The internal representation of vegetation and landscape structure, and processes governing this structure, may be configured in two alternative ways in LPJ-GUESS. Cohort mode is the standard and recommended for normal purposes. The alternative population mode is provided mainly for testing purposes (as it runs much faster) and as a contrast to examine the consequences for model behaviour and performance of the cohort mode features of explicit size structure and landscape heterogeneity.
Separate instruction files are provided for cohort and population mode. Fresh copies are normally copied to the workspace directory when a new workspace is created. Choose the appropriate instruction file through the Simulation settings form when you run the model.
You may also switch between modes by setting the value of the vegmode parameter in the instruction file:
vegmode "cohort"
vegmode "population"
However, this requires care as cohort mode requires additional settings not defined in population mode. The most suitable values for certain parameters may also be different depending on the vegetation mode.
Cohort mode
Each average individual represents an age class (cohort) of a PFT in a patch. Grass PFTs are, however, represented by a single average individual, similar to population mode. Multiple patches are simulated to accomodate variation due to stochastic (random) processes, including disturbances that destroy all or part of the standing biomass on an affected patch. Vegetation processes are simulated in a more realistic way, but the simulation will take longer to complete, compared with population mode.
It is possible to increase simulation speed by reducing the number of simulated patches, but this results in more stochastic variation. Smoother results are obtained by increasing the number of patches. 100 patches are recommended for meaningful results. You can change the number of patches to simulate by modifying the npatch setting in the instruction file:
npatch 100
Population mode
Each PFT is described by a single ‘average individual’. Age classes of trees, or patches affected by different histories of disturbance, are not distinguished. This configuration is based on the vegetation dynamics representation in LPJ, a representative early DGVM.