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Site Index estimates by Site Series (SIBEC) - second
approximation
For a particular target species, site index is the height of the largest diameter (at breast height) site tree on a 0.01 ha plot at breast height age 50, provided the tree is suitable. A suitable site tree is a vigorous dominant or co-dominant tree that has a full crown and a straight, disease-free, undamaged stem. It is free of suppression above breast height, is not repressed, and is not a wolf, open-grown, or veteran tree.
2.2 Why is Site Index Important in Forest Management? Site index provides a numeric description of site productivity that enables forest managers to predict forest stand growth and the yield of timber at harvest. Estimates of site index are important in the following areas of forest management.
Site index allows the comparison of productive potential between sites across a broad range of existing stand conditions. For example, a stand of Douglas-fir that is taller at breast height age 50 than another similar-aged stand of the same species will have a higher site index. It should achieve a greater timber volume, and therefore be more productive, than the stand with the lower site index. As measures of site productivity, these estimates influence timber supply analyses and support the Chief Forester’s decisions on allowable annual cut. By allowing forest managers to predict the outcome of a particular forest practice, site index estimates are also important inputs to land-use decisions and analyses of silviculture investments.
2.3 What Tools are Available to Measure Site Index? For many stands, more than one acceptable way exists to estimate site index. The methods either involve "direct" measurement of site trees and the subsequent calculation of site index, or the "indirect" estimation of site index from various site factors. These two types of methods differ both in their suitability for different stand conditions and their accuracy. The most common site index estimation tools available in British Columbia involve the use of models-mathematical equations that allow one attribute to be estimated from other attributes (e.g., in growth intercept models, site index is estimated from height and breast height age). The three models briefly described here are each suited to specific stages of stand development (see Figure 1). Height-age Models: Also known as site index models, or curves, the primary purpose of these models is to describe height development over time for different levels of site index. These models also allow estimates of site index from height and breast height age data. Each tree species requires a different model because height growth characteristics are not the same for all species. This method is best used for stands that have between 50- and 140-years growth above breast height. Growth Intercept Models: These species-specific models are designed explicitly for young stands (5-50 years breast height age). The growth intercept technique estimates site index from the average annual height growth of site trees, which is determined either from the distance between annual branch whorls or from the height and breast height age of the tree. Site Index-Biogeoclimatic Ecosystem Classification (SIBEC) Models: This
comprehensive tool correlates site index with site series within biogeoclimatic ecosystem
classification (BEC) units and site series. The BEC system is designed specifically for
British Columbia’s ecosystems. This model is best used for very young stands, very
old stands, and stands not suitable for other methods.
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2008 Approximation
