Abstract
Abstract. Processes that describe the distribution of vegetation and ecosystem succession after disturbance are an important component of dynamic global vegetation models (DGVMs). The vegetation dynamics module (ORC-VD) within the process-based ecosystem model ORCHIDEE (Organizing Carbon and Hydrology in Dynamic Ecosystems) has not been updated and evaluated since many years and is known to produce unrealistic results. This study presents a new parameterization of ORC-VD for mid- to high-latitude regions in the Northern Hemisphere, including processes that influence the existence, mortality and competition between tree functional types. A new set of metrics is also proposed to quantify the performance of ORC-VD, using up to five different data sets of satellite land cover, forest biomass from remote sensing and inventories, a data-driven estimate of gross primary productivity (GPP) and two gridded data sets of soil organic carbon content. The scoring of ORC-VD derived from these metrics integrates uncertainties in the observational data sets. This multi-data set evaluation framework is a generic method that could be applied to the evaluation of other DGVM models. The results of the original ORC-VD published in 2005 for mid- to high-latitudes and of the new parameterization are evaluated against the above-described data sets. Significant improvements were found in the modeling of the distribution of tree functional types north of 40° N. Three additional sensitivity runs were carried out to separate the impact of different processes or drivers on simulated vegetation distribution, including soil freezing which limits net primary production through soil moisture availability in the root zone, elevated CO2 concentration since 1850, and the effects of frequency and severity of extreme cold events during the spin-up phase of the model.
Highlights
The terrestrial biosphere plays an important role in the carbon (Schimel, 1995; Ciais et al, 2013), water (Oki and Kanae, 2006) and energy balances of Earth (Trenberth et al, 2009)
We use satellite observations of land cover translated into the plant functional types (PFTs) of ORCHIDEE to evaluate the simulated vegetation distribution
The PFT maps were aggregated at 2◦ × 2◦, matching the resolution run by ORCHIDEE in this study
Summary
The terrestrial biosphere plays an important role in the carbon (Schimel, 1995; Ciais et al, 2013), water (Oki and Kanae, 2006) and energy balances of Earth (Trenberth et al, 2009). To simulate past and future changes on long timescales, Earth system models must represent how the distribution and structure of ecosystems respond to changes in climate, CO2 and land use This need provides the motivation for the development of dynamic global vegetation models (DGVM). Since the first model description by Krinner et al (2005), the representation of existing processes has been improved and new processes have been implemented, such as a physically based multi-layer soil hydrology scheme (de Rosnay et al, 2002) and a scheme describing soil freezing and its effects on root-zone soil moisture and soil thermodynamics (Gouttevin et al, 2012) These new parameterizations have been evaluated for static runs in which the geographical distribution of PFTs is specified based on observed satellite land-cover information. Because the initial distribution of the vegetation in 1850 is sensitive to preindustrial climate conditions, we tested the effect of the return frequency of cold extremes relating to tree mortality during the spin-up phase of the model and discussed its implications
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