simulated from mid rotation measurements to a rotation length of 50 to 55 years using the individual-tree growth model ORGANON (SMC variant; Hann 2011). The biomass and N content of Douglas-fir tree components was estimated by first determining the biomass of the stemwood using specific gravity of Douglas-fir at a particular study site, according to the western wood density survey (Forest Products Laboratory 1965), and stemwood volume output from ORGANON. Based on individual-tree stem biomass estimates, total aboveground biomass and component biomass (i.e., stem wood, stem bark, coarse roots, and foliage) was estimated using the component biomass ratio equations of Jenkins et al. (2003). The N content of total aboveground biomass and components was estimated for each site using biomass estimates in combination with N content equations from Augusto et al. (2000). N stability ratios (N removal / N store)were then calculated for each site where N removal is defined as N content of the total aboveground tree (WTH) or N content of stem wood+stem bark (SOH) and N store were estimated as the sum of soil, forest floor, and residual tree component N content. Stability ratios were used to rate the risk of N depletion as suggested by Evans (1999, 2009). Figure 3.Stability ratios for whole-tree and stem-only harvest in 68 coastal Pacific Northwest Douglas-fir plantations, by site N store (total soil N to 1.0-m plus forest floor). Figure 1. Locations of 68 Douglas-fir plantations evaluated in this study by site N store (total N content of soil to 1.0-m depth, or compacted layer, plus forest floor). References Augusto L, Ranger J, Ponette Q, Rapp M (2000) Relationships Between Forest Tree Species, Stand Production, and Stand Nutrient Amount. Ann. For. Sci. 57:313-324. Evans, J. 1999. Sustainability of Forest Plantations: The Evidence—A Review of Evidence Concerning Narrow-sence Sustainability of Planted Forests. London, United Kingdom: Department for International Development. 64 p. Forest Products Laboratory (US). (1965). Western wood density survey: report (No. 1). US Dept. of Agriculture, Forest Service, Forest Products Laboratory. Hann, D.W. 2011. ORGANON User’s Manual Edition 9.1. Available from http://www.cof.orst.edu/cof/fr/research/organon/downld.htm#man. Jenkins, J.C., D.C. Chojnacky, L.S. Heath, and R.A. Birdsey. 2003. National-scale Biomass Estimators for United States Tree Species. For. Sci. 49(1):12-35.