Treatment Plots

Plot conditions for treatments studied at time of sampling.

Bole-only without vegetation control

BO-VC

Total Tree Plus with vegetation control

TTP+VC

Bole-only with vegetation control BO+VC

ObjectiveAssess the effects of organic matter removal and vegetation control on soil C for a 12 year old Douglas-fir plantation in western Washington, comparing:• bole-only versus total tree plus harvest (BO+VC vs. TTP+VC)• bole-only harvest with and without vegetation control (BO+VC vs. BO-VC)

Paul Footen drives the soil corer into one of the three composite sample locations within a TTP+VC subplot.

Effects of organic matter removal and competing vegetation control on soil C in a Pacific Northwest Douglas-fir plantation

Erika Knight1,4, Paul Footen1, Robert Harrison1, Thomas Terry2, and Scott Holub3

(1) School of Environmental and Forest Sciences, University of Washington, Seattle, WA; now Shannon and Wilson, Inc. Anchorage, AK(2) Weyerhaeuser Corporation (retired) and USDA Forest Service Pacific Northwest Research Station, Olympia, WA(3) Weyerhaeuser Co., Springfield, OR

Fall River Long-term Soil Productivity site, spring 2012

Forest floor sampling in a TTP+VC plot. Fall River LTSP

Study Site

Fall River Long-term Soil Productivity (LTSP) site:: Randomized block design Installed and base levels of C established in 1999 Mixed Douglas-fir and western hemlock prior to harvest 10-15% slopes Age 12 Douglas-fir plantation at time of sampling

Soil characteristics: Boistfort series Formed from weathered

basalt, some volcanic ash Deep and well-drained Silt loam to silty clay loam

texture, few stones High organic matter content Low bulk density

Conclusions No significant difference in total soil C content

between treatments (α=0.10). Numerical treatment differences in the carbon

content we observed primarily below 20 cm. No loss in C storage from post-harvest to 2012, but

there was a loss of forest floor C and gain of mineral soil C.

Methods• Each plot was divided into 1.0 m2 subplots. Six subplots

were randomly chosen for measurement.

• Three forest floor samples and three mineral soil cores were composited per subplot.

• Samples were dried, sieved (4.75mm), and analyzed to obtain %C, pH, LOI, bulk density, and C content.

Fall River plot layout; stars indicate plots sampled. Modified from Ares et al., 2007.

Soil sampling depths: Forest floor 0-15 cm 15-30 cm 30-45 cm 45-60 cm 60-100 cm

Results

C content to 100 cm was 18 Mg/ha greater for BO+VC than for TTP+VC

C content to 20 cm was 7.5 Mg/ha greater for BO+VC than for TTP+VC

Harvest IntensityBO+VC vs. TTP+VC

C content to 20 cm was 1.4 Mg/ha greater for BO+VC than for BO-VC

C content to 100 cm was 16 Mg/ha greater for BO+VC than for BO-VC

Vegetation ControlBO+VC vs. BO-VC

Cumulative C content to 100 cm was 18 Mg/ha greater in the BO+VC harvest than TTP+VC. This difference was not statistically significant (α=0.10).

Cumulative C content to 100 cm was 16 Mg/ha greater in the BO+VC harvest than BO-VC. This difference was not statistically significant (α=0.10).

Forest floor C content decreased and mineral soil C increased from the initial post-harvest sampling to 2012.