Sub-grid scale representation of vegetation in global land surface schemes: implications for estimation of the terrestrial carbon sink

J. R. Melton,V. K. Arora

doi:10.5194/bg-11-1021-2014

Abstract

Abstract. Terrestrial ecosystem models commonly represent vegetation in terms of plant functional types (PFTs) and use their vegetation attributes in calculations of the energy and water balance as well as to investigate the terrestrial carbon cycle. Sub-grid scale variability of PFTs in these models is represented using different approaches with the "composite" and "mosaic" approaches being the two end-members. The impact of these two approaches on the global carbon balance has been investigated with the Canadian Terrestrial Ecosystem Model (CTEM v 1.2) coupled to the Canadian Land Surface Scheme (CLASS v 3.6). In the composite (single-tile) approach, the vegetation attributes of different PFTs present in a grid cell are aggregated and used in calculations to determine the resulting physical environmental conditions (soil moisture, soil temperature, etc.) that are common to all PFTs. In the mosaic (multi-tile) approach, energy and water balance calculations are performed separately for each PFT tile and each tile's physical land surface environmental conditions evolve independently. Pre-industrial equilibrium CLASS-CTEM simulations yield global totals of vegetation biomass, net primary productivity, and soil carbon that compare reasonably well with observation-based estimates and differ by less than 5% between the mosaic and composite configurations. However, on a regional scale the two approaches can differ by > 30%, especially in areas with high heterogeneity in land cover. Simulations over the historical period (1959–2005) show different responses to evolving climate and carbon dioxide concentrations from the two approaches. The cumulative global terrestrial carbon sink estimated over the 1959–2005 period (excluding land use change (LUC) effects) differs by around 5% between the two approaches (96.3 and 101.3 Pg, for the mosaic and composite approaches, respectively) and compares well with the observation-based estimate of 82.2 ± 35 Pg C over the same period. Inclusion of LUC causes the estimates of the terrestrial C sink to differ by 15.2 Pg C (16%) with values of 95.1 and 79.9 Pg C for the mosaic and composite approaches, respectively. Spatial differences in simulated vegetation and soil carbon and the manner in which terrestrial carbon balance evolves in response to LUC, in the two approaches, yields a substantially different estimate of the global land carbon sink. These results demonstrate that the spatial representation of vegetation has an important impact on the model response to changing climate, atmospheric CO2 concentrations, and land cover.

Highlights

Terrestrial ecosystem models (TEMs) or dynamic global vegetation models (DGVMs), with their associated land surface schemes (LSSs), are used in Earth system models (ESMs) to simulate the CO2 flux between the land surface and the atmosphere’s lower boundary
In our equilibrium spin-up simulations using CLASSCTEM, in either the composite or mosaic configurations, we see no large differences in the global sums of model variables like vegetation biomass, GPP, net primary productivity (NPP), soil C and litter mass between the two approaches (< 5 %)
These differences are most apparent in regions with high heterogeneity of land cover where the mosaic and composite representations are less comparable

Summary

Introduction

Terrestrial ecosystem models (TEMs) or dynamic global vegetation models (DGVMs), with their associated land surface schemes (LSSs), are used in Earth system models (ESMs) to simulate the CO2 flux between the land surface and the atmosphere’s lower boundary. Vegetation in ESMs is commonly represented in terms of broad plant functional types (PFTs) Appropriate representation of these PFTs’ spatial distribution presents a challenge to modellers, as the area of climate model grid cells is often on the order of 100 000 km. A grid cell with a land cover that is 20 % treed and 80 % herbaceous may represent a typical savannah landscape with intermittent trees, or a closed-canopy forest surrounded by prairie grasslands. In reality, these two landscapes represent greatly different physical and hydrological environments for the plants growing within them. In response to this requirement, the Earth system modelling community has adopted three main approaches to represent sub-grid scale vegetation variability within LSS frameworks, which are termed: (i) composite, (ii) mosaic, and (iii) mixed (following Li and Arora, 2012)

Methods

Results

Conclusion