Description:
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%).