Treatment Plots Plot conditions for treatments studied at time of sampling.

1. Effects of organic matter removal and competing vegetation control on soil C in a Pacific NorthwestDouglas-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 LTSP • 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. • Objective • Assess 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) Soil sampling depths: • Forest floor • 0-15 cm • 15-30 cm • 30-45 cm • 45-60 cm • 60-100 cm C content to 20 cm was 7.5 Mg/ha greater for BO+VC than for 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 18 Mg/ha greater for BO+VC than for TTP+VC C content to 100 cm was 16 Mg/ha greater for BO+VC than for BO-VC 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 Forest floor sampling in a TTP+VC plot. Fall River plot layout; stars indicate plots sampled. Modified from Ares et al., 2007. 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 Paul Footen drives the soil corer into one of the three composite sample locations withina TTP+VC subplot. Results Vegetation Control BO+VC vs.BO-VC Harvest Intensity BO+VC vs. TTP+VC Total Tree Plus with vegetation control TTP+VC Bole-only with vegetation control BO+VC Bole-only without vegetation control BO-VC Forest floor C content decreased and mineral soil C increased from the initial post-harvest sampling to 2012. • 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. Treatment Plots Plot conditions for treatments studied at time of sampling. 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). 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). Fall River Long-term Soil Productivity site, spring 2012