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Impacts of organic matter removal and vegetation control on nutrition and growth of Douglas-fir at three Pacific Northwestern Long-Term Soil Productivity sites

Littke, K. M., T. B. Harrington, R. A. Slesak, S. M. Holub, J. A. Hatten, A. C. Gallo, W. R. Littke, R. B. Harrison and E. C. Turnblom. 2020.

Abstract

As intensive management of forest plantations and interest in harvesting biomass for energy continue to increase, there is a need to investigate the longer-term effects of harvest-related disturbances and intensive treatments on soil and site productivity. This research focused on three Pacific Northwestern Douglas-fir soil productivity studies around 15 years since harvest that spanned a range in soil nutrients: high soil N and low base cations (Fall River), low soil N and base cations (Matlock), and high soil N and base cations (Molalla). The studies, which had similar organic matter and vegetation control treatments, were compared for differences in belowground and aboveground nutrients as well as differences in periodic stand volume growth. Five years of annual vegetation control (AVC) resulted in the greatest losses of belowground N and base cations compared to one year of vegetation control (IVC) at planting, but also resulted in significantly greater stand volume growth at Fall River and Matlock. Whole tree removal (WT) resulted in lower soil NO3- at Fall River but greater soil NO3- at Matlock due to greater colonization by N-fixing Scotch broom. There was also a decrease in soil exchangeable K due to WT and WT plus coarse woody debris removal (WT   ) at Fall River, which had the lowest initial soil exchangeable K. There was either no effect (Matlock and Molalla) (0-15 years) or a decrease (Fall River) (0-5 years) in stand volume growth due to WT removal. At Fall River, WT, WT , and AVC treatments had no detectable effect on volume growth from 10 to 15 years. Overall, longer-term effects of organic matter removals and vegetation control on soil and site productivity were variable at each site due to pre-treatment soil nutrition and competition from understory vegetation.

Key Words

Douglas-fir Pacific Northwest Organic matter removal Vegetation control Soil nutrition Tree nutrition Stand volume growth