Acknowledgments

1. Effects of Five-Year Vegetation Control on Aboveground Biomass and Nitrogen Allocation in Douglas-Fir Plantations on Three Contrasting Sites Warren D. Devine1, Timothy B. Harrington2, Thomas A. Terry3, Robert B. Harrison1, Robert A. Slesak4, David H. Peter2, Constance A. Harrington2, Carol J. Shilling5 and Stephen H. Schoenholtz6 (1) University of Washington, Seattle, WA; (2) USDA Forest Service Pacific Northwest Research Station, Olympia, WA; (3) Weyerhaeuser Corporation (retired); (4) MN Forest Resources Council, St. Paul, MN; (5) Glen Burnie High School, Glen Burnie, MD; (6) Virginia Tech, Blacksburg, VA Abstract On three Pacific Northwest sites affiliated with the North American Long-Term Soil Productivity Study, we evaluated effects of presence/absence of five years of annual vegetation control (VC) treatments on allocation of aboveground biomass and N between planted coast Douglas-fir (Pseudotsuga menziesii var. menziesii) and competing vegetation. Sites differed in soil texture, soil N content, and soil water-holding capacity, as well as understory species composition; however, equations for predicting tree stem, branch, foliar, and total aboveground dry weights based on stem diameter at a 15-cm height and total tree height did not differ significantly among sites or between VC treatments. Estimated whole-tree biomass among the six site/VC combinations at plantation-year 5 ranged from 0.8 to 7.5 Mg ha-1. Increases in tree biomass associated with VC ranged from 62 to 173 percent among the three sites. Across sites, there were positive, linear relationships between soil total N content to a depth of 60 cm and both N content of aboveground vegetation (trees plus competing vegetation) and Douglas-fir foliar N concentration; this supports the premise that soil N content is strongly linked to N uptake and plant growth at the study sites. Tree N content increased by 8.4, 8.2, and 40.0 kg ha-1 with VC at the three sites, whereas competing vegetation N content decreased with VC by 0.9, 18.8, and 32.0 kg ha-1, respectively, at the same sites. In addition to the differences in N availability among sites, trends in biomass and N allocation were clearly influenced by species composition of the understory vegetation community and efficacy of the VC treatments. Phase 1: Develop equations to predict tree biomass Background We are aware of only one published set of biomass equations for young Douglas-fir grown with and without competing vegetation control (Petersen et al. 2008) Objective To develop individual-tree biomass equations for planted Douglas-fir, with and without control of competing vegetation, on three contrasting sites Approach We destructively sampled 119 trees to develop biomass equations; we used D15 (diameter at a 15-cm height) rather than DBH as a predictor of biomass because some trees had not reached breast height • Findings • Individual-tree biomass equations based on D15 (predicting bole, branch, foliar, and total aboveground dry weight) did not differ by site or vegetation control treatment. Equations in Table 2 are for pooled data from all sites and treatments (also see Fig. 3). • For Douglas-fir at plantation age 5, D15 was a more robust predictor of tree biomass than DBH would have been because it captured variation in stem basal taper (here associated with vegetation control) that was not captured by DBH. Figure 3. Relationships between diameter at 15 cm (D15) and total aboveground dry weight for trees from 5-year-old Douglas-fir plantations on three sites. Equation for line is shown in Table 2. Study Sites This study incorporates 3 randomized, block-design experiments that are ancillary sites in the Long-Term Soil Productivity Study Network (Table 1; Fig. 1); locations were selected to represent diverse Douglas-fir plantation sites in the Pacific Northwest. Douglas-fir seedlings were planted on a 2.5- x 2.5-m grid in spring 2000 (Fall River) and a 3- x 3-m grid (Matlock & Molalla) in spring 2004 Treatments +VC 5 years of annual vegetation control using herbicides (Fig. 2) -VC No annual vegetation control (pre-planting control was necessary at Matlock and Molalla) Phase 2: Assess aboveground biomass and nitrogen allocation • Background • Accumulation and distribution of biomass and nutrients are of key importance in forest plantations during the years between planting and canopy closure, when trees are capable of achieving exponential growth • Objective • Determine how vegetation control (VC) affects allocation of aboveground biomass and N between young trees and competing vegetation on three contrasting Douglas-fir sites in the Pacific Northwest • Approach • We estimated biomass of trees and competing vegetation using the equations from Phase 1, measurements of all study trees, and sampling of competing vegetation on “clip plots” • Findings • Estimated whole-tree dry biomass among the six site/VC combinations at year 5 ranged from 0.8 to 7.5 Mg ha-1 (Fig. 4). • Increases in tree biomass associated with VC ranged from 62% to 173% among the sites (Fig. 4). • There was a positive, linear relationship across sites between soil total N content and aboveground vegetation N (trees + other veg.) (Fig. 5A). • Aboveground vegetation N allocated to trees was <50% except in the +VC treatment at Fall River (Fig. 5B), where VC efficacy was very high (Fig. 2). • Douglas-fir foliar N concentration was positively related to soil N content across sites in both VC treatments (Fig. 5C). Figure 4. Total year-5 dry weight of Douglas-fir trees and competing vegetation on 3 sites with (+VC) and without (-VC) annual vegetation control. Mean comparisons are for total aboveground dry biomass (trees plus competing vegetation). • Summary • Across sites, vegetation N content reflected soil total N. • Whereas tree N content and biomass were substantially increased by VC at all sites, N content of competing vegetation was unchanged, halved, and nearly eliminated by VC at Matlock, Molalla, and Fall River, respectively. Thus, VC did not lead to a direct compensatory tradeoff of aboveground N content betweentrees and other vegetation. • Biomass and N allocation were influenced by species composition of the understory vegetation community and the efficacy of the VC treatments. Figure 1. Locations of three study sites in western WA and OR. Reference Petersen, K.S., Ares, A., Terry, T.A., Harrison, R.B., 2008. Vegetation competition effects on aboveground biomass and macronutrients, leaf area, and crown structure in 5-year old Douglas-fir. New Forests. 35, 299-311. This poster is based on a paper in an upcoming issue of Forest Ecology and Management (in press): http://dx.doi.org/10.1016/j.foreco.2011.08.010 • Acknowledgments • This study is a product of the Sustainable Forestry Component of Agenda 2020, a joint effort of the USDA Forest Service Research and Development Program and the American Forest and Paper Association. • Financial support provided by National Council for Air and Stream Improvement, Inc. and Pacific Northwest Stand Management Cooperative. • Study sites and field support were provided by Weyerhaeuser Company, Green Diamond Resource Company, and Port Blakely Tree Farms LLC. • We are grateful for support from the Oregon State University College of Forestry and the University of Washington School of Forest Resources. Figure 5. Relationships between soil total N and (A) year-5 aboveground N in all living vegetation, (B) percentage of N allocated to planted trees, and (C) tree foliar N concentration determined from whole-crown samples, for -VC and +VC treatments. Figure 2. Percentage cover of herbaceous and woody competing vegetation (sum of every species’ cover) and crowns of planted trees at year 5; Fall River tree cover scaled from 1,600 to 1,111 trees/ha. Mean comparisons are for total competition cover.