This study uses a 12-year time series (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) of eddy covariance measurements to investigate the long-term net ecosystem exchange (NEE) of carbon dioxide (CO 2 ) and interannual variations in relation to abiotic drivers in a boreal fen in northern Sweden. The peatland was a sink for atmospheric CO 2 in each of the twelve study years with a 12-year average (± standard deviation) NEE of −58 ± 21 g C m −2 yr −1 . For ten out of twelve years, the cumulative annual NEE was within a range of −42 to −79 g C m −2 yr −1 suggesting a general state of resilience of NEE to moderate inter-annual climate variations. However, the annual NEE of −18 and −106 g C m −2 yr −1 in 2006 and 2008, respectively, diverged considerably from this common range. The lower annual CO 2 uptake in 2006 was mainly due to late summer emissions related to an exceptional drop in water table level (WTL). A positive relationship (R 2 = 0.65) between pregrowing season (January to April) air temperature (Ta) and summer (June to July) gross ecosystem production (GEP) was observed. We suggest that enhanced GEP due to mild pregrowing season air temperature in combination with air temperature constraints on ecosystem respiration (ER) during the following cooler summer explained most of the greater net CO 2 uptake in 2008. Differences in the annual and growing season means of other abiotic variables (e.g. radiation, vapor pressure deficit, precipitation) and growing season properties (i.e. start date, end date, length) were unable to explain the inter-annual variations of NEE. Overall, our findings suggest that this boreal fen acts as a persistent contemporary sink for atmospheric CO 2 that is, however, susceptible to severe anomalies in WTL and pre-growing season air temperature associated with predicted changes in climate patterns for the boreal region.

Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate-carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO 2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid-and high-latitudes, (2) a strong function of dryness at mid-and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45 • N). The sensitivity of NEE to mean annual temperature breaks down at ∼16 • C (a threshold value of mean annual temperature), above which no further increase of CO 2 uptake with temperature was observed and dryness influence overrules temperature influence.

[1] Mires are key landscape elements at high latitudes and have certainly accumulated carbon during the Holocene, but their current carbon balance at the present time is very unclear. The major carbon flux is the land-atmosphere CO 2 exchange and full-year data are still limited. Here we present data from 3 a (2001)(2002)(2003) of continuous Eddy Covariance measurements at Degerö Stormyr (64°11 0 N, 19°33 0 E) an oligotrophic, minerotrophic mire in Sweden. The climate at the site is defined as cold temperate humid, with 30-a annual precipitation and temperature means of 523 mm and +1.2°C, respectively, while the mean temperatures in July and January are +14.7°C and À12.4°C, respectively. The length of the vegetation period was 153 ± 15 d during the measured years. The minerotrophic mire represented a net sink for the vertical exchange of atmospheric CO 2 -C during the 3 a, with an average net uptake of 55 ± 7 g (mean ± SD) CO 2 -C m À2 a À1 . The growing season average uptake was 92 ± 10 g CO 2 -C m À2 , of which approximately 40% (37 ± 5 g CO 2 -C m À2 ) was lost during the nongrowing season. The daily average uptake over the growing season was 0.65 ± 0.57, 0.73 ± 0.61, and 0.68 ± 0.62 g CO 2 -C m À2 d À1 in 2001, 2002, and 2003, respectively. The daily average net uptake for the month with highest uptake was 1.10 ± 0.33, 1.11 ± 0.63, and 1.22 ± 0.55 g CO 2 -C m À2 d À1

A B S T R A C T Net CO 2 exchange measured under well-mixed atmospheric conditions in four different mires in Sweden and Finland were used to analyse which factors were controlling photosynthesis and respiration. The parameters of a light response function showed strong seasonal variations with similar behaviour for all mires. The half-monthly nighttime respiration rates in the central part of the growing season were about two times higher in the southernmost, warmest site, Fäje, as compared to the northernmost, coldest site, Kaamanen. However, Kaamanen had high photosynthesis rates, and this in combination with the long daylight periods in the middle of the summer caused Kaamanen to have the largest net ecosystem exchange (NEE) during the summer period. Fäje that showed the highest productivity had also the highest respiration and therefore, the lowest NEE during summer. Correlation between half-monthly components and different environmental variables showed the highest correlation between the components themselves. Thereafter came temperature except for Fäje where water table depth (WTD) explained most of the variance both for detrended and temperature-normalized components. All sites showed dependencies between WTD and the respective components during drying up periods. Temperature sensitivity was higher for productivity than for respiration indicating that CO 2 uptake would increase during global warming.

[1] In mires, which occupy large areas of the boreal region, net ecosystem CO 2 exchange (NEE) rates vary significantly over various timescales. In order to examine the effect of one of the most influencing variables, the water table depth, on NEE the general ecosystem model GUESS-ROMUL was modified to predict mire daily CO 2 exchange rates. A simulation was conducted for a lawn, the most common microtopographical feature of boreal oligotrophic minerotrophic mires. The results were validated against eddy covariance CO 2 flux measurements from Degerö Stormyr, northern Sweden, obtained during the period [2001][2002][2003]. Both measurements and model simulations revealed that CO 2 uptake was clearly controlled by interactions between water table depth and temperature. Maximum uptake occurred when the water table level was between 10 and 20 cm and the air temperature was above 15°C. When the water table was higher, the CO 2 uptake rate was lower, owing to reduced rates of photosynthetic carbon fixation. When the water table was lower, NEE decreased owing to the increased rate of decomposition of organic matter. When the water table level was between 10 and 20 cm, the NEE was quite stable and relatively insensitive to both changes within this range and any air temperature changes above +15°C. The optimal water table level range for NEE corresponds to that characteristic of mire lawn plant communities, indicating that the annual NEE will not change dramatically if climatic conditions remain within the optimal range for the current plant community.