Abstract
The representation of carbon allocation (CA) in ecosystem differs tremendously among models, resulting in diverse responses of carbon cycling and storage to global change. Several studies have highlighted discrepancies between empirical observations and model predictions, attributing these differences to problems of model structure. We analyzed the mathematical representation of CA in models using concepts from dynamical systems theory; we reviewed a representative sample of models of CA in vegetation and developed a model database within the Python package bgc-md. We asked whether these representations can be generalized as a linear system, or whether a more general framework is needed to accommodate nonlinearities. Some of the vegetation systems simulated with the reviewed models have a fixed partitioning of photosynthetic products, independent of environmental forcing. Vegetation is often represented as a linear system without storage compartments. Yet, other structures with nonlinearities have also been proposed, with important consequences on the temporal trajectories of ecosystem carbon compartments. The proposed mathematical framework unifies the representation of alternative CA schemes, facilitating their classification according to mathematical properties as well as their potential temporal behaviour. It can represent complex processes in a compact form, which can potentially facilitate dialog among empiricists, theoreticians, and modellers.
Highlights
Current and expected changes in climate and atmospheric CO2 concentration have prompted the study of the mechanisms used by vegetation to survive to changing environmental conditions
In terms of time resolution, the processes described in the published model descriptions (PMDs) often take place at different scales; there can be daily simulations of photosynthesis, evapotranspiration, and soil water dynamics, and annual updates of vegetation structure and plant functional type population densities (Sitch et al 2003)
Models with fixed coefficients for C partitioning and cycling would fail to simulate a vegetation that responds to environmental cues by adjusting its partitioning scheme: How can such a model be used to simulate the prioritization of a particular compartment when the environmental conditions change? the compartments in models with fixed partitioning coefficients will always receive the same proportion of the photosynthetic input, which means that these models may not be able to simulate observations such as the prioritization of the canopy of tropical forests a year after drought (Doughty et al 2015)
Summary
Current and expected changes in climate and atmospheric CO2 concentration have prompted the study of the mechanisms used by vegetation to survive to changing environmental conditions. One of these mechanisms is the adjustment of their carbon allocation scheme (Lacointe 2000; Franklin et al 2012; Xia et al 2017). Photosynthesis, for example, is a metabolic pathway that takes place in the chloroplast and requires light, CO2, and water. Ontogeny plays an important role, because older plants have slower metabolism than younger ones (Hartmann et al 2018). Not all leaves in a plant are exposed
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