Northern wetlands and tundra ecosystems exist in the extremely cold desert climate of the Arctic. Yet, they often remain waterlogged during the summer, as the water balance is positive—precipitation exceeds evaporation. This combination of factors affects the seasonality and rates of carbon exchange experienced by northern wetlands and tundra (Figure 15.9). Despite extreme cold temperatures during the winter, these ecosystems continue to respire (Vourlitis et al. 2000). Respiration occurs because microbes can remain active and decompose organic matter deep in the soil in unfrozen zones.
Since the growing season is very short and the amount of vegetation is low, rates of net carbon uptake during the summer are low. Functionally, soil temperature and solar radiation control net CO2 balance during the period from snow melt to early autumn. On a seasonal and interannual basis, these ecosystems are sensitive and vulnerable to changes in the water table, as large stores of carbon remain. Lowering the water table allows stored carbon pools to oxidize, causing the ecosystem to be a net source of carbon during the growing season (Vourlitis et al. 2000; Lloyd 2001).
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