Site index has many uses in:
In inventory, SI is used as a basic descriptor of site quality and used to grow the inventory to keep height and volume estimates current.
In silviculture, accurate estimates of SI are required to adequately describe site quality, formulate appropriate prescriptions, schedule and prioritize treatments and to predict stand growth and yield.
In many cases, site index is the single most important factor determining stand growth and yield. To illustrate how site index affects stand development, Figure 2.3 shows WinTIPSY (version 1.3) output for a coastal Fd stand planted at 1100 stems/ha on site index 25, 30, and 35 m.
Timber Supply Analysis
In timber supply analysis, SI is used to estimate years to green-up, size of operable land base, minimum harvestable age, yield of regenerated stands and growth of existing stands.
In many management units, our estimate of future timber supply depends heavily on our site index estimates. In the Kingcome TSA, a sizable portion of the land base is considered inoperable due to low site index (Figure 2.4A). The B.C. Ministry of Forests' analysis shows that timber supply is very sensitive to the size of the harvestable land base (Figure 2.4B). Clearly, accurate site index estimates are essential to ensure that the size of the area inoperable due to low site index is accurately estimated. Site index largely determines the yield expected from regenerated stands. Figure 2.4C shows that the Kingcome TSA timber supply analysis is sensitive to our yield expectations for regenerated stands. Clearly, we must ensure that we have accurate site index estimates so that we can develop accurate expectations for the yield of regenerated stands. Site index partially determines the number of years it will take a stand to reach green-up height. Figure 2.4D shows that our estimate of future timber supply in the Kingcome TSA is significantly affected by our estimates of time to green-up. Clearly, accurate site index estimates are required to increase the accuracy with which we can predict years to green-up.
In some management units, recent sampling programs have confirmed that site index is underestimated. Figure 2.5 provides an example of two management units where SI was found to be underestimated, improved SI estimates were obtained, and these estimates were used in timber supply analysis. Figure 2.5A shows the change in estimated timber supply in MacMillan Bloedel's TFL 44 after improved site index estimates were obtained. Figure 2.5B shows the change in estimated timber supply in the Lakes TSA after improved site index estimates were obtained.