Long-term sequestration of carbon in Alaskan Arctic tundra ecosystems was reversed by warming and drying of the climate in the early 1980s, resulting in substantial losses of terrestrial carbon. But recent measurements suggest that continued warming and drying has resulted in diminished CO2 efflux, and in some cases, summer CO2 sink activity. Here we compile summer CO2 flux data for two Arctic ecosystems from 1960 to the end of 1998. The results show that a return to summer sink activity has come during the warmest and driest period observed over the past four decades, and indicates a previously undemonstrated capacity for ecosystems to metabolically adjust to long-term (decadal or longer) changes in climate. The mechanisms involved are likely to include changes in nutrient cycling, physiological acclimation, and population and community reorganization. Nevertheless, despite the observed acclimation, the Arctic ecosystems studied are still annual net sources of CO2 to the atmosphere of at least 40 g C m(-2) yr(-1), due to winter release of CO2, implying that further climate change may still exacerbate CO2 emissions from Arctic ecosystems.

[1] The investigation of the microtopographic controls on thermal and hydrologic conditions of the soil and consequently the carbon dynamics from Arctic regions is of major importance. Ecosystem respiration (ER) between microsites of the same tundra type could differ more than ER in different tundra types even at different latitudes. In this study we investigated the microtopographic effect on soil temperature, thaw depth, pH, oxidation reduction potential (ORP), electrical conductivity (EC), dissolved CO 2 , vegetation types, and ER rates from different features forming the low-center polygon structure. Most of these environmental variables significantly differ particularly between areas with higher elevation (polygon rims) and with lower elevation (low-center polygons). Polygon rims presented the lowest water table and showed the lowest thaw depth and the highest ER (a seasonal average of 1 mmol CO 2 m −2 s −1 ), almost double than the ER in the low-center polygons (a seasonal average of 0.6 mmol CO 2 m −2 s −1 ). The microtopographic gradient from polygon rims to low-centers led to a very consistent pattern in pH, EC, ORP and dissolved CO 2 , with low-centers presenting the highest pH, the highest EC, the highest dissolved CO 2 , and the lowest ORP. Based on vegetation measurements, we also showed that microtopography controls the lateral flow of organic matter, and that vascular plant material accumulates as litter in the lower elevation areas, possibly contributing to the higher dissolved CO 2 in the low-center polygons. Microtopography, and the ramifications discussed here, should be considered when evaluating landscape scale environmental controls on carbon dynamics in the Arctic.

[1] Temporal and spatial variability in the Arctic introduces considerable uncertainty in the estimation of the current carbon budget and Arctic ecosystem response to climate change. Few representative measurements are available for land-surface parameterization of the Arctic tundra in regional and global climate models. In this study, the eddy covariance technique was used to measure net ecosystem CO 2 exchange (NEE) of Alaskan wet sedge tundra and moist tussock tundra ecosystems during the summer (i.e., 1 June to 31 August) from 1999 to 2003 in order to quantify the seasonal and spatial variability in NEE and to determine controlling factors on NEE in these tundra ecosystems. Warmer and drier conditions prevailed for the moist tussock tundra compared with that of the wet sedge tundra. Over the 5-year period, the wet sedge tundra was a sink for carbon of 46.4 to 70.0 gC m À2 season À1 , while the moist tussock tundra either lost carbon of up to 60.8 gC m À2 season À1 or was in balance. The contrasting patterns of carbon balance at the two sites demonstrate that ecosystem difference can be more important in determining landscape NEE than intraseasonal and interseasonal variability due to environmental factors with respect to NEE. The wet sedge tundra showed an acclimation (e.g., over days) to temperature, while the moist tussock tundra illustrated a strong temperature dependence. Warming and drying accentuated ecosystem respiration in the moist tussock tundra, causing a net loss of carbon. Better characterization of spatial variability in NEE and associated environmental controls is required to improve current and future estimates of the Arctic terrestrial carbon balance.Citation: Kwon, H.-J., W. C. Oechel, R. C. Zulueta, and S. J. Hastings (2006), Effects of climate variability on carbon sequestration among adjacent wet sedge tundra and moist tussock tundra ecosystems,

Summary Net CO2 flux measurements conducted during the summer and winter of 1994–96 were scaled in space and time to provide estimates of net CO2 exchange during the 1995–96 (9 May 1995–8 May 1996) annual cycle for the Kuparuk River Basin, a 9200 km2 watershed located in NE Alaska. Net CO2 flux was measured using dynamic chambers and eddy covariance in moist‐acidic, nonacidic, wet‐sedge, and shrub tundra, which comprise 95% of the terrestrial landscape of the Kuparuk Basin. CO2 flux data were used as input to multivariate models that calculated instantaneous and daily rates of gross primary production (GPP) and whole‐ecosystem respiration (R) as a function of meteorology and ecosystem development. Net CO2 flux was scaled up to the Kuparuk Basin using a geographical information system (GIS) consisting of a vegetation map, digital terrain map, dynamic temperature and radiation fields, and the models of GPP and R. Basin‐wide estimates of net CO2 exchange for the summer growing season (9 May−5 September 1995) indicate that nonacidic tundra was a net sink of −31.7 ± 21.3 GgC (1 Gg = 109 g), while shrub tundra lost 32.5 ± 6.3 GgC to the atmosphere (negative values denote net ecosystem CO2 uptake). Acidic and wet sedge tundra were in balance, and when integrated for the entire Kuparuk River Basin (including aquatic surfaces), whole basin summer net CO2 exchange was estimated to be in balance (−0.9 ± 50.3 GgC). Autumn to winter (6 September 1995–8 May 1996) estimates of net CO2 flux indicate that acidic, nonacidic, and shrub tundra landforms were all large sources of CO2 to the atmosphere (75.5 ± 8.3, 96.4 ± 11.4, and 43.3 ± 4.7 GgC for acidic, nonacidic, and shrub tundra, respectively). CO2 loss from wet sedge surfaces was not substantially different from zero, but the large losses from the other terrestrial landforms resulted in a whole basin net CO2 loss of 217.2 ± 24.1 GgC during the 1995–96 cold season. When integrated for the 1995–96 annual cycle, acidic (66.4 + 25.25 GgC), nonacidic (64.7 ± 29.2 GgC), and shrub tundra (75.8 ± 8.4 GgC) were substantial net sources of CO2 to the atmosphere, while wet sedge tundra was in balance (0.4 + 0.8 GgC). The Kuparuk River Basin as a whole was estimated to be a net CO2 source of 218.1 ± 60.6 GgC over the 1995–96 annual cycle. Compared to direct measurements of regional net CO2 flux obtained from aircraft‐based eddy covariance, the scaling procedure provided realistic estimates of CO2 exchange during the summer growing season. Although winter estimates could not be assessed directly using aircraft measurements of net CO2 exchange, the estimates reported here are comparable to measured values reported in the literature. Thus, we have high confidence in the summer estimates of net CO2 exchange and reasonable confidence in the winter net CO2 flux estimates for terrestrial landforms of the Kuparuk river basin. Although there is larger uncertainty in the aquatic estimates, the small surface area of aquatic surfaces in the Kuparuk river basin (≈ 5%) presumably reduces the potential f...

Carbon flux in arid and semiarid area shrublands, especially in old-growth shrub ecosystems, has been rarely studied using eddy covariance techniques. In this study, eddy covariance measurements over a 100-year old-growth chamise-dominated chaparral shrub ecosystem were conducted for 7 years from 1996 to 2003. A carbon sink, from À96 to À155 g C m À2 yr À1 , was determined under normal weather conditions, while a weak sink of À18 g C m À2 yr À1 and a strong source of 207 g C m À2 yr À1 were observed as a consequence of a severe drought. The annual sink strength of carbon in the 7-year measurement period was À52 g C m À2 yr À1 . The results from our study indicate that, in contrast to previous thought, the old-growth chaparral shrub ecosystem can be a significant sink of carbon under normal weather conditions and, therefore, be an important component of the global carbon budget.