ABOVEGROUND BIOMASS AND NITROGEN NUTRITION IN A CHRONOSEQUENCE OF PRISTINE DAHURIAN LARIX STANDS IN EASTERN SIBERIA

Measurements of aboveground biomass and nitrogen (N) nutrition were ma de during July 1993 in 50-, 130-, and 380-year-old stands of Larix gme linii (Rupr.) Rupr. in eastern Siberia. Constituting six forest types based on understorey plants, the stands were representative of vegetat ion throughout the Yakutsk region. Average tree height, diameter, and density ranged from 2 m, 23 mm, and 50 800 stems/ha in the 50-year-old stand to 11 m, 160 mm, and 600 stems/ha in the oldest stand. Abovegro und biomass in the 50-year-old stand was 4.4 kg . m(-2), and the above ground N pool was 1.1 mol . m(-2). This was slightly higher than the N pool in a 125-year-old stand with a Ledum understorey (1.0 mol . m(-2 )), despite its higher biomass (7.2 kg . m(-2)). The highest observed aboveground biomass in a 125-year-old stand (characterized by the N-2- fixing understorey plant Alnaster fruticosa) reached 12.0 kg . m(-2), but the corresponding N pool was only 1.6 mol . m(-2). In the oldest s tand, aboveground biomass was 8.9 kg . m(-2) and the N pool was 1.1 mo l . m(-2). There was thus a relatively constant quantity of N in the a boveground biomass of stands differing in age by almost 400 years. We postulate that N sets a limit on carbon accumulation in this boreal fo rest type. Trees were extremely slow growing, and there was essentiall y no aboveground biomass accumulation between the ages of 130 and 380 years because of a lack of available N. This conclusion was supported by graphical analysis indicating that the self-thinning process in our stands was not governed by the availability of radiation according to allometric theory. Much of the available N was used in the production of tree stems where 86% of the aboveground N (and 96% of aboveground biomass) was immobilized in the oldest stand. N in wood of the old sta nd exceeded the N pool in the litter layer and was 20% of the N pool i n the Ah horizon. The processes of carbon and N partitioning were furt her explored by the estimation of carbon and N fluxes during three per iods of forest development. We calculated a loss of ecosystem N during the period of self-thinning, while in the mature stands the N cycle a ppeared to be very tight. The immobilized N is returned from the wood into a plant-available form only by a recurrent fire cycle, which rege nerates the N cycle. Thus fire is an essential component for the persi stence of the L. gmelinii forest.