Predicting future yields normally requires an estimate of site quality.  A commonly used measure is site index (SI). SI is often incorrectly quantified operationally due to the ambiguity associated with selecting “site” trees.  Plus, error in the measurement of height itself occurs.  This study quantifies the impacts on the number and timing of thinnings, and the final harvest ages, as well as financial returns when incorrectly determining SI.  Three values of SI (base age 25 years) were examined using two older generation loblolly pine plantation growth and yield simulation models from the Western Gulf, USA; 16.76 m, 21.34 m, and 25.91 m.  Firstly, a particular SI was assumed to be the “true” value, growth and yield estimates were obtained, and financial assessments were conducted.  The same process was then conducted again, but assuming that the SI was incorrectly determined by varying positively and negatively the SI by up to 1.22 m from the assumed “true” value. For these older generation plantations, incorrectly determining SI did impact the age of the first thinning by as much as 5 years.  In some cases, errors of +/- 1.22 m in SI estimation had little impact on the estimated timing of the first thinning.  Errors in SI of up to +/- 1.22 m had little impact on the number of thinnings across economic rotation ages. For both unthinned and twice-thinned stands, final harvest (clearcut) ages differed by as much as 4 years for SI errors up to +/- 1.22 m.  These errors led to differences in Land Expectation Value (LEV) up to $406.50 ha-1.  Across the three SI (16.76, 21.34, and 25.91 m), differences in LEV ranged from $237.49 to $406.50 ha-1.  These differences in LEV could be enough to incorrectly not conduct, or incorrectly conduct, a silvicultural operation such as an herbicide treatment or a fertilization treatment across a rotation, among other treatments.

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