Incorporating the effects of interspecific competition and vegetation management treatments in stand table projection models for Douglas-fir saplings

A stand table projection system based on individual-tree and stand-level models for young Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii ) plantations was developed from and evaluated with remeasurement data from sites in the Siskiyou Mountains of southwestern Oregon and in the Coast Ranges of Oregon and Washington. A projection equation was developed for relative tree size, defined as the ratio of individual-tree diameter at 15 or 30 cm above ground level (depending on the study location) to quadratic mean diameter. The relative size projection equation for the Coast Ranges study included the effect of total vegetation control, which indicated that diameters of Douglas-fir receiving total vegetation control tended to become more uniform over time in the Coast Ranges. An additional equation was developed to project quadratic mean diameter so that individual-tree diameters could be estimated from projected relative size. The effect of vegetation management treatments on projected quadratic mean diameters in the Siskiyou study was expressed as an interaction between proportion of cover removed by treatments (intensity) and dominant height of Douglas-fir at time of treatment relative to current dominant height. In 1- and 2-year projection periods, the stand table projection system performed similarly to a diameter distribution prediction system based on a Weibull distribution function. However, the difference between projected and predicted diameter distributions became more pronounced as the projection period increased to 5 years.