Douglas-fir plantation productivity: effects of organic matter retention, competing vegetation, and soil compaction Investigators: Tim Harrington and Connie Harrington, USDA Forest Service Tom Terry, Weyerhaeuser Company Rob Harrison, University of Washington Stephen Schoenholtz, Virginia Tech and those scientists in training...
Some of the participating organizations...
Study context: extending the LTSP network Does manipulating organic matter, soil porosity, and the complexity of the plant community affect a forest site s productive capacity? Powers et al. 2004
Approach: compare productivity across a wide range of organic matter, vegetation, and soil treatments
Site characteristics Fall River Matlock Molalla Matlock soils silt loam very gravelly loamy sand cobbly loam Fall River rainfall 90 100 70 site index 138 110 120 planted 2000 2004 2004 Molalla Grove soil series near Matlock
Precipitation and temperature differences Fall River: coldest Matlock: wettest Molalla: warmest & driest
Dominant effect: competing vegetation Competing vegetation effects Organic matter retention effects
100 Matlock Molalla Fall River How does Douglas-fir development compare among sites? Survival (%) Stem diameter (mm) 95 90 85 35 30 25 20 15 10 5 0 Stem only, NWC Stem only, WC Piled, NWC Piled, WC Whole tree, NWC Whole tree, WC Height (cm) 150 125 100 75 50 25 0 0 1 2 3 0 1 2 3 Years since treatment 0 1 2 3
Interacting effects of organic matter retention and competing vegetation Douglas-fir stem volume (cm 3 ) 2500 2000 1500 1000 500 0 Matlock Molalla 0% debris, NWC 40% debris, NWC 80% debris, NWC 0% debris, WC 40% debris, WC 80% debris, WC 0 1 2 3 0 1 2 3 Years since treatment Individual-tree study
Soil water: primary driver of productivity 30 25 Direct relationship of diameter growth to soil water content at Fall River Diameter growth (mm) 20 15 10 5 r = 0.66 p < 0.001 0 0.0 0.1 0.2 0.3 0.4 0.5 Soil water content (m 3 m -3 ) Competing vegetation effects Organic matter retention effects
Organic matter retention effects on competing vegetation: Reduced herb abundance Competing vegetation cover (%) 100 80 60 40 20 0 Matlock Herbs, NWC Herbs, WC Woody, NWC Woody, WC Molalla 0 40 80 0 40 80 Logging debris cover (%) Individual-tree study: vegetation present
Nitrogen losses: small and short term Organic matter retention effects Competing vegetation effects Nitrate-N (mg L -1 ) Soil water nitrate-n (60cm depth) by herbicide treatment and site 7.0 Matlock NWC 6.0 Matlock WC Molalla NWC 5.0 Molalla WC 4.0 3.0 2.0 1.0 0.0 Jan-06 Mar-06 May-06 Jul-06 Sep-06 Nov-06 Jan-07 Mar-07 May-07 Jul-07
Soil compaction effects: small to non-detectable Stem diameter (mm) 35 30 25 20 160 control compacted Height (cm) 140 120 100 Fall River Matlock Molalla Third-year size of Douglas-fir: differences not statistically significant
Soil microbial community responses: fate of carbon Microbial respiration by woody-debris retention at Molalla 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0 debris 40 debris 80 debris Organic matter retention effects CO2 flux (umol m 2 sec -1 ) 0.5 0.0 Microbial respiration by herbicide treatment at Molalla Sep-05 Nov-05 Jan-06 Mar-06 May-06 Jul-06 Sep-06 Nov-06 Jan-07 Mar-07 May-07 Jul-07 Sep-07 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 CO2 flux (umol m 2 sec -1 ) Competing vegetation effects 1.0 NWC WC 0.5 0.0 Sep-05 Nov-05 Jan-06 Mar-06 May-06 Jul-06 Sep-06 Nov-06 Jan-07 Mar-07 May-07 Jul-07 Sep-07
The Next Steps linking similar data sets collaborative relationship 5 th -yr data in 2008 characterizing the drivers how do they work? formulating decision-analysis systems how will this work be supported? predicting management consequences model validation