Carbon exchange in Western Siberian watershed mires and implication for the greenhouse effect : A spatial temporal modeling approach

Abstract

The vast watershed mires of Western Siberia formed a significant sink of carbon during the Holocene. Because of their large area these mires might play an important role in the carbon exchange between terrestrial ecosystems and the atmosphere. However, estimation of the Holocene and future carbon balance of whole Western Siberian mires is hampered by the lack of spatially resolved models. The main objective was to assess the carbon exchange fluxes of the mires using a 3-D dynamic approach. These exchange fluxes comprise the sequestration of carbon dioxide (CO2) by peat growth, the emission of methane (CH4) by anaerobic peat decay and the emission of CO2 by aerobic peat decay. From the detailed analysis of peat cores from different sites in the southern taiga of Western Siberia, it emerged that Holocene peat growth and carbon accumulation had different trends, caused by variations in vegetation succession. These differences were strongly influenced by the position in the landscape. Therefore, the effect of climatic change on mire development varied spatially. The indirect effects of climate change through local hydrology appeared to be more important than direct influences of changes in precipitation and temperature. Mire development is closely connected to hydrological dynamics. In the thesis a 3-D dynamic modeling approach is described that makes use of groundwater modeling. In successive timesteps peat growth and decay as well as mire type distribution were calculated, depending on hydrological conditions. The model was forced with a paleo-precipitation record to include variable climatic input. The model results show the Holocene development of a watershed mire from a few small spots to a contiguous mire landscape. As hydrology is the major limiting factor, the mire development is most sensitive to precipitation and evapotranspiration. Under unchanged conditions the mire will grow further, eventually reaching its maximum peat thickness around 11400 yr A.D. Under wetter climatic conditions the mire growth could continue much longer, whereas under dryer conditions an earlier termination will occur. Mire drainage leads to aerobic decay of peat and thus to CO2 emission instead of uptake. In a modeling study in a mire catchment containing both drained and undrained parts, the effect of drainage was quantified. The results show that the water table drawdown not only affected the drained mire part, but also influenced the undrained part over a zone of 1-1.5 km. In this zone peat growth and carbon accumulation were decreased. The contribution of a mire system to the greenhouse effect is depending on the exchange of the greenhouse gases CO2 and CH4. CH4 is a much stronger greenhouse gas than CO2, but has a much shorter atmospheric lifetime. With a stocks-and-flows approach the radiative forcing of both gases could be calculated. This approach has been applied in two cases: (1) the northward shift of bioclimatic zones in Western Siberia under 21st century warming and (2) the carbon exchange in a Western Siberian mire resulting from the 3-D dynamic model. The results show that Western Siberian mires form a sink of greenhouse gases at present and have therefore a negative contribution to the greenhouse effect. Under 21st century warming or drainage conditions the mires will turn into a source of greenhouse gases, thus enhancing the greenhouse effect. However, in the case of warming the increase in CH4 emission will be overruled by the increase in CO2 uptake on the long term, thus leading to a negative contribution again