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SOJ?e Patterns in Crop Structureand Productivity for UnthinnedSitka SpruceG. Gallagher lABSTRACTPatterns of crop structure in unthinned Sitka spruce stands areexamined by multiple and polynomial regression. Productivitycategories for top height are evolved on the basis of confidencebelts. Wide variation in volume to 8cm top diameter amounting to160-200m 3 /ha is shown within a range of top heights 7m to over20m. · Volume production per ha to 8 cm top diameter for topheight is greater in young unthinned stands in Ireland thanindicated in the Forest Management Tables (1971). Total stemwoodvolume is equivalent to timber volume plus 20m3jha. Topheight/age patterns indicate a range of Yield classes to over24m 3 /hajan. Mortality is not severe up to 14m top height. Variationin site conditions meant that volume/top height patterns could noteasily be explained by the site factors. elevation, aspect and slope.INTRODUCTIONForest yield classification by height and age alone has limitationsin that crops cannot be readily identified with their productivecapacities. This difficulty has been already largely overcome withthe concept of yield classes or classes of maximum mean annualincrem~nt. (Bradley, Johnston and Christie 1967, Hamilton andChristie 1971). The identification of volume producing forestcategories is not always a straightforward process due to variationwithin apparently similar crops (Gallagher 1970). Difficulties canarise in expressing variation in yield by equations and graphs(Christie 1970). The study of crop productivity and the problemsof correlating this with site factors in regions and localities hasbeen undertaken with varying degrees of success. (Page 1970,Oswald 1969).In Ireland the limiting factor in crop production studies has been1 Research Branch, Forest and Wildlife- Service, Department of Lands,Dublin.