Representing canopy structure dynamics within the LPJ-GUESS dynamic global vegetation model (revision 13221)

Abstract. The competition for light is a fundamental determinant of the structure and composition of a forest. Large-scale forest models must balance real-world complexity with computational demand and poorly constrained parameters. The LPJ-GUESS dynamic global vegetation model has a strong track record of simulating forest composition and tree demography with a simple representation of forest canopies. The current approach, however, is limited in its ability to explore functional co-existence of trees within forest patches or to represent the full implications of forest management actions which create heterogeneous light conditions on the forest floor. The current representation of forest canopy in LPJ-GUESS is based on vertically overlapping crowns with no horizontal structure. Whilst computationally efficient, this approach does not allow for a realistic representation of forest floor light distribution following tree death or harvest.

Here we describe the implementation of a new scheme with spatially explicit canopies, where tree cohorts have a fixed position within a patch, enabling more realistic simulation of forest floor light conditions, especially following disturbances such as tree death or harvest. Additionally, we introduce a lower-complexity model version based on a perfect plasticity-like approximation.

To evaluate these developments, we conduct four assessments. First, we evaluate the model's performance against field observations of aboveground woody biomass, mortality, and productivity across diameter size classes. Second, we examine the ability to represent tree functional co-existence. Third, we explore how forest harvest influence the re-establishment of a woody understory. Lastly, we conduct two sensitivity tests.

Results show that the spatially explicit canopy schemes improve representation of forest size structure and dynamics across boreal, temperate, and tropical regions. It also enables representation of functional co-existence without the influence of large-scale disturbances and captures the interplay of forest gap dynamics with the establishment of a recruitment layer, capabilities not achievable with the standard canopy approach.

These advances significantly enhance the model's capacity to explore forest management effects and functional co-existence, and improve its alignment with observational data. The new canopy schemes offer a more robust foundation for modelling forest dynamics under historical, current, and future environmental conditions.