Comparing Basal Area Growth Rates in Repeated Inventories: Simpson’s Paradox in Forestry

Recent analyses of radial growth rates in southern commercial forests have shown that current rates are lower than past rates when compared diameter class by diameter class. These results have been interpreted as an indication that the growth rate of the forest is declining. In this paper, growth rates of forest populations in Alabama are studied. Basal area growth (a function of both radius and radial growth squared) by diameter classes is examined for plantation and natural stands. Basal area growth and population distribution8 for the 1962-1972 and 1972-1982 measurement periods are presented. Also, significance of Simpson’s paradox in these analyses is discussed. Basal area growth proves to be consistent with changes that have occurred in tree frequency in diameter classes, i.e., stand structure. In Alabama’s natural stands, basal area growth is shown to be relatively constant over the most recent inventories, while it has increased in plantations. Simple comparison of radial growth would be misleading. FOR. Ser. 35(4):1029-1039. ADDITIONAL