Site Index Curves and Old Growth
When old-growth stand height and age are input to a site index curve, the resulting site index usually underestimates the true growth potential of the site. Most inventory files and the forest cover map labels contain site index values that were obtained in this way. As a result, the site index estimates for old-growth stands in most inventory files and forest cover map sheets are biased.
Figure 7.4 depicts the relationship between the site index curve estimate of SI for old Pl stands and our best current estimate of the true site potential of these stands.
The poor performance of site index curves, when applied to old stands, is due to several factors. First, the height growth of old stands frequently flattens off more than the height growth assumed by site index curves. Figure 7.5A illustrates this effect in a Ss stand on site index 40 where top height growth stops due to repeated top breakage after the stand is 100years old. When this stand is 300 years old and its age and height are input to the Ss site index curve, site index is estimated at 25 m.
Height repression is the second factor contributing to site index underestimates in some old stands. Repression, most common in Pl, is when top height growth is slowed by high establishment density. Figure 7.5B shows WinTIPSY (version 2.0) predictions of the effect on Pl top height development of excessive establishment density. When regenerated at 200,000/ha, top height on site index 23 m develops as though the stand was site index 20 m.
Suppression is the third factor contributing to poor SI estimates in old stands. In some forest types, today's candidate SI sample trees grew up under an overstory. These trees have reached their dominating position in the stand today from a history of suppression and release as canopy gaps opened and closed above them. Figure 7.5C illustrates this effect as a dominant Ss tree reaches its current canopy position through a repeating cycle of suppression and release. Over the 300 years, site index estimated by site index curves varies from 10 m to almost 40 m.
Use the SI curve for coastal Douglas-fir. If the breast height age of the tree is 105 and the top height is 44 m, what is the SI indicated by this one tree?