Accounting for forest age in the tile-based dynamic global vegetation model JSBACH4 (4.20p7; git feature/forests) – a land surface model for the ICON-ESM

Abstract

Natural and anthropogenic disturbances, in particular forest management, affect forest age-structures all around the globe. Forest age-structures in turn influence biophysical and biogeochemical interactions of the vegetation with the atmosphere. Yet, many dynamic global vegetation models (DGVMs), including those used as land surface models (LSMs) in Earth system models (ESMs), do not account for subgrid forest age structures, despite being used to investigate land-use effects on the global carbon budget or simulating land-atmosphere interactions. In this paper we present a new scheme to introduce 5 forest age-classes in hierarchical tile-based DGVMs combining benefits of recently applied approaches. Our scheme combines a computationally efficient age-dependent simulation of all relevant processes, such as photosynthesis and respiration, without loosing the information about the exact forest age, which is a prerequisite for the implementation of age-based forest management. This combination is achieved by using the hierarchy to track the area fraction for each age on an aggregated plant functional type level, whilst simulating the relevant processes for a set of age-classes. We describe how we implemented this 10 scheme in JSBACH4, the LSM of the ICON-ESM. Subsequently, we compare simulation output against global observationbased products for gross primary production, leaf area index and above-ground biomass to assess the ability of simulations with and without age-classes to reproduce the annual cycle and large-scale spatial patterns of these variables. The comparisons show differences exponentially decreasing with the number of distinguished age-classes and linearly increasing computation costs. The results demonstrate the benefit of the introduction of age-classes, with the optimal number of age-classes being a 15 compromise between computation costs and accuracy.Geosci. Model Dev. Discuss., https://doi.

Perspectives on the Future of Land Surface Models and the Challenges of Representing Complex Terrestrial Systems

“…Similarly, where natural systems are subject to ongoing disturbance from natural mortality, in any given grid cell, anthropogenic disturbance also gives rise to a range of ages of secondary forest where biomass recovery also proceeds in a nonlinear fashion after abandonment. Shevliakova et al (2009) and Nabel et al (2019) illustrate the importance of capturing the regrowth after disturbance in anthropogenically disturbed ecosystems for simulating the terrestrial carbon sink. At larger scales of soil heterogeneity, some models also implement tiling regimes for soil type.…”

“…The shift toward representing the agents of change has led groups to represent microbial types and their population dynamics in soil biogeochemical models as well (Treseder et al, 2012; Wieder et al, 2013). The role of both natural and anthropogenic disturbance, missing in early land surface models, has been a major focus of developments in order to represent the many direct effects that humans have on modifying the land surface (P. J. Lawrence et al, 2012; Nabel et al, 2019; Pongratz et al, 2018; Shevliakova et al, 2009; Yue et al, 2018). Many further dimensions of process complexification exist as well including canopy radiative transfer, trace gases, fire, permafrost, boundary layer turbulence, and rivers.…”

Evaluating two soil carbon models within a global land surface model using surface and spaceborne observations of atmospheric CO<sub>2</sub> mole fractions

“…It is furthermore also used in JSBACH4, a re-implementation of JSBACH for the ICON-ESM (Giorgetta et al, 2018;Nabel et al, 2019).…”

Ecosystem age-class dynamics and distribution in the LPJ-wsl v2.0 global ecosystem model

“…The most novel advancement in this study is a new method of age-class transition modeling, which we call 'vector-140 tracking of fractional transitions' (VTFT), which improves the computational efficiency of modeling age-classes in global models; this is a similar approach independently conceived by Nabel et al (2019). The method is a transparent and simple solution to the problem of dilution, which manifests as an advective process when state variables, such as carbon stocks or tree density, are made to merge by area-weighted averaging.…”

“…Recent model developments in JSBACH4 (Nabel et al 2019) and ED-2.2 (Longo et al 2019) could point the way forward for incorporating a greater amount of vertical heterogeneity in LPJ-wsl, as well as in other models. In any case, the age-module in LPJ-wsl v2.0 now contributes to an ensemble of global models with demographic capabilities.…”