Growth traits, stem form and branch characteristics in three series of well-replicated trials, comprising open-pollinated families of Pinus elliottii Engelmann, were analysed to investigate whether the absolute value of the component of variance for family effect and its relative value (which is equivalent to one-quarter of heritability on individual values) were related, with trial averages of the traits studied taken as productivity indices of the sites tested. The first series, called the foreign selection series, consisted of four trials comprising open-pollinated families from single clones from a Florida seed orchard and from selected trees in Queensland, Australia. Two trials were established in the eastern Transvaal, at Tweefontein and Frankfort State Forests (SF), and two in Zululand, at Dukuduku and KwaMbonambi SF. The second series, called the Zululand selection series, consisted of open-pollinated families from single trees selected in Zululand that were compared in three trials: in the eastern Transvaal at Wilgeboom SF, in Zululand at Dukuduku SF and in the southern Cape at Lottering SF. The third series, called the South African selection series, consisted of open-pollinated families from single trees selected in the different forest areas of South Africa and compared in four trials: two trials in the eastern Transvaal at Tweefontein and Mariti SF and two in the Natal province at Dukuduku and Weza SF. Some positive and linear relationships between absolute sizes of the variance component for family and trial average for all traits studied, except stem form, were discernible for the foreign selections and for the Zululand selections. The South African selections presented a more erratic pattern of variation. However, when the origin of the genetic material was disregarded, the positive trends were undeniable. Curvilinear relationships between relative values and site averages were discernible, when the origin of the genetic material was disregarded. Further research is needed in order to confirm the suggested effect of site on genetic expressivity of open-pollinated families of slash pine grown in South Africa.