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Improving site index estimates for pine and spruce plantations: a case study in the sub-boreal spruce zone in British Columbia

Site index (SI) expresses the growth potential for a species on a given site. Three SI determination methods were used for each managed stand sampled in the southern portion of the Prince George Timber Supply Area (TSA): forest inventory, calculated growth intercept or height over age, and site index biogeoclimatic ecosystem classification (SIBEC). Interior spruce and lodgepole pine leading stands were sampled in the Bowron Valley and only pine stands were sampled in the rest of the TSA. In the Bowron Valley for both species, calculated SI was greater than SIBEC SI, which was greater than inventory SI. Increases were greater for spruce. In the remaining pine areas sampled, the same relationship was observed. Pine yield model projections indicate mean annual increment (MAI) culmination occurred 5.7 and 16.4 years sooner for calculated SI compared with SIBEC and inventory SI, respectively, with yields increasing by 8% and 13%. The increases could be due to improved silviculture, tree response to climate change or a combination of the two. Differences could result in a mid-term timber supply available sooner or a greater yield at planned harvest age. This is particularly important in areas attacked by mountain pine beetle.