Comparison of 5th-and 14th-year Douglas-fir and understory vegetation responses to selective vegetation removal

WOS: 000292428600002The effects of early vegetation management on the survival and growth of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] were examined 5 and 15 years after planting in the Oregon Coast Range. Our first objective was to document the effects of vegetation species competition upon key ecosystem properties. The second objective was to document the effects of vegetation removal during early Douglas-fir stand establishment upon long-term tree growth and on biomass production by vegetation components. Seven levels of manual vegetation removal were maintained for the first 5 years after planting: 0%, 25%, 50%, 75%, and 100% shrub removal; and 100% shrub removal combined with 50% or 100% herbaceous vegetation removal. Shrub and herb removal did not affect Douglas-fir survival at year five, but treatments providing less than 75% shrub removal significantly reduced Douglas-fir survival by year 15. Removing shrubs and herbs completely (100S + 100H) during the 5 years following tree planting allowed successful tree establishment, with a 366% increase in biomass accumulation per hectare for Douglas-fir in that treatment at the end of 14 years of growth. At 15 years stand age, even with shrub removal alone, a 304% gain in tree biomass per hectare was obtained compared to no vegetation removal (NVR). By stand age 15 years, any increase in the degree of understory removal beyond 75% did not contribute significantly to additional tree survival and growth. The understory vegetation on NVR treatment plots and the herbaceous vegetation on 100% shrub removal (100S) treatment plots, contained >90% and >80% of aboveground biomass N at 5 years, respectively, indicating possible competition for soil N. Soil moisture was not different among treatments at 5 years. Complete vegetation removal (100S + 100H) for 5 years resulted in a significant increase in soil bulk density (P < 0.05), a significant decrease in total soil C (P < 0.05) and no change in total soil N in the upper 15 cm of the mineral soil. By 14 years, however, only the soil bulk density remained greater (P < 0.05) on the 100S + 100H treatment. We conclude that greater tree survival and growth occurred with at least 75% shrub removal. Our results suggest that managers may have substantial flexibility in maintaining a partial understory component suitable for ecosystem productivity, canopy cover and wildlife habitat, while maintaining forests productive for timber resources. (C) 2011 Elsevier B.V. All rights reserved.Duzce University, TurkeyDuzce University; Oregon State University Forest Science Department; US Forest Service Pacific Northwest Research StationUnited States Department of Agriculture (USDA)United States Forest Service; US Environmental Protection AgencyUnited States Environmental Protection AgencySupport for the senior author was received from Duzce University, Turkey. Research support came from the Oregon State University Forest Science Department, the US Forest Service Pacific Northwest Research Station, and the US Environmental Protection Agency. We gratefully acknowledge the assistance of Robert G. Wagner, who began the original study and shared key data for tree survival and growth from year 5. The following individuals gave helpful advice on research methods and data analysis: Carol Glassman, Jillian Gregg, Erik Hobbie, Marc Kramer, Dave Myrold, Nan Posavatz, and Chris Swanston. Dan Binkley, Elizabeth C. Cole, Douglas A. Maguire, Vicente J. Monleon, John C. Tappeiner, II, and Richard H. Waring provided helpful manuscript suggestions. Angeline S. Cromack and Kevin T. Cromack edited and formatted the final draft of the paper