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Hypotheses: Soil nitrogen content is negatively correlated with N fertilizer response (Figure 1).

Predicting Nitrogen Fertilizer Response in Douglas-fir Plantations. Kim Littke SMC , Rob Harrison SMC , Dave Briggs SMC , Scott Holub WY , Doug Maguire OSU , Darlene Zabowski UW. Sites: 15-25 year old Douglas-fir stands in British Columbia, Oregon, and Washington (Figure 2).

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Hypotheses: Soil nitrogen content is negatively correlated with N fertilizer response (Figure 1).

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  1. Predicting Nitrogen Fertilizer Response in Douglas-fir Plantations Kim LittkeSMC, Rob HarrisonSMC, Dave BriggsSMC,Scott HolubWY, Doug MaguireOSU, Darlene ZabowskiUW • Sites: • 15-25 year old Douglas-fir stands in British Columbia, Oregon, and Washington (Figure 2). • Located in Sitka spruce and western hemlock vegetation zones. • Range of landform, elevation, and position on slope. • Range of parent materials. • Introduction: • Nitrogen fertilization is a common practice in forests of the Pacific Northwest. • The Regional Forest Nutrition Research Project was implemented in the 1960s to study the effect of N fertilization over western Washington and Oregon. • However, growth response to N fertilization was widely variable. • This study will attempt to determine soil and non-soil factors that best predict N fertilization response. • Hypotheses: • Soil nitrogen content is negatively correlated with N fertilizer response (Figure 1). • Sites where soil moisture is low enough to limit transpiration will not be able to utilize N fertilization. • Sites with high LAI will not respond to N fertilization. • . Figure 2. Current Paired Tree Study locations in the Pacific Northwest. Parent materials are labeled as follows: Glacial, Sedimentary, Igneous. Figure 1. Nitrogen Cycles in Stands Expected to be Responsive and Unresponsive to N Fertilization • Materials and Methods: • Select 48 circular plots (80 m^2) in a stand with uniform slope and aspect (Figure 3). • Pair trees according to the best matches for the DBH and live crown height. • Fertilize one tree of each pair with 227 kg N/ha. • Determine LAI, elevation, slope, aspect, and slope shape. • Collect precipitation using rain gauges. • Measure soil moisture and temperature at different temperatures. • Sample soil for bulk density and nutrient content to 1 meter. • Sample known area of forest floor. • Use a pole pruner to collect terminal flush and all current year lateral branches. • Preliminary Results: Determining Differences between Installations Air Temperature by Installation Relative Humidity by Installation DBH (cm) Range by Installation Figure 3. Paired Tree Treatment Plots on a 15 m Grid Soil Nitrogen Content (kg/ha) by Installation Soil Moisture Content (m3/m3) at 5 cm by Installation Soil Moisture Content (m3/m3) at 30 cm by Installation

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