Robust Ecosystem Demography (RED version 1.0): a parsimonious approach to modelling vegetation dynamics in Earth system models

Abstract

Abstract. A significant proportion of the uncertainty in climate projections arises from uncertainty in the representation of land carbon uptake. Dynamic global vegetation models (DGVMs) vary in their representations of regrowth and competition for resources, which results in differing responses to changes in atmospheric CO2 and climate. More advanced cohort-based patch models are now becoming established in the latest DGVMs. These models typically attempt to simulate the size distribution of trees as a function of both tree size (mass or trunk diameter) and age (time since disturbance). This approach can capture the overall impact of stochastic disturbance events on the forest structure and biomass – but at the cost of increasing the number of parameters and ambiguity when updating the probability density function (pdf) in two dimensions. Here we present the Robust Ecosystem Demography (RED), in which the pdf is collapsed onto the single dimension of tree mass. RED is designed to retain the ability of more complex cohort DGVMs to represent forest demography, while also being parameter sparse and analytically solvable for the steady state. The population of each plant functional type (PFT) is partitioned into mass classes with a fixed baseline mortality along with an assumed power-law scaling of growth rate with mass. The analytical equilibrium solutions of RED allow the model to be calibrated against observed forest cover using a single parameter – the ratio of mortality to growth for a tree of a reference mass (μ0). We show that RED can thus be calibrated to the ESA LC_CCI (European Space Agency Land Cover Climate Change Initiative) coverage dataset for nine PFTs. Using net primary productivity and litter outputs from the UK Earth System Model (UKESM), we are able to diagnose the spatially varying disturbance rates consistent with this observed vegetation map. The analytical form for RED circumnavigates the need to spin up the numerical model, making it attractive for application in Earth system models (ESMs). This is especially so given that the model is also highly parameter sparse.

Linking plant hydraulics and the fast–slow continuum to understand resilience to drought in tropical ecosystems

“…The accurate representation of the diverse ecological strategies that exist in tropical forests and their impact on vegetation processes and distribution still require extensive fieldwork, which will enable a more reliable and general description of tropical PFTs and the trade‐offs that control their trait distribution. In addition, accurately modeling the consequences of hydraulic vulnerability on the plant life ‐historymight require a more explicit representation of certain demographic processes, such as plant mortality, using individual (Christoffersen et al ., 2016) or even cohort‐based approaches (Argles et al ., 2020). However, our simulations show that a simple PFT‐based representation of the end‐members of a trade‐off can already improve the simulation of tropical forest responses to drought.…”

Coupled regional Earth system modeling in the Baltic Sea region

“…(Fisher et al, 2018;Pugh et al, 2019), it is important to incorporate the size-and age-structure and demography of vegetation and ecosystems explicitly, and to account for the competitive interactions of growing vegetation stands comprising individuals or cohorts of different plant functional types. A number of DGVMs and land surface models (LSMs) are now moving in this direction, away from a traditional tiled or area-based (Smith et al, 2001) land surface representation, including the CLM4.5(ED) LSM (Moorcroft et al, 2001;Fisher et al, 2015Fisher et al, , 2018 and the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) vegetation demography submodel (Koven et al, 2020), the POP (Population Orders Physiology) module for woody demography in the CABLE LSM (Haverd et al, 2013(Haverd et al, , 2014(Haverd et al, , 2018, the RED module in the JULES LSM (Argles et al, 2020) and the SEIB-DGVM (Sato et al, 2007). To date, however, no demo-graphic model has been applied to study the Baltic Sea region specifically (see Kumkar et al, 2020, for an application using CLM4.5) apart from the LPJ-GUESS DGVM (Smith et al, 2001.…”

“…Socioeconomic development is a major driver for nutrient loads into the Baltic Sea (Arheimer et al, 2012;Bartosova et al, 2019) that contribute to eutrophication and hypoxic conditions (e.g., Saraiva et al, 2019a, b). So far, only a few hydrological models include nutrient cycling to provide explicit estimates of nutrient inputs to the sea (Hundecha et al, 2016).…”

Representation of phosphorus cycle in Joint UK Land Environment Simulator (vn5.5_JULES-CNP)

“…mature trees are less responsive to the nutrient limitations) (De Lucia et al, 2007;Norby et al, 2016). However, a recent development of Robust Ecosystem Demography (RED) model into JULES (Argles et al, 2020) and its integration into JULES-CNP in the future can resolve this issue.…”