The topography-related variation and plot-to-plot variation of above-ground forest biomass in a central Appalachian watershed and in a southeast Australian watershed and its surroundings were compared. The Appalachian site was covered with typical Appalachian hardwoods, ca 85 years old, and the Australian site was covered with an eucalyptus forest (Eucalyptus regnans), ca 53 years old, when the study was initiated. Both forests are unmanaged. Biomass data were collected on 112 800 m(2) plots at the Appalachian site and on 30 plots of the same size at the Australian site. The oven-dry above-ground biomass average was 322 t/ha at the Appalachian site and 476 t/ha at the Australian site. At the Appalachian site, the single plot biomass varied from minus 67% to plus 85% of the mean, and at the Australian site the corresponding numbers are minus 50% and plus 61%. At both sires, the coefficient of variation shows a more or less steady value when the number of plots (n) exceeds 20. For n = 20, coefficients of variation are 28% and 22% for the Appalachian site and for the Australian site, respectively. The smallest above-ground biomass per unit area was observed on southwest-facing slopes at the Appalachian site and on west-facing slopes at the Australian site. The greatest above-ground biomass per unit area was observed on east-facing slopes at the Appalachian site and on south-facing slopes at the Australian site. In general, the sites facing east and the poles are more productive than those facing west and the equator. Copyright (C) 1996 Elsevier Science Ltd.