No abstract

The functioning of Arctic ecosystems is not only critically affected by climate change, but it also has the potential for major positive feedback on climate. There is, however, relatively little information on the role, patterns, and vulnerabilities of CO 2 fluxes during the nonsummer seasons in Arctic ecosystems. Presented here is a year-round study of CO 2 fluxes in an Alaskan Arctic tussock tundra ecosystem, and key environmental controls on these fluxes. Important controls on fluxes vary by season. This paper also presents a new empirical quantification of seasons in the Arctic based on net radiation. The fluxes were computed using standard FluxNet methodology and corrected using standard Webb-Pearman-Leuning density terms adjusted for influences of open-path instrument surface heating. The results showed that the nonsummer season comprises a significant source of carbon to the atmosphere. The summer period was a net sink of 24.3 g C m À2 , while the nonsummer seasons released 37.9 g C m À2 . This release is 1.6 times the summer uptake, resulting in a net annual source of +13.6 g C m À2 to the atmosphere. These findings support early observations of a change in this particular region of the Arctic from a long-term annual sink of CO 2 to an annual source from the terrestrial ecosystem and soils to the atmosphere. The results presented here demonstrate that nearly continuous observations may be required in order to accurately calculate the annual net ecosystem CO 2 exchange of Arctic ecosystems and to build predictive understanding that can be used to estimate, with confidence, Arctic fluxes under future conditions.

The atmospheric methane (CH 4 ) concentration, a potent greenhouse gas, is on the rise once again, making it critical to understand the controls on CH 4 emissions. In Arctic tundra ecosystems, a substantial part of the CH 4 budget originates from the cold season, particularly during the "zero curtain" (ZC), when soil remains unfrozen around 0°C. Due to the sparse data available at this time, the controls on cold season CH 4 emissions are poorly understood. This study investigates the relationship between the fall ZC and CH 4 emissions using long-term soil temperature measurements and CH 4 fluxes from four eddy covariance (EC) towers in northern Alaska. To identify the large-scale implication of the EC results, we investigated the temporal change of terrestrial CH 4 enhancements from the National Oceanic and Atmospheric Administration monitoring station in Utqiaġvik, AK, from 2001 to 2017 and their association with the ZC. We found that the ZC is extending later into winter (2.6 ± 0.5 days/year from 2001 to 2017) and that terrestrial fall CH 4 enhancements are correlated with later soil freezing (0.79 ± 0.18-ppb CH 4 day −1 unfrozen soil). ZC conditions were associated with consistently higher CH 4 fluxes than after soil freezing across all EC towers during the measuring period (2013)(2014)(2015)(2016)(2017). Unfrozen soil persisted after air temperature was well below 0°C suggesting that air temperature has poor predictive power on CH 4 fluxes relative to soil temperature. These results imply that later soil freezing can increase CH 4 loss and that soil temperature should be used to model CH 4 emissions during the fall.

The aim of this study was to systematically analyze the potential and limitations of using plant functional trait observations from global databases versus in situ data to improve our understanding of vegetation impacts on ecosystem functional properties (EFPs). Using ecosystem photosynthetic capacity as an example, we first provide an objective approach to derive robust EFP estimates from gross primary productivity (GPP) obtained from eddy covariance flux measurements. Second, we investigate the impact of synchronizing EFPs and plant functional traits in time and space to evaluate their relationships, and the extent to which we can benefit from global plant trait databases to explain the variability of ecosystem photosynthetic capacity. Finally, we identify a set of plant functional traits controlling ecosystem photosynthetic capacity at selected sites. Suitable estimates of the ecosystem photosynthetic capacity can be derived from light response curve of GPP responding to radiation (photosynthetically active radiation or absorbed photosynthetically active radiation). Although the effect of climate is minimized in these calculations, the estimates indicate substantial interannual variation of the photosynthetic capacity, even after removing site‐years with confounding factors like disturbance such as fire events. The relationships between foliar nitrogen concentration and ecosystem photosynthetic capacity are tighter when both of the measurements are synchronized in space and time. When using multiple plant traits simultaneously as predictors for ecosystem photosynthetic capacity variation, the combination of leaf carbon to nitrogen ratio with leaf phosphorus content explains the variance of ecosystem photosynthetic capacity best (adjusted R 2 = 0.55). Overall, this study provides an objective approach to identify links between leaf level traits and canopy level processes and highlights the relevance of the dynamic nature of ecosystems. Synchronizing measurements of eddy covariance fluxes and plant traits in time and space is shown to be highly relevant to better understand the importance of intra‐ and interspecific trait variation on ecosystem functioning.

No abstract