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Millennia peatland model


Peatlands in the UK have mostly been 'growing' for the last 8-10k years, since after the ice-age. Basically, plants take up carbon from the atmosphere through photosynthesis and put this carbon into plant biomass (leaves and roots). However, plants or parts of their biomass die annually, therefore annual litter cohorts are laid down and as plant matter decays only very slowly under wet and cold conditions in the uplands, the undecomposed organic matter is 'locked away' as long as conditions do not change (i.e. high water tables and low temperatures). Consequently, over millennia organic matter accumulates, leading to peat formation. The MILLENNIA model (Heinemeyer et al., 2010) captures this long-term peat accumulation and underlying changes in vegetation and water table dynamics over time. It has been validated and successfully compared to other UK peatland models by Clark et al. (2010) who pointed out several advantages of this model over others (mainly the dynamic (and total) peat depth, dynamic water table and vegetation feedbacks). We have since further developed the monthly model in collaboration with M. Carroll on his UKPopNet cranefly work (i.e. Carroll et al., 2011). This model performs well in comparison to site hydrological data (see previous figure), a prerequisite for modelling accurate carbon dynamics. However, the winter months still show some discrepancies to site data as there is no modelled snow representation. However, annual mean temperatures also seem crucial for key decomposers in peatlands, enchytraids (see Briones et al., 2007), which will be subjected to climate change alonside other biota crucial for bird polulations, such as crane flies (Carroll et al., 2011). We will therefore run the MILLENNIA model with climate change scenarios to determine likely impacts on such key species.

Importantly, the field flux measurements and the empirical models of carbon uptake (photosynthesis) and release (respiration) in connection with root exclusion will allow comparing field based estiamtes of NPP to predictions by the MILLENNIA model based on the real site climate data. Currently, the MILLENNIA model derives net carbon inputs (NPP) based on a simple empirical relationship to actual evapotranspiration (AET). This will offer an important validation for the MILLENNIA model and enable comparison of the simple versus a more process level driven calculation of NPP (considering light conditions and vegetation specific NPP estimates). A further opportunity will be the model inclusion of available vegetation and treatment specific water flux data. This will enable an estimation of the treatment impact on the water balance and discharge (flow patterns) in connection with runoff relationships to water table dynamics and vegetation. This will be achieved in a combination of GIS and the MILLENNIA model, also as part of the up-scaling attempts to larger regions (making use of the variance estimates from the ‘between site’ comparison to provide an uncertainty measure on model input parameters and processes).


The MILLENNIA model, considering past climate, dynamic vegetation and water table.
Bottom: preliminary modeled water table depth (WTD) data compared to site measurements from the three sites
(note the good overall agreement, but lower WTD at the Nid and Whit sites - likely due to drainage - STDEV ~4 cm)
v1 (left) unadjusted model runs; v2 (right) adjusted model runs, increasing drainage (hydraulic conductivity and specific yield in bedrock) at Nid and Whit