Equations for predicting height-to-crown-base, 5-year diameter-growth rate, 5-year height-growth rate, 5-year mortality rate, and maximum size-density trajectory for Douglas-fir and western hemlock in the coastal region of the Pacific Northwest

Using existing permanent research plot data, we developed equations for predicting height-to-crown-base (HCB), 5-yr diametergrowth rate (delta D), 5-yr height-growth rate (delta H), 5-yr mortality rate (PM), and the maximum size-density trajectory for Douglasfir and western hemlock in the coastal region of the Pacific Northwest. With the exception of the HCB equation, the equations developed for predicting trees from untreated plots agreed in predictive behavior with previously published equations for the study area. The HCB equation predicted shorter HCB (and therefore longer crown lengths [CL]) than previously published equations for the study area. Western hemlock showed no response to fertilization. Modifiers for fertilization response were incorporated into the final equations for predicting delta D, delta H, and PM in Douglas-fir. All three modifiers for Douglas-fir predicted an increase in growth and mortality rates with the amount of nitrogen applied and a decrease with number of years since fertilization, with most of the fertilization effect gone within 15 yr of application. For the delta D and delta H modifiers, the size of the increase varied by the site index (SI) of the plot, with plots of lower site quality showing greater increases. For delta D, fertilization response did not appear to vary by plot density, tree size, or tree position within the plot. Modifiers for thinning response were incorporated into the final equations for predicting tree delta D for both species and delta H for Douglas-fir. For both species, the delta D thinning-effects modifier predicted an increased growth rate with the proportion of the BA removed and a decrease with years since thinning; most of the thinning effect was gone within 10 yr. For Douglas-fir, the delta H thinning-effects modifier predicted a reduced growth rate immediately after thinning, with the size of the reduction increasing with the intensity of thinning. Most of the reduction was gone by about 10 yr. For Douglas-fir, the combined effect on delta D and delta H of applying both thinning and fertilization could be adequately characterized by the product of the thinning modifier and the fertilization modifier. The percent increase in predicted growth rate due to a combined treatment thus was greater than the sum of the percent increases for each treatment alone. Analysis of the maximum size-density trajectory data strongly suggests that plots of neither species approach a single maximum stand density index value (SDI) as they develop. The potential yield for a given site therefore depends, not only on its SI, but also on its maximum SDI. Fertilization does not appear to affect the intercept of the maximum size-density line for Douglas-fir. The strengths and weaknesses of the existing data sets and the modeling and analytical approaches tested during development of these equations are presented to aid future modelers, and alternative modeling approaches are explored.