The goal of this project goal was to address the question “what controls the temporal and spatial variability of water, net C and energy exchange between arctic ecosystems and the atmosphere?” Our multiscale studies in Sweden and Finland addressed this goal by the integrated deployment of a range of process and modelling studies.
Extreme winter warming events may significantly reduce the C sink capacity of sub-arctic dwarf shrub heathland. Short-lived, sudden winter warming events are increasing in frequency in some arctic regions, and past work has shown that these can damage arctic vegetation by triggering spring-like loss of cold hardiness in plants mid-winter. Analysis by the ABACUS team of C fluxes within dwarf shrub heathland subject to simulated extreme winter warming events showed that these climate change phenomena can significantly push this widespread arctic ecosystem type towards being a net source of C. This occurs primarily due to a reduction in gross primary productivity from the damaged plant canopy, rather than (as hypothesised) a stimulation of soil respiration from enhanced litter inputs from damaged plant communities. Given the arctic may warm more in winter than summer, this places greater uncertainly as to the greening of the arctic (and its capacity to increase C sequestration) in a future warmer world.
Seasonal controls on mountain birch CO2 assimilation. We identified strong seasonal and environmental controls on both branch-level photosynthesis and respiration by analysing the parameters of light response curves (Poyatos et al., in preparation). Photosynthetic parameters showed a delayed response to temperature, and were well described by a state of acclimation model with time constants of 7 days for maximum photosynthesis and 10 days for quantum efficiency. High temperature-independent values of the respiration parameter during leaf and shoot expansion were consistent with associated higher growth respiration rates. Growing season CO2 uptake of individual branches on a leaf area basis did not differ across two sub-Arctic sites (Abisko, Sweden and Kevo, Finland), and averaged ca. 640 g CO2 m-2. (WP3
Carbon allocation in mosses ABACUS PhD student Lorna Street undertook a series of isotopic tracer experiments that quantified the fate of fixed carbon in moss. We showed significant differences in C turnover between two contrasting arctic moss species (Polytrichum piliferum and Sphagnum fuscum). A modelling analysis indicated that the ratio of net to gross primary production differed significantly between the two species, 23% in Polytichum and 43% in Sphagnum. These results are the first to show differences in C partitioning between arctic bryophyte species in situ and highlight the importance of modelling C dynamics of this group separately from vascular plants for a realistic representation of vegetation in arctic C models.
Landscape-scale calculations of net methane flux. Chamber measurements were used to calculate net methane fluxes in the main plant communities found at our Kevo field site. Based on plant community coverage within a tower footprint, these fluxes were compared with those calculated by eddy covariance, and exceptional agreement between the two approaches was observed. This allowed us to scale measurements up the landscape (10 x 10 km) based on the results of an intercomparison between high-resolution aerial photography provided by the Edinburgh aircraft and earth observation data. As expected mires, and graminoid lawns in particular, were important point sources of methane, but other landscape units, including mountain birch forest, were methane sinks. Non-mire ecosystems may oxidise approximately 10% of the methane released from the wetlands. We calculated that the mean rate of methane release at the landscape level was 9 mg CH4 per metre square per day, during the growing season.