Forest Stand300 kg N/ha

Forest Floor104 kg N/ha

Forest Floor243 kg N/ha

Soil9800 kg N/ha

Litterfall9.7 kg N/ha/yr

Uptake5.3 kg N/ha/yr

Leaching5.1 kg N/ha/yr

Uptake24.1 kg N/ha/yr

Leaching21.4 kg N/ha/yr

Litterfall29.4 kg N/ha/yr

Responsive Stand Unresponsive Stand

Soil1960 kg N/ha

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.

•.

Predicting Nitrogen Fertilizer Response in Douglas-fir Plantations

Kim LittkeSMC, Rob HarrisonSMC, Dave BriggsSMC, Scott HolubWY, Doug MaguireOSU, Darlene ZabowskiUW

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.

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.

Figure 2. Current Paired Tree Study locations in the Pacific Northwest. Parent materials are labeled as follows: Glacial, Sedimentary, Igneous.

Soil Nitrogen Content (kg/ha) by Installation

Air Temperature by Installation

Figure 3. Paired Tree Treatment Plots on a 15 m Grid

Figure 1. Nitrogen Cycles in Stands Expected to be Responsive and Unresponsive to N Fertilization

542 3 6

11108 9 12

171614 15 18

232220 21 24

1

7

13

19

25 292826 27 30

10 meters

15 meters

Preliminary Results: Determining Differences between Installations

Relative Humidity by InstallationDBH (cm) Range by Installation

Soil Moisture Content (m3/m3) at 5 cm by Installation Soil Moisture Content (m3/m3) at 30 cm by Installation

35

30

25

20

15

10

DB

H

8 4 0

Frequency

840 8 4 0 840 8 4 0 840 8 4 0

35

30

25

20

15

10

840

829828826825824823822821

Installation