TREED (v1.0): a trait- and optimality-based eco-evolutionary vegetation model for the deep past and the present

Abstract. We present the TREED model (TRait Ecology and Evolution over Deep time), a trait- and optimality-based vegetation model to simulate vegetation structure, carbon cycling and eco-evolutionary adaptation dynamics to climate and CO₂ changes across geologic time scales. The global grid-based vegetation model represents plant carbon allocation and trait evolution as a set of carbon economic trade-offs. Based on optimality principles, it is assumed that functional traits of the modelled community-representative average plants evolve towards an optimum that maximizes height growth while maintaining a positive carbon balance. The considered trait trade-offs resolve the potential plant height, leaf carbon pool size, leaf longevity, and phenology as the major axes of plant trait variation. Based on these key traits, whole-plant structure and functioning are derived using functional and allometric relationships. In its eco-evolutionary mode, vegetation-mediated carbon cycling can be tracked over the course of climatic transitions, testing the effects of the speed of evolutionary trait adaptation and dispersal dynamics. Moreover, with its generalized plant physiology, continuous trait space, and lack of pre-defined functional types, the model can be used to calculate metrics of biodiversity, including indices of the functional diversity and species richness potential. With a low computational demand, a flexible time stepping scheme and scalable adaptation parameters, TREED is intended to simulate biological and environmental transitions across time scales spanning from centuries to millions of years. Here, we present the underlying theory and model functions and evaluate model outputs against present-day observations. We show that the trait- and optimality-based approach captures major patterns in present-day vegetation-mediated carbon and water fluxes, biomass carbon storage, vegetation height, leaf traits, as well as the global distribution of plant biodiversity. Finally, we illustrate its application in the context of paleoclimate and palaeoecological research using the Paleocene-Eocene Thermal Maximum as a case study and show how eco-evolutionary adaptation dynamics of terrestrial ecosystems may strongly affect global carbon cycle dynamics during hyperthermal events. The TREED model is a step towards a more self-consistent and parameter-scarce representation of vegetation dynamics under environmental conditions that are fundamentally different from the present. In combination with geochemical and paleobotanical data, the model may help to better constrain the resilience of vegetation-mediated Earth system functions to perturbations in the geologic past and at present.