ORIGINAL PAPER A hybrid model for intensively managed Douglas-fir plantations in the Pacific Northwest, USA Aaron R. Weiskittel • Douglas A. Maguire • Robert A. Monserud • Gregory P. Johnson Received: 6 July 2009 / Revised: 28 October 2009 / Accepted: 30 November 2009 / Published online: 30 December 2009 Ó Springer-Verlag 2009 Abstract Recent advances in traditional forest growth models have been achieved by linking growth predictions to key ecophysiological processes in a hybrid approach that combines the strengths of both empirical and process-based models. A hybrid model was constructed for intensively managed Douglas-fir plantations in the Pacific Northwest, USA, by embedding components representing fundamental physiological processes and detailed tree allometrics into an empirical growth model for projecting individual tree and stand development. The simulated processes operated at a variety of scales ranging from individual branches to trees and stands. The canopy structure submodel improved predictions of leaf area index at the stand level when compared to allometric and other empirical approaches (reducing mean square error by 30–42%). In addition, the hybrid model achieved accuracy in short-term volume growth prediction comparable to an empirical model. Biases in 4-year stand growth predictions from the hybrid model were similar to those from the empirical model under thinning, fertilization, and the combination of these treatments; however, volume growth predictions in unmanaged plantations averaged approximately 36% less bias. These improvements were attributed to detailed information on crown structure (i.e. size, location, and foliage mass of primary branches), simple representation of key physiological processes, and improved site character- ization. Soil moisture, temperature, and nitrogen mineral- ization predicted by the hybrid model also agreed closely with observed values from several previous studies. Overall, the model framework will be helpful for future analyses as it can lend insight into the influence of weather and site edaphic factors on growth, help identify mecha- nisms of response to silvicultural treatments, and facilitate the design of sound management regimes for Douglas-fir plantations across the Pacific Northwest region. Keywords Hybrid model Á Thinning Á Fertilization Á Growth and yield Á Oregon Á Washington Á Leaf area index Á Soil temperature Á Soil moisture Á Soil nitrogen mineralization Á Net primary production Introduction The effects of intensive management on tree growth and stand yield are difficult to quantify empirically because natural variability necessitates extensive replication of silvicultural treatments across the full range of sites. Many years must also pass before growth trajectories and impli- cations for final yields become evident. Empirical growth and yield models have been very effective for summarizing Communicated by Uta Berger. Electronic supplementary material The online version of this article (doi:10.1007/s10342-009-0339-6) contains supplementary material, which is available to authorized users. A. R. Weiskittel (&) School of Forest Resources, The University of Maine, Orono, ME 04469, USA e-mail: aaron.weiskittel@umit.maine.edu D. A. Maguire Department of Forest Engineering, Resources & Management, Oregon State University, Corvallis, OR 97331, USA R. A. Monserud PNW Research Station (retired), USDA Forest Service, 620 SW Main, Suite 400, Portland, OR 97205, USA G. P. Johnson Weyerhaeuser Company, P.O. Box 9777, Federal Way, WA 98063, USA 123 Eur J Forest Res (2010) 129:325–338 DOI 10.1007/s10342-009-0339-6