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While substantial cold-season respiration has been documented in most arctic and alpine ecosystems in recent years, the significance of cold-season photosynthesis in these biomes is still believed to be small. In a mesic, subartic heath during both the cold and warm season, we measured in situ ecosystem respiration and photosynthesis with a chamber technique at ambient conditions and at artificially increased frequency of freeze–thaw (FT) cycles during fall and spring. We fitted the measured ecosystem exchange rates to respiration and photosynthesis models with R2-values ranging from 0.81 to 0.85. As expected, estimated cold-season (October, November, April and May) respiration was significant and accounted for at least 22% of the annual respiratory CO2 flux. More surprisingly, estimated photosynthesis during this period accounted for up to 19% of the annual gross CO2 uptake, suggesting that cold-season photosynthesis partly balanced the cold-season respiratory carbon losses and can be significant for the annual cycle of carbon. Still, during the full year the ecosystem was a significant net source of 120 ± 12 g C m−2 to the atmosphere. Neither respiration nor photosynthetic rates were much affected by the extra FT cycles, although the mean rate of net ecosystem loss decreased slightly, but significantly, in May. The results suggest only a small response of net carbon fluxes to increased frequency of FT cycles in this ecosystem. [ABSTRACT FROM AUTHOR]

Terrestrial ecosystem respiration ( R eco ) represents a major component of the global carbon cycle. It consists of many sub-components, such as aboveground plant respiration and belowground root and microbial respiration, each of which may respond differently to abiotic factors, and thus to global climate change. To correctly predict future carbon cycles in forest ecosystems, R eco must therefore be partitioned and understood for each of its various components. In this study we used the eddy covariance technique together with manual and automated closed-chambers to quantify the individual components of R eco in a temperate beech forest at diel, seasonal and annual time scales. R eco was measured by eddy covariance while respiration rates from soil, tree stems and isolated coarse tree roots were measured bi-hourly by an automated closed-chamber system. Soil respiration ( R soil ) was measured in intact plots, and heterotrophic R soil was measured in trenched plots. Tree stem ( R stem ) and coarse root ( R root ) respiration were measured by custom made closed-chambers. We found that the contribution of R stem to total R eco varied across the year, by only accounting for 6% of R eco during winter and 16% during the summer growing season. In contrast R soil was approximately half of R eco during winter (52%), spring (45%) and summer (49%), while the contribution increased to 79% during autumn. Based on observed fluxes in the trenched and intact soil plots, we found that autotrophic R soil accounted for 34% of R soil during summer, i.e. a relatively low fractional estimate compared to findings from other studies. It is likely that dead roots were still decomposing in the trenched soil plots thus causing overestimation of heterotrophic R soil . Diel R stem and R root measurements showed a distinct pattern during summer with the highest respiration rates around 13:00-15:00 CET for R stem , and the highest respiration seen from 9:00–15:00 for R root . In contrast, R soil showed the lowest respiration during daytime with no clear difference in the diel pattern between the intact and trenched soil plots. Finally, we calculated annual R soil for different transects, and found that annual R soil estimated from the previously used transect at the site was underestimated due to R soil of the transect not being representative for the spatial heterogeneity of R soil at the site. This highlights the importance of performing a sufficient number of chamber measurements at a site to adequately capture the spatial variation and estimate R soil correctly. [ABSTRACT FROM AUTHOR]