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

Abstract

Abstract. To improve the simulation of vegetation-climate feedbacks in the high latitudes, three new circumpolar Plant Functional Types (PFTs) were added in the ORCHIDEE land surface model, namely non-vascular plants (NVPs) representing bryophytes and lichens, arctic shrubs, and arctic C3 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 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 C3 grasses have the same equations than in the original ORCHIDEE version, but different parameter values, optimized from in-situ observations of biomass and 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 optimization procedure. With the new PFTs, we obtain a lower Net Primary Productivity (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 a larger 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 y−1), owing to less evapotranspiration. Future developments should focus on the competition between these three PFTs and boreal trees PFTs, in order to simulate their area changes in response to climate change, and the effect of carbon-nitrogen interactions.

Modeling the Vegetation Dynamics of Northern Shrubs and Mosses in the ORCHIDEE Land Surface Model

“…ORCHIDEE was first described in Krinner et al (), with the vegetation dynamics module directly introduced from the Lund‐Potsdam‐Jena model (Sitch et al, ). The version of ORCHIDEE used in this study (ORC‐HL‐VEGv1.0, Druel et al, ) includes key processes relevant for high latitudes: (i) a soil‐freezing scheme and its effect on root water availability and soil thermodynamics in order to represent permafrost (Gouttevin et al, ), (ii) an improved snow scheme describing the snow pack with three explicit snow layers (Wang et al, ) based on the ISBA‐ES LSM (Boone, ), and (iii) nonvascular plants (bryophytes and lichens, referred as moss in this article) and shrubs as two new plant functional types (PFTs) compared to the original version of ORCHIDEE, with specific equations and parameters (Table S1). Using a prescribed constant spatial distribution of the PFTs from a satellite land cover map, Druel et al () analyzed the impact of including these new PFTs on the net and gross carbon fluxes and the surface energy budgets over the boreal and Arctic zone.…”

“…The Rubisco‐limited carboxylation rate is given by the parameter Vc max(25) at 25 °C and the electron transport limited rate Vj max is assumed to be proportional to Vc max . The formulation of Vc max depends upon temperature (Yin & Struik, ), with an acclimation to seasonal temperature conditions through the monthly mean temperature as detailed in Druel et al (). Boreal vegetation has a lower Vc max(25) than standard PFTs (Table S1) and thus lower potential productivity in optimal conditions.…”

“…For mosses, Vc max(25) = 28 μmol·m −2 ·s −1 (cf. to 70 μmol·m −2 ·s −1 for original grasses) and water stress occurs because of restricted access to soil water (99% of “roots” are considered above 11 cm for mosses vs. 50 cm for C3 grasses), as defined after a calibration of key model parameters with in situ data in Druel et al (). The higher long‐term resistance of mosses to soil water stress is due to their ability to dry out and restart as soon as favorable conditions return.…”

“…This reversible desiccation process was modeled by prescribing a lower mortality rate for mosses and a Vc max limitation based on a monthly soil moisture stress factor ( w s ) rather than a daily/hourly one. The stress factor is defined by the convolution of the moss root fraction in each layer with the relative soil water content (Druel et al, ). To take into account the higher water stress of grasses compare to mosses in water‐logged soils, we decrease the Vc max(25) of grasses when the soil is water saturated (1 ≥ w s > w s_ max ) by a so‐called anoxic factor ( a f ) defined by equation .…”

The pyrogeography of eastern boreal Canada from 1901 to 2012 simulated with the LPJ-LMfire model

“…While a previous study showed that the abundance of Picea decreases with latitude in the Tundra region and is coupled with the occurrence of dwarf shrubs in the Ericaceae and herbs (Gajewski et al, 1993), such species were not parametrized in the current version of LPJ-LMfire, due to a lack of information on their physiological and biogeographical preferences. Future research could incorporate recently developed parameterizations for 20 boreal shrubs and non-vascular plants into LPJ-LMfire (Druel, 2017;Druel et al, 2017).…”