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Arctic terrestrial ecosystems may be contributing to increasing atmospheric carbon dioxide (CO2) concentrations under amplified climate change at high‐latitudes. This research investigates how summer net ecosystem CO2 exchange (NEE) and its component fluxes, gross primary production (GPP), and ecosystem respiration (Reco) varied over five years (2008, 2009, 2010, 2012, and 2014) at the Cape Bounty Arctic Watershed Observatory (74.92°N, 109.58°W). The eddy covariance technique was used to measure NEE and a combined light and temperature response model was used to partition NEE into GPP and Reco. Total summer NEE varied from −19.8 to 7.9 g C m−2. Despite two summers with net CO2 uptake, this tundra was likely a source of CO2 on an annual basis. In most years, growing degree days with a base 0°C (GDD0) had more predictive power than other environmental variables in random forest analyses of daily NEE. Interannual variability in total summer NEE over the five study years was also attributed to greater variability in GPP than Reco (coefficient of variation: 62% and 27%, respectively). However, total summer GDD0 significantly correlated with total summer Reco but not NEE nor GPP. Instead, summer total NEE and GPP significantly correlated with satellite‐derived normalized difference vegetation index (NDVI), which may have exhibited carry‐over effects from one year to the next to limit the impact of current year GDD0 on total summer NEE at this tundra site. Plain Language Summary: Arctic ecosystems currently store a vast amount of carbon within frozen soils. Over the last two decades, Arctic surface air temperatures have increased at a rate twice the global average, which has the potential to shift terrestrial carbon cycling, ultimately impacting atmospheric carbon dioxide (CO2), a potent greenhouse gas. Whether Arctic terrestrial ecosystems are releasing more CO2 than they are absorbing is important, as this can impact the global climate system. This research measured CO2 exchange between the atmosphere and the tundra over five summers at the Cape Bounty Arctic Watershed Observatory (74.92°N, 109.58°W) on Melville Island, Nunavut, Canada. The ecosystem acted as a CO2 source for three out of five summers and is likely an annual net CO2 source to the atmosphere. Interannual differences in weather appeared to impact plant productivity and CO2 uptake more than decomposition and respiration and CO2 emissions to the atmosphere at this site. The amount of CO2 absorbed or lost from this tundra each summer was correlated to satellite observations, which could help us monitor the carbon balance of these remote but important ecosystems. This research highlights the importance of multiyear observations to constrain understanding of High Arctic tundra CO2 dynamics. Key Points: Over five summers, a High Arctic tundra site with low productivity varied from being a net source to a net sink for atmospheric CO2Summer net CO2 uptake depended more on photosynthesis than respiration and correlated well with satellite‐derived NDVIDaily net ecosystem CO2 exchange could be predicted using growing degree days and other environmental variables using random forests [ABSTRACT FROM AUTHOR]

Climate warming is affecting terrestrial ecosystems in the Canadian Arctic, potentially altering the carbon balance of the landscape and contributing additional CO2 to the atmosphere. High spatial resolution remote sensing data can enhance models of net ecosystem exchange (NEE) and its component fluxes, gross ecosystem exchange (GEE), and ecosystem respiration (ER) by quantifying vegetation structure and function over time. In this study, we explored the variability of daytime CO2 exchange rates for three vegetation types along a natural moisture gradient at ecologically distinct mid- and high Arctic sites. We demonstrated that for the two sites studied, there was no statistically significant variation in CO2 exchange rates for the vegetation types through the peak growing season. Hence, the capacity to model these rates with a limited number of satellite data acquisitions is feasible. Simple bivariate models relating the Normalized Difference Vegetation Index (NDVI) to CO2 exchange processes (GEE, ER, and NEE) were developed independent of vegetation type and geographic location and validated using independent data. The spectral models explain between 33 and 94 percent of the variation in CO2 exchange rates at each site, indicating a high level of functional convergence in ecosystem-level structure and function within Arctic landscapes. [ABSTRACT FROM AUTHOR]