An assessment of geographical distribution of different plant functional types over North America simulated using the CLASS–CTEM modelling framework

Rudra K Shrestha,Vivek K Arora,Joe R Melton,Laxmi Sushama

doi:10.5194/bg-14-4733-2017

Abstract

Abstract. The performance of the competition module of the CLASS–CTEM (Canadian Land Surface Scheme and Canadian Terrestrial Ecosystem Model) modelling framework is assessed at 1° spatial resolution over North America by comparing the simulated geographical distribution of its plant functional types (PFTs) with two observation-based estimates. The model successfully reproduces the broad geographical distribution of trees, grasses and bare ground although limitations remain. In particular, compared to the two observation-based estimates, the simulated fractional vegetation coverage is lower in the arid southwest North American region and higher in the Arctic region. The lower-than-observed simulated vegetation coverage in the southwest region is attributed to lack of representation of shrubs in the model and plausible errors in the observation-based data sets. The observation-based data indicate vegetation fractional coverage of more than 60 % in this arid region, despite only 200–300 mm of precipitation that the region receives annually, and observation-based leaf area index (LAI) values in the region are lower than one. The higher-than-observed vegetation fractional coverage in the Arctic is likely due to the lack of representation of moss and lichen PFTs and also likely because of inadequate representation of permafrost in the model as a result of which the C3 grass PFT performs overly well in the region. The model generally reproduces the broad spatial distribution and the total area covered by the two primary tree PFTs (needleleaf evergreen trees, NDL-EVG; and broadleaf cold deciduous trees, BDL-DCD-CLD) reasonably well. The simulated fractional coverage of tree PFTs increases after the 1960s in response to the CO2 fertilization effect and climate warming. Differences between observed and simulated PFT coverages highlight model limitations and suggest that the inclusion of shrubs, and moss and lichen PFTs, and an adequate representation of permafrost will help improve model performance.

Highlights

The terrestrial ecosystem plays an important role in regulating climate and weather through land–atmosphere exchange of water and energy (Cramer et al, 2001; Garnaud et al, 2015; Pielke et al, 1998; Ran et al, 2016) and in mitigating climate change by sequestering atmospheric CO2 (Bonan, 2008; Timmons et al, 2016)
This study evaluates the CLASS–CTEM-simulated fractional coverages of plant functional types (PFTs), when driven with observed meteorological forcing, against the observation-based estimates from MODIS and the modified WANG06 data sets over the North American region
Performance of the competition module of the CLASS–CTEM modelling framework has been assessed at a global scale, at a coarse spatial resoluwww.biogeosciences.net/14/4733/2017/

Summary

Introduction

The terrestrial ecosystem plays an important role in regulating climate and weather through land–atmosphere exchange of water and energy (Cramer et al, 2001; Garnaud et al, 2015; Pielke et al, 1998; Ran et al, 2016) and in mitigating climate change by sequestering atmospheric CO2 (Bonan, 2008; Timmons et al, 2016). The projected sink of atmospheric CO2 is uncertain due to disagreements among the Earth system models (Arora et al, 2013; Friedlingstein et al, 2006) primarily due to differing responses of their terrestrial ecosystem modules to future changes in atmospheric CO2. This uncertainty arises primarily because of the differences in the strength of the CO2 fertilization effect on the land carbon cycle components (Arora et al, 2013; Cramer et al, 2001; Friend et al, 2013) and because of differences in the response of vegetation.

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