1DONE: Structural assumptions¶

Every process accounted for in ORCHIDEE comes with at least one, if not several, assumptions. As such, there are 100’s of assumptions underlying the ORCHIDEE model. The complexity of this kind of model makes it no longer feasible to state all assumptions Rykiel, 1996. The focus is therefore on listing the six structural assumptions of the ORCHIDEE model, in other words, the assumptions that determine the applicability of ORCHIDEE. Unless mentioned otherwise, these assumptions are common to all ORCHIDEE versions.

1. Assumption on vegetation demography. Although ecosystem attributes such as demography, growth rates, mortality, extinction rates, and community structure are all emergent properties of individual-based systems Benton et al., 2006Uchmanski et al., 2008, the ORCHIDEE model, assumes that interactions between individuals can be represented through statistical-averaging. For the purpose of the ORCHIDEE land surface model, simulating ecosystem dynamics in forests, grasslands and croplands does not require the inclusion of every individual within the community. From an ecological point of view, the simulation unit of ORCHIDEE is thus the community level which sets ORCHIDEE apart from individual-based models, as is the case for most land surface models except for SEIB-DGVM Sato et al., 2007 which is an individual-based land surface model. Following a broadening of the purpose of ORCHIDEE v4.2 to study the resilience of ecosystems, a hybrid approach Beckage et al., 2011Eamus et al., 2016Grimm & Berger, 2016 was needed in which the community is represented by a few individual model trees. Such an approach was introduced in ORCHIDEE v3.0: Vuichard et al., 2019 and developed further in ORCHIDEE v4.2 Naudts et al., 2015. The introduction of this hybrid approach did not change the classification of the ORCHIDEE model as a community-level model, partly due to the following two assumptions.

2. Assumption on vegetation diversity. Although different tree, grass and crop species may respond differently to similar environmental conditions, it is assumed that for the purpose of ORCHIDEE, the daunting diversity, i.e., already more than 60,000 tree species Beech et al., 2017, can be represented by making use of a limited number of plant functional types Krinner et al., 2005, which in turns relies on the assumption that all species within a single functional type show sufficiently similar land–atmosphere interactions irrespective of their geographical location Chapin III et al., 1996Brovkin et al., 1997Bonan et al., 2002Eamus et al., 2016. In ORCHIDEE, as in many other land surface models Fisher & Koven, 2020, the assumption on vegetation diversity hinders the model to address the response of the land surface to changes in species diversity. The assumption on vegetation diversity strengthens the classification of the ORCHIDEE land surface model as a community-level model (see Assumption on vegetation demography).

3. Assumption on interactions. Different vegetal communities compete for light, water and nutrients, affect each other’s demography, and affect the atmosphere in different ways resulting in micro-climates. Minus few exceptions, these interactions between communities are not accounted for as it is assumed that for the purpose of the ORCHIDEE model, landscape level interactions within and between grid cells can be ignored. Landscape level interactions within a grid cell, e.g., roughness as a function of the lay-out of the distribution of different vegetation types, are simulated assuming that statistical-averaging of the communities’ properties can be used. Landscape level interactions between grid cells, e.g., river routing, are explicitly simulated. The competition for soil water within an ORCHIDEE grid cell is the result of the discretisation of the model (section 1) rather than a deliberate scientific representation of a land surface process. The assumption on landscape interactions sets ORCHIDEE, as most other land surface models, apart from the landscape models and strengthens the classification of the ORCHIDEE land surface model as a community-level model (see Assumption on vegetation demography and diversity).

4. Assumption on vegetation evolution. Although natural selection operates in every generation and can often not be ignored when studying ecological phenomena Hairston et al., 2005 at least for species with generation times substantially less than the time frame of the model application Eamus et al., 2016, it is assumed that for the purpose of ORCHIDEE, micro-evolution and adaptation can be ignored. This is reflected by mostly using spatially and temporally constant parameters within a plant functional type. The diversity and evolutionary assumptions have been challenged and trait-based solutions Van Bodegom et al., 2012Van Bodegom et al., 2014Sakschewski et al., 2015Sakschewski et al., 2016, informed by manipulation experiments dekauwe2014, global datasets Wright et al., 2004Chave et al., 2009kattge2011Dı́az et al., 2016Tavşanoğlu & Pausas, 2018, and optimality theory Wang et al., 2017Wright et al., 2017Franklin et al., 2020 begin to provide the land surface community with the insights required to overcome several of the limitations resulting from the assumptions on vegetation diversity and evolution. Stomatal closure under soil moisture stress, for example, could be refined by incorporating vegetation acclimation to long‐term vapour pressure deficit conditions Abadie et al., 2025 and the maximum tree height was estimated from long-term precipitation Kempes et al., 2011

5. Assumption on the canopy structure. Although vegetation canopies are three-dimensional heterogeneous media, the ORCHIDEE model, as well as many other land surface models Best et al., 2011Lawrence et al., 2011Clark et al., 2011Haverd et al., 2018 assume that a bulk canopy approach adequately represents canopy processes, such as transpiration, and the exchange of sensible heat. The bulk canopy approach assumes that the canopy is an infinitesimal thin layer between the soil and the atmosphere. From a physical point of view, the simulation unit of ORCHIDEE is thus the bulk canopy. Broadening the purpose of ORCHIDEE v4.2 to study the resilience of ecosystems required representing three-dimensional heterogeneous canopies in ORCHIDEE Naudts et al., 2015. This new representation is used in the calculation of the radiative transfer in ORCHIDEE v4.2 and efforts to use it in the calculations of the energy budget Ryder et al., 2016Chen2016 are ongoing.

6. Assumption on spatial heterogeneity. With a range of a few to hundreds of metres, the heterogeneity of the land surface is 10$^{3}$ to 10$^{5}$ higher than the current grid resolution of typical land surface models. Hence, land surface models, including ORCHIDEE, rely on simplified, statistical sub-grid tiling schemes that treat complex, interconnected landscapes as disconnected sets of patches. In ORCHIDEE v4.2 only one energy budget is calculated for the entire grid-cell. For the water budget, three soil columns are considered, respectively, for short vegetation, tall vegetation, and bare soil, which prevent soil water competition between these three groups of plant functional types. For carbon and nitrogen, the budgets are calculated for each plant functional type separately.