Despite widespread use of intensive vegetation control (VC) in forest management, the effects of VC on allocation of biomass and nutrients between young trees and competing vegetation are not well understood. On three Pacific Northwest sites differing in productivity, soil parent material, and understory vegetation community, we evaluated year-5 effects of presence/absence of 5years of VC on allocation of aboveground biomass and nitrogen (N) between planted Douglas-fir (Pseudotsuga menziesii var. menziesii) and competing vegetation. Equations for predicting bole, branch, foliar, and total dry weights based on stem diameter at a height of 15cm and total tree height did not differ significantly among sites or by presence or absence of VC. This contrasts with previous research, using diameter at breast height rather than at 15cm, which found that separate equations were warranted for trees with and without competing vegetation. Estimated whole-tree biomass among the six site/VC combinations ranged from 0.8 to 7.5Mgha-1, and increases in tree biomass associated with VC ranged from 62% to 173% among sites. Among the three sites, there were positive, linear relationships between soil total N content to a depth of 60cm and both N content of aboveground vegetation (trees plus competing vegetation) and Douglas-fir foliar N concentration. Tree N content increased by 8.4, 8.2, and 40.0kgNha-1 with VC at the three sites, whereas competing vegetation N content decreased with VC by 0.9, 18.8, and 32.0kgNha-1, respectively, at the same sites. Thus, VC did not lead to a direct compensatory tradeoff between aboveground N content of trees and other vegetation. However, soil N content was linearly related to N accumulation and plant growth across the three sites. In addition to differences in N availability among sites, the effect of VC on the redistribution of resources among trees and competing vegetation also was influenced by vegetation community composition and efficacy of VC treatments.
Bibliographical noteFunding Information:
This study is a product of the Sustainable Forestry Component of Agenda 2020, a joint effort of the US Department of Agriculture, Forest Service, Research and Development Program and the American Forest and Paper Association. The authors gratefully acknowledge the financial support provided by the National Council for Air and Stream Improvement, Inc. and the Pacific Northwest Stand Management Cooperative. Study sites and support for experimental treatments were provided by Weyerhaeuser Company, Green Diamond Resource Company, and Port Blakely Tree Farms LLC; the authors are particularly grateful to Rodney Meade, Randall Greggs and Jeff Madsen. We are grateful for support from the Oregon State University College of Forestry and the University of Washington School of Forest Resources. We are grateful for study installation and data collection assistance from many employees of these institutions, and of the USDA Forest Service Pacific Northwest Research Station, particularly James Dollins, Barry Flaming, Paul Footen, Irae Guerrini, Diana Livada, Kyle Peterson, Brian Strahm, and David Stephens.