Abstract
Abstract. The Canadian Terrestrial Ecosystem Model (CTEM) is the interactive vegetation component in the Earth system model of the Canadian Centre for Climate Modelling and Analysis. CTEM models land–atmosphere exchange of CO2 through the response of carbon in living vegetation, and dead litter and soil pools, to changes in weather and climate at timescales of days to centuries. Version 1.0 of CTEM uses prescribed fractional coverage of plant functional types (PFTs) although, in reality, vegetation cover continually adapts to changes in climate, atmospheric composition and anthropogenic forcing. Changes in the spatial distribution of vegetation occur on timescales of years to centuries as vegetation distributions inherently have inertia. Here, we present version 2.0 of CTEM, which includes a representation of competition between PFTs based on a modified version of the Lotka–Volterra (L–V) predator–prey equations. Our approach is used to dynamically simulate the fractional coverage of CTEM's seven natural, non-crop PFTs, which are then compared with available observation-based estimates. Results from CTEM v. 2.0 show the model is able to represent the broad spatial distributions of its seven PFTs at the global scale. However, differences remain between modelled and observation-based fractional coverage of PFTs since representing the multitude of plant species globally, with just seven non-crop PFTs, only captures the large-scale climatic controls on PFT distributions. As expected, PFTs that exist in climate niches are difficult to represent either due to the coarse spatial resolution of the model, and the corresponding driving climate, or the limited number of PFTs used. We also simulate the fractional coverage of PFTs using unmodified L–V equations to illustrate its limitations. The geographic and zonal distributions of primary terrestrial carbon pools and fluxes from the versions of CTEM that use prescribed and dynamically simulated fractional coverage of PFTs compare reasonably well with each other and observation-based estimates. The parametrization of competition between PFTs in CTEM v. 2.0 based on the modified L–V equations behaves in a reasonably realistic manner and yields a tool with which to investigate the changes in spatial distribution of vegetation in response to future changes in climate.
Highlights
Dynamic global vegetation models (DGVMs) are considered an integral component of Earth system models (ESMs)
The performance of the original L–V-based competition parametrization (LVCOMP) and its modified version as implemented in Canadian Terrestrial Ecosystem Model (CTEM) (CTCOMP) is evaluated at the most basic level in Fig. 1a comparing the simulated coverage of trees and grasses, and the bare and vegetated areas, at the global scale with observation-based estimates of these quantities from the modified W2006 and Moderate-Resolution Imaging Spectroradiometer (MODIS)-derived data sets
Simulated coverage by CTCOMP lies in-between the observation-based estimates based on the modified W2006 and the MODIS-derived products, except for the area covered by grasses, which is lower than both observation-based estimates (13 and 24 %, respectively)
Summary
Dynamic global vegetation models (DGVMs) are considered an integral component of Earth system models (ESMs). DGVM-based competition schemes are either based on (i) the Lotka–Volterra (L–V) equations (e.g. TRIFFID, Cox, 2001; University of Sheffield Conifer Model (USCM), Brentnall et al, 2005; and CTEM, Arora and Boer, 2006a, b), (ii) simpler approaches that assume PFTs occupy area in proportion to their net primary productivity (NPP) (e.g. LPJ, Sitch et al, 2003; the Organizing Carbon and Hydrology in Dynamic Ecosystems model (ORCHIDEE), Krinner et al, 2005; Zhu et al, 2015) or (iii) account for competitiveness via both NPP and mortality (e.g. Jena Scheme for Biosphere–Atmosphere Coupling in Hamburg (JSBACH); Brovkin et al, 2009). A detailed model description of CTEM v. 2.0 is provided in the Appendix
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