PHYTOMASS (LIVE BIOMASS) AND CARBON OF SIBERIAN FORESTS

The results of the phytomass (live biomass) estimates inventory for th e Siberian forests are presented. These results are based on the follo wing: (i) models estimating basic phytomass fractions for eight main f orest-forming species (pine, spruce, fir, larch, Russian cedar, birch, aspen, oak); (ii) an ecoregional division of the territory into 63 ec ological regions; and (iii) data from the State Forest Account (SFA) o f 1993. The models are in the form of multidimensional regression equa tions for the ratio R-fr = M-fr/GS. where M-fr, is the mass of a phyto mass fraction in teragrams (Tg), and GS is (green) growing stock in cu bic meters (m(3)). The independent variables used are age, site index and relative stocking of stands. The fractions evaluated are wood and bark of the stems, bark, wood and bark of branches, foliage, stump and roots, understorey and green forest floor. The final results are pres ented by 18 administrative units and three economic regions. The total phytomass of the vegetation of forest ecosystems of the total foreste d areas in Siberia is estimated to be 48 253.8 Tg of dry organic matte r, of which 59.2% are stems, 18.4% stump and roots, 8.4% branches, 5.6 % green forest cover, 3.4% foliage, 2.1% understorey and an additional 2.9% of the total phytomass is in the form of shrubbery areas. Due to Russian forest terminology, forested areas, i.e. closed forests, incl ude forests generated by: (i) so-called main forest-forming species co mbined in three groups (coniferous, hard-leaved deciduous and soft-lea ved deciduous); (ii) other species (rare, valuable and introduced spec ies) with small area; (iii) shrubbery areas, considered as forested ar eas for territories where forests are not able to grow due to severe c limatic conditions (zonal and altitudinal tree lines). The average den sity of phytomass (as an average of the total forested areas) is 4.04 kg of carbon (C) per square meter and varies from 2.0 kg C/m(2) (in ec oregions of the forest tundra) to 5.7-5.9 kg C/m(2) (in southern taiga and mixed broadleaved coniferous forests in the Far East). The C dyna mics over time that have been estimated based on official forest inven tory data for 1961-1993 reveal that during this period the Siberian fo rests were, on average, a small source of C emissions (about 20 Tg C/y ear). For 'reconstructed' dynamics of the growing stock, which take in to account systematic errors in the forest inventory data, the Siberia n forests were estimated on average to sequester carbon (51 Tg C/year) . Both approaches provide the conclusion that during the 10 years betw een 1983 and 1992 the Siberian forests have been a net source of atmos pheric carbon (between 81 and 123 Tg C/year). (C) 1998 Published by El sevier Science Ltd. All rights reserved.