Incorporating grassland management in ORCHIDEE: model description and evaluation at 11 eddy-covariance sites in Europe

J F Chang,P Ciais,J.-F Soussana,R Lardy,T Wang,N Viovy,A.-I Graux,K Klumpp,N Vuichard,A Cozic,R Martin

doi:10.5194/gmd-6-2165-2013

Abstract

Abstract. This study describes how management of grasslands is included in the Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) process-based ecosystem model designed for large-scale applications, and how management affects modeled grassland–atmosphere CO2 fluxes. The new model, ORCHIDEE-GM (grassland management) is enabled with a management module inspired from a grassland model (PaSim, version 5.0), with two grassland management practices being considered, cutting and grazing. The evaluation of the results from ORCHIDEE compared with those of ORCHIDEE-GM at 11 European sites, equipped with eddy covariance and biometric measurements, shows that ORCHIDEE-GM can realistically capture the cut-induced seasonal variation in biometric variables (LAI: leaf area index; AGB: aboveground biomass) and in CO2 fluxes (GPP: gross primary productivity; TER: total ecosystem respiration; and NEE: net ecosystem exchange). However, improvements at grazing sites are only marginal in ORCHIDEE-GM due to the difficulty in accounting for continuous grazing disturbance and its induced complex animal–vegetation interactions. Both NEE and GPP on monthly to annual timescales can be better simulated in ORCHIDEE-GM than in ORCHIDEE without management. For annual CO2 fluxes, the NEE bias and RMSE (root mean square error) in ORCHIDEE-GM are reduced by 53% and 20%, respectively, compared to ORCHIDEE. ORCHIDEE-GM is capable of modeling the net carbon balance (NBP) of managed temperate grasslands (37 ± 30 gC m−2 yr−1 (P < 0.01) over the 11 sites) because the management module contains provisions to simulate the carbon fluxes of forage yield, herbage consumption, animal respiration and methane emissions.

Highlights

Grassland is a widespread vegetation type, which covers 20 to 40 percent (26.8 to 56 million km2) of the whole land surface on the earth, depending on the grassland definition (Suttie et al, 2005), and plays a significant role in the global carbon (C) cycle
The average growing season specific leaf area (SLA) across the 11 sites in ORCHIDEE-GM is of 0.0424 ± 0.0010 m2 g−1C, which is close to the observed value of 0.0201 m2 g−1 dry matter (DM) reported by Kattge et al (2011) in the TRY database for 594 species (5033 observations) around the world
This paper is an attempt to realistically represent the impact of management on the C fluxes of European grasslands in a dynamic global vegetation models (DGVMs)

Summary

Introduction

Grassland is a widespread vegetation type, which covers 20 to 40 percent (26.8 to 56 million km2) of the whole land surface on the earth, depending on the grassland definition (Suttie et al, 2005), and plays a significant role in the global carbon (C) cycle. Grasslands were estimated to be a net C sink of about 0.5 PgC per year (Scurlock and Hall, 1998), but with considerable uncertainty. Schulze et al (2009) recently inferred a net C sink in European grasslands of 57 ± 34 gC m−2 yr−1 from a small sample of flux tower net ecosystem exchange (NEE) measurements, completed by C imports/exports at each site to estimate net biome production (NBP). When accounting for emissions of nonCO2 greenhouse gases (GHGs) such as methane (CH4) from grazing animals and nitrous oxide from soil nitrogen (N) nitrification/denitrification processes, the European grasslands were estimated to be nearly neutral for their radiative forcing, with a net balance of −14±18 g CO2-C eq m−2 y−1 (Schulze et al, 2010). Grasslands sequester C in soils – and sequestration is likely favored by high belowground C allocation and root turnover, and possibly by N fertilization (Schulze et al, 2010).

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