Towards a more detailed representation of high-latitude vegetation in the global land surface model ORCHIDEE (ORC-HL-VEGv1.0)

Abstract

Abstract. Simulation of vegetation-climate feedbacks in high latitudes in the ORCHIDEE land surface model was improved by the addition of three new circumpolar plant functional types (PFTs), namely non-vascular plants representing bryophytes and lichens, Arctic shrubs and Arctic C 3 grasses. Non-vascular plants are assigned no stomatal conductance, very shallow roots, and can desiccate during dry episodes and become active again during wet periods, which gives them a larger phenological plasticity (i.e. adaptability and resilience to severe climatic constraints) compared to grasses and shrubs. Shrubs have a specific carbon allocation scheme, and differ from trees by their larger survival rates in winter, due to protection by snow. Arctic C 3 grasses have the same equations as in the original ORCHIDEE version, but different parameter values, optimised from in situ observations of biomass and net primary productivity (NPP) in Siberia. In situ observations of living biomass and productivity from Siberia were used to calibrate the parameters of the new PFTs using a Bayesian optimisation procedure. With the new PFTs, we obtain a lower NPP by 31 % (from 55 • N), as well as a lower roughness length (−41 %), transpiration (−33 %) and a higher winter albedo (by +3.6 %) due to increased snow cover. A simulation of the water balance and runoff and drainage in the high northern latitudes using the new PFTs results in an increase of fresh water discharge in the Arctic ocean by 11 % (+140 km 3 yr −1 ), owing to less evapotranspiration. Future developments should focus on the competition between these three PFTs and boreal tree PFTs, in order to simulate their area changes in response to climate change, and the effect of carbon-nitrogen interactions.

Distribution modelling of vegetation types based on area frame survey data

“…The advantage of simple ERMs is that they facilitate exploring responses of VTs to the individual important EVs -to find thresholds, cut-off values and limiting environmental conditions. This may, in turn, prove valuable for finding and improving the parameterization of variables that quantify plant functional types (PFTs) in dynamic vegetation modules of earth system models (Fisher et al, 2018), for the parameterization of new PFTs (Druel et al, 2017), as well as for validating the outputs from such models (Fisher et al, 2015). Simple ERMs also have the advantage of being more easily implemented in an automated procedure by which a large number of models may be produced in a computationally manageable process.…”

ORCHIDEE-MICT (v8.4.1), a land surface model for the high latitudes: model description and validation

“…S5). An explicit representation for non-vascular plants in ORCHIDEE (Druel et al, 2017) has been worked in parallel with this study at the moment, but would be incorporated into ORCHIDEE-MICT in the future developments. Fig.…”

“…However, a recent work (Chadburn et al, 2017) shows a late onset even in the absence of snow persistence on site simulations by ORCHIDEE. This calls for a revisit of the phenology-related thresholds at the high latitudes, perhaps by introducing new PFTs (arctic C3 grass and shrub, and non-vascular plants), with their separate set of parameters calibrated, to better represent Arctic vegetation and their phenology (Druel et al, 2017). There is also a lag of GPP (Fig.…”

“…mosses and lichens which could have respiration under winter low temperatures (Atanasiu, 1971). This suggests that we should either allow decomposition below the freezing point, perhaps to account for heat produced in the soil by microbial decomposition (Zimov et al, 1993;Hollesen et al, 2015), improve the snow insulation, prescribe an organic layer of insulating topsoil (e.g., mosses, O-horizons observed in boreal forests; see O'Donnell et al, 2011) into the thermal module, or explicitly represent the moss/lichen plants, including their carbon cycle and physical effects (Porada et al, 2016;Druel et al, 2017).…”

ORCHIDEE-PEAT (revision 4596), a model for northern peatland CO<sub>2</sub>, water, and energy fluxes on daily to annual scales

“…This simplification may cause discrepancies between model output and observations. Druel et al (2017) added non-vascular plants (bryophytes and lichens), boreal grasses, and shrubs into ORC-HL-VEGv1.0. Their work is parallel to our model and will be incorporated into the model in the future.…”