Biomass dynamics were studied in isolated relict stands of Scots pine (Pinus sylvestris L.) on the dry steppe of Kazakhstan (53-54° N), where potential evaporation is 500-600 mm yr-1 and the rainfall is 250-260 mm yr-1. Samples were taken from seven plots in natural stands on sandy forest soils (aged 13-110 yrs) and ten plots in plantations on dark-chestnut-coloured soils (aged 5-50 yrs). Nine or ten sample trees were taken from each plot, giving a total of 68 and 96 sample trees in natural and plantation stands respectively. Root systems were excavated and fractionated in 11 plots. Analyses indicated that the stability of these stands becomes critical at 10-20 yrs, when foliage biomass reaches its maximum (7-13 t ha-1 dry weight), both in plantations and natural stands. Self-regulating mechanisms in natural stands provide stability that may not develop in some plantations. Natural stands may show an abrupt decrease in foliage biomass at the time of canopy closure, but it increases again by age 40-50 yrs. In plantations this critical period may cause die-back and may trigger stand collapse before maturity. Stem and root biomass increases monotonically and does not depend upon stand origin. The total biomass production is influenced by ground water level and the presence of and depth to the clay layer underlying the sandy sediments.