Introduction to process-based forest modelling

Miquel De Cáceres, Rodrigo Balaguer

Ecosystem Modelling Facility, CREAF

Outline

  1. Fundamental concepts
  2. Modelling cycle
  3. Overview of process-based forest models

M.C. Escher - Ascending and Descending, 1960

1. Fundamental concepts

Models: What are they?

  • Model - A simplification of reality constructed to gain insights into a set of attributes of a physical, biological, economic, or social system.
  • Conceptual model - A hypothesis regarding the important factors that govern the behavior of an object or a process of interest.
  • Statistical model - A numerical model built using observations within a probabilistic framework.
  • Mechanistic (or process-based) model - A numerical model that explicitly represents the understanding of physical, chemical or biological processes.
  • Simulation model - A numerical model that represents the development of a solution by incremental steps through the model domain.
  1. Fundamental concepts

Model components

  • Modules or sub-models - An independent or self-contained component of a model (e.g. Farquhar’s C3 photosynthesis model).
  • State variables - The dependent variables calculated within a model, which often change over the simulation (e.g. soil moisture).
  • Parameters - Terms in the model that are fixed during a model run but can be changed in different runs (e.g. soil texture).
  • Constants - Terms that are fixed values under all runs, representing known physical, biological or ecological activities (e.g. the speed of light).
  1. Fundamental concepts

Model assessment

  • Verification - Examination of the implementation to ascertain that they truly represents the conceptual model and there are no inherent numerical problems.
  • Qualitative assessment - Uncertainty in model predictions that cannot be quantified (i.e. about the theory underlying the model or the model design).
  • Uncertainty analysis - Investigation of the effects of lack of knowledge or potential errors of inputs (e.g. climate forcing) on the model output.
  • Robustness - The capacity of a model to perform well across the full range of conditions for which it was designed.
  • Sensitivity - The degree to which the model outputs are affected by changes in input parameters.
  • Transparency - The clarity and completeness with which data, assumptions, and methods of analysis are documented.

  1. Fundamental concepts

2. Modelling cycle

Modelling tasks: Development (1)

  1. Problem formulation

    • Definition of objectives
    • Definition of the spatio-temporal physical domain

  2. Model design and formulation

    • Data availability
    • Conceptual model
    • Use of existing vs. new model formulation
    • Compatibility with other modules

  3. Implementation

    • Algorithmic design
    • Model coding (e.g. Python, C++)
    • Profiling and code optimization

  1. Modelling cycle

Modelling tasks: Development (2)

  1. Parameterization and calibration

    • Sources for direct parameter estimation
    • Sources for parameter calibration
    • Meta-modelling (estimation from the output of other models)

  2. Model analysis

    • Verification and qualitative assessment
    • Sensitivity/uncertainty analysis
    • Formal evaluation (validation)

  3. Model application

    • Simulation and documentation
    • Quantifying uncertainty
    • Evidence for decision

  1. Modelling cycle

3. Overview of process-based forest models

A typology of forest processes

Processes

  1. Overview of process-based forest models

A typology of forest processes

Forest gap models

FORCLIM, FORCEEPS, GREFOS

  1. Overview of process-based forest models

A typology of forest processes

Soil-vegetation-atmosphere transfer model

BILJOU, MuSICA, CANVEG

  1. Overview of process-based forest models

A typology of forest processes

Forest biochemical model

CASTANEA, GOTILWA+, FOREST-BGC

  1. Overview of process-based forest models

A typology of forest processes

Watershed ecohydrological model

RHESYS, ECH2O, Tethys-Chloris

  1. Overview of process-based forest models

M.C. Escher - Ascending and Descending, 1960