Phytomass (live plant mass) and net primary productivity are major components of the terrestrial carbon balance. A major location for phytomass storage is the subcontinent of Siberia, which is dominated by extensive reaches of taiga (boreal forest). The responsiveness of the phytomass component of the carbon pool is examined by comparing vegetation in the mid-Holocene (4600--6000 years before present) to modern potential vegetation. The mid-Holocene was warmer and moister in middle and northern Siberia than today, producing conditions ideal for boreal forest 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 Siberia and the highlands of central Siberia, with its central and eastern components in the mid-Holocene replaced today by light-demanding light-needled species with lower productivity and phytomass. Total phytomass in Siberia in the mid-Holocene was 105.0¿3.1 Pg, compared to 85.9¿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 results in a loss of 28.8 Pg, while the expansion of the corresponding light-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 potential vegetation and not adjusted for agriculture and other anthropogenic disturbances. Given long periods for vegetation to approach equilibrium with climate, the phytomass component of the carbon pool is responsive 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%) has occurred under our cooler modern climate, primarily due to the shift from dark-needled taiga in the mid-Holocene to light-needled taiga today. ¿ American Geophysical Union 1995