Description
This research focuses on the spatial and temporal patterns of, and controls on, CO_ in the Alaskan Arctic tundra ecosystem. The sites investigated--wet sedge, moist acidic, and low tussock tundra--represent the dominant land cover types in the Arctic tundra ecosystem, yet none have previously been investigated continuously throughout the year. In the first part of the research presented here, new definitions of season are presented, which will allow better comparisons across sites, seasons, and years in the Arctic tundra, where season length varies among years and locations. The results of this, the first, continuous, yearlong Arctic tundra study in a moist acidic tundra region, show that while summer uptake was detected (-11 g C m__ yr__), the annual carbon signal was overwhelmed by the non-summer seasons, resulting in a net annual carbon release of nearly 38 g C m__ yr__. Winter showed low metabolic rates over a long season resulting in a net source of carbon to the atmosphere. The transitional seasons of spring and fall demonstrated active rates over short durations and were also sources of carbon to the atmosphere. In addition to the variable pattern of carbon exchange, the controls on carbon varied by season as well. For example, the effect of increasing soil temperature was negatively related to net ecosystem exchange (NEE) during winter and summer, but positively related to NEE during spring and fall. These results indicate that continuous monitoring of carbon, and related environmental variables, is important in accurate estimation of the current total annual and seasonal carbon budgets. This information, in turn, is critical to our ability to predict, with confidence, future carbon budgets. In the second part of the research, three years of continuous carbon measurements are presented for a low tussock tundra region. This southern site is especially vulnerable to climate change effects because it is at the southern extent of the tundra ecosystem near the graminoid-shrub boundary and increased rates of decomposition, and the region is likely to undergo community compositional changes in the near future. This region is likely to experience deeper active layers in the future, potentially exposing large stocks of carbon. This southern system was a net source of carbon over the three-year period of study, with only two of the three summer seasons acting as net carbon sinks. In one year, drought was so severe that even during the summer season, respiration overwhelmed photosynthesis, leading to a large (87 g C m__ yr__) annual efflux compared to the other years of the study (which had 0.04 and 49 g C m__ yr__ annual carbon release). In the last part of this research, NEE was measured at three sites located along a latitudinal gradient that spanned the North Slope of Alaska. Only in the northernmost site at Barrow was net annual carbon uptake detected, leading to an average uptake rate of 80 g C m__ yr__. Increased temperatures and decreased rainfall led to greater uptake in this, the coldest and least well drained of the sites. The two inland sites were both net sources of carbon to the atmosphere over the three-year period, resulting in an average of 30 and 45 g C m__ yr__ at each of the sites. Site differences were the primary controls on carbon variation among the sites, but inter and intra-annual variation were also significant. These data represent the first continuous measurements in the Arctic tundra ecosystem, and highlight the high degree of heterogeneity in the tundra ecosystem. These data may be used to validate and further develop climate and ecosystem models and to more accurately depict the variability, both spatially and temporally, in the Arctic tundra ecosystem.
