Reanalysis of the SMC-ORGANON equations for diameter-growth rate, height-growth rate, and mortality rate of Douglas-fir Public Deposited
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Using existing data from untreated research plots, we developed equations for predicting 5-yr diameter-growth rate (ΔD₅), 5-yr height-growth rate (ΔH₅), and 5-yr mortality rate (PM₅) for Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] in the coastal region of the Pacific Northwest. These equations are revisions of the equations constructed in 1995–1997 for the Stand Management Cooperative's (SMC) version of the ORGANON growth-and-yield model, and they have been developed with substantially larger and more comprehensive data sets than were available in 1995–1997. The new ΔD₅ and ΔH₅ equations were validated with an independent data set. The PM₅ equation was evaluated by comparing 100-yr predictions of Reineke's (1933) stand density index to behavior previously reported from measurements taken on long-term research plots. The new ΔD₅, ΔH₅, and PM₅ equations appear to be considerably superior in predictive ability and behavior to the original equations. The effects of the new equations on stand-level predictions were evaluated by comparing the maximum mean annual increments (MAI) in total stem volume (ft³) and associated rotation ages (RA) predicted from the original SMC-ORGANON model to predictions from the revised SMC-ORGANON model. This analysis was done by making 100-yr projections using 170 plots in young stands from the SMC data sets. Some of the ending values for average crown ratio (CR) after 100 yr of projection were near 15%, however, and predictions of basal area (BA) for some of these stands peaked and then declined over stand age. Substituting the HCB equation published by Hann and Hanus in 2004 for predicting crown recession (ΔHCB₅) eliminated the problem with BA peaking over stand age and resulted in somewhat larger average ending CRs. The 100-yr projections were then made again with the 2004 HCB equation of Hann and Hanus. On average, the revised model reduced RA by 2.1 yr (or 4.3%) and maximum MAI by 55.7 ft³/ac/yr (18.9%).
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