model represents the permanently frozen soil consid- ered as permafrost limit north of 50 As a direct consequence, the water content of surface layers may thus be underestimated, which directly impacts the soil. In ORCHIDEE, a first step is needed with the computation of a soil water budget for each PFT and not for the entire herbaceous layer as currently done, before we can properly represent this effect. (2015) chose to use a suction equation from Brooks and Corey (1964) to com- pute the plant water uptake and represent the "spongy" ef- fect of NVPs. Moreover, NVPs have an impact on the surface soil water dynamics, not currently explicitly modelled in ORCHIDEE. Also note that, while the NVP heat conductivity and heat capacity used in this study are in accordance with other experiments ( Soudzilovskaia et al., 2013 Chadburn et al., 2015), the av- erage thickness of mosses in our simulation is lower than the one prescribed in Chadburn et al. A further improvement will be to model explicitly the energy budget of the moss layer (and heat transfer). Further investigations are re- quired to determine whether this is an artefact of our choice to replace the standard soil thermal capacity and conductivity by intermediate values between those from NVPs and min- eral soil. This is at odds with results reported elsewhere (Jorgenson et al., 2010 Soudzilovskaia et al., 2013 Chadburn et al., 2015 Porada et al., 2016). Overall, we obtain a weak or negative impact of the new boreal vegetation implementation on the permafrost extent. NVPs insulate the soil as modelled in previous studies (Porada et al., 2016) but also increase the soil thermal conductivity through an increase of soil humidity due to a global decrease of tran- spiration. 13) and permafrost extent are impacted by the NVPs through two competing effects. substitution of a fraction of trees by shrubs largely contributes to the summer transpiration decrease.
With this formula- tion, we can take into account the impact of a drought on a monthly.
Indeed, leaf main- tenance respiration defined in ORCHIDEE being a function of the leaf carbon content (biomass) and LAI, should then be reduced when NVPs get desiccated. The maximum rate of electron transport (Vj max ) is scaled through Vc max. 3 scales the maximum rate of carboxylation (Vc max ) as well as the maintenance respiration. The function d ess (w s ) illustrated in Fig. 3) varying between 1 (no impact) and a minimum value d off, when w s decreases to zero under maxi- mum water stress. We defined a desiccation function, d ess, as a linear function of w s (Eq. In ORC-HL-VEGv1.0 we chose to use a monthly running mean hydric stress factor (w s ) computed from the relative water content in each soil layer weighted by the specific water uptake profile of NVPs defined in Fig. (2009) reduced gross primary production as a function of the annual mean water table position.
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