CHANGE IN PHYTOMASS AND NET PRIMARY PRODUCTIVITY FOR SIBERIA FROM THEMIDHOLOCENE TO THE PRESENT

Phytomass (live plant mass) and net primary productivity are major com ponents of the terrestrial carbon balance. A major location for phytom ass storage is the subcontinent of Siberia, which is dominated by exte nsive reaches of taiga (boreal forest). The responsiveness of the phyt omass component of the carbon pool is examined by comparing vegetation in the mid-Holocene (4600-6000 years before present) to modern potent ial vegetation. The mid-Holocene was warmer and moister in middle and northern Siberia than today, producing conditions ideal for boreal for est growth. As a result, both northern and middle taiga were dominated by shade-tolerant dark-needled species that thrive in moist climates. Today, shade-tolerant dark-needled taiga is restricted to western Sib eria and the highlands of central Siberia, with its central and easter n components in the mid-Holocene replaced today by light-demanding lig ht-needled species with lower productivity and phytomass. Total phytom ass in Siberia in the mid-Holocene was 105.0 +/- 3.1 Pg, compared to 8 5.9 +/- 3.2 Pg in modern times, a loss of 19.1 +/- 3.1 Pg of phytomass . The reduction in dark-needled northern and middle taiga classes resu lts in a loss of 28.8 Pg, while the expansion of the corresponding lig ht-needled taiga results in a gain of 13.5 Pg, a net loss of 15.3 Pg. The loss is actually greater, because the modern figures are for poten tial vegetation and not adjusted for agriculture and other anthropogen ic disturbances. Given long periods for vegetation to approach equilib rium with climate, the phytomass component of the carbon pool is respo nsive to climate change. Changes in net primary productivity (NPP) for Siberia between the mid-Holocene and the present were not as large as changes in phytomass. A minor decrease in NPP (0.6 Pg yr(-1), 10%) ha s occurred under our cooler modern climate, primarily due to the shift from dark-needled taiga in the mid-Holocene to light-needled taiga to day.