Towards a representation of complex ecosystems in the ORCHIDEE Land Surface Model

Abstract

Complex ecosystems, such as mixed forests or savannahs, are poorly represented in Land Surface Models (LSM). Those models mainly use simple and efficient representations such as the “big leaf” model for the energy budget in order to minimize time calculation. However, this approach prevents them from modelling more complex processes such as intra-canopy climate or competition for water between different vegetation strata which are highly important processes in order to understand the behavior and the responses of complex and mixed ecosystems in a changing climate. Although some ecosystem-specific models start to represent the 3D structure of complex ecosystems, including competition for light, water and nutrient between species and vertical / horizontal organization, these approaches are still too complex to be fully included in global LSM. However, first steps can be made towards this direction by representing the exchanges and interactions of biophysical fluxes such as water, carbon and energy. This study proposes some first steps towards this direction. We refined the computation of the energy and water transfers in the soil –plant – atmosphere continuum, working both on the horizontal and vertical heterogeneity. On the water transfers side, we implemented the soil-plant-atmosphere continuum model developed by Tuzet et al. (2017) which introduces a proper representation of the water flow inside the vegetation and a stronger coupling between plant water status and stomatal conductance. On the energy budget point of view, we implemented the multi-layer energy budget developed by Ryder et al. (2016) which represents the exchanges and turbulent transport of light and energy within a canopy. Finally, those two works being adapted for site-level modelling, we introduced a sub-grid heterogeneity representation of the energy and water budget in order to implement those developments for global applications. The study focuses on the two first developments which are firstly tested over several forest sites where intra-canopy gradients of humidity and temperature have been measured. A model inter-comparison between two LSM who have developed a vertical multi-layer energy budget, ORCHIDEE and CLM5 (Lawrence et al. (2019)), and the forest model MuSICA (Ogée et al. (2003)) allowed to highlight some of the model strengths and weaknesses. Finally, the expected improvements for complex ecosystems modelling and future developments in ORCHIDEE based on those representations will be presented.