STAND BIOMASS DYNAMICS OF PINE PLANTATIONS AND NATURAL FORESTS ON DRYSTEPPE IN KAZAKHSTAN

Biomass dynamics were studied in isolated relict stands of Scots pine (Pinus sylvestris L.) on the dry steppe of Kazakhstan (53-54 degrees N ), where potential evaporation is 500-600 mm yr(-1) and the rainfall i s 250-260 mm yr(-1). Samples were taken from seven plots in natural st ands on sandy forest soils (aged 13-110 yrs) and ten plots in plantati ons on dark-chestnut-coloured soils (aged 5-50 yrs). Nine or ten sampl e 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 s tability of these stands becomes critical at 10-20 yrs, when foliage b iomass reaches its maximum (7-13 t ha(-1) dry weight), both in plantat ions and natural stands. Self-regulating mechanisms in natural stands provide stability that may not develop in some plantations. Natural st ands may show an abrupt decrease in foliage biomass at the time of can opy closure, but it increases again by age 40-50 yrs. In plantations t his critical period may cause die-back and may trigger stand collapse before maturity. Stem and root biomass increases monotonically and doe s not depend upon stand origin. The total biomass production is influe nced by ground water level and the presence of and depth to the clay l ayer underlying the sandy sediments.