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Arctic Biosphere Atmosphere Coupling at Multiple Scales

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Hypothesis 3: Temporal transitions

The timing and rapidity of snowmelt in spring, and the onset of soil freezing in autumn, are critical controls on process, but their effects on energy balances and C cycling, and differential effects on plant and decomposer organisms, are not well understood. For example, evidence suggests that internal constraints limit the response of some plant species to lengthened growing seasons (Starr et al, 2000), while the activities of decomposer communities may be more closely related to overall length of thaw season. Nonetheless, temperature responsiveness of soil respiration varies substantially through the thaw period and growing season (Sjögersten & Wookey 2002a), and there is considerable uncertainty in apportioning seasonal variation in the relative contributions of recently fixed versus older C in soil respiration (Hirsch et al., 2002; Schimel & Mikan 2005). We suspect that in sites with larger SOM stores, decomposition will be responsive to wet, warm periods, while soil effluxes from sites with low SOM will be driven largely by plant activity, which has a stronger deterministic component (e.g. photoperiod) than do microbial processes.

H3a: Plant and soil C fluxes in dry sites are more closely coupled over time than in wet sites.

H3b: Wetter sites will have significant pulse losses of CO2, originating from older C, during spring thaw. We are also largely ignorant of seasonal changes in arctic surface energy balances, which we expect are closely coupled to snow-melt and local hydrological changes.

H3c: Land-atmosphere energy coupling is seasonally dependent through coupling to snow dynamics and local soil moisture.

WP1 will track seasonal patterns of plant activity, WPs 3, 4, 5 will monitor the impact of seasonality on chamber, tower and aircraft C/energy flux data. WP5 and 6 will provide imagery of seasonal land cover changes (vegetation and snow). WP2 will compare the mean residence time of soil respired CO2 (and thus the ‘age’ of SOM being metabolised) during spring thaw and at peak biomass (see 6 below), while the aircraft will capture air samples for 14C analysis, for comparison with soil-respired samples. WP7 will provide simulations of soil moisture, snow melt, C and energy fluxes for wet and dry sites for comparison with observations. WP4 will provide continuous measurements of soil moisture.

Outcome: Improved modelling of snow dynamics, energy balances, and seasonality of CO2 effluxes.

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Last modified: 27 Jan, 2006
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