A height increment equation for young ponderosa pine plantations using precipitation and soil factors

A height increment equation was used to determine the effects of site quality and competing herbaceous vegetation on the development of ponderosa pine seedlings ( Pinus ponderosa var. scopulorum Engelm.). Study areas were established in 36 plantations across northwest and west-central Montana on Champion International Corporation’s timberland (currently owned by Plum Creek Timber Company). Site quality indices used in the equation were available water index (available water capacity multiplied by the natural logarithm of annual precipitation) and site index (SI) of the previous stand. Three-year height increment was modeled as a function of tree size, vigor, and competition by using available water index (AWI) and SI separately as site quality indicators in the same equation form. Comparison of the two equations suggest that soil factors in combination with precipitation data may be a useful alternative to traditional tree-based site quality indices in predicting height increment of young stands where site index is not known or is poorly estimated.