Low sensitivity of three terrestrial biosphere models to soil texture over the South American tropics

Drought stress is an increasing threat for vegetation in tropical regions, within the context of human-induced increase of drought frequency and severity observed over South American forests. Drought stress is induced when a plant's water demand is not met with its water supply through root water uptake. The latter depends on root and soil properties, including soil texture (i.e. the soil clay and sand fractions) that determines the soil water availability and its hydraulic properties. Hence, soil clay content is responsible for a significant fraction of the spatial variability in forest structure and productivity. Soil-textural properties largely vary at the spatial resolution used by Terrestrial Biosphere Models (TBMs) and it is currently unclear how this variability affects the outputs of these models used to predict the response of vegetation ecosystems to future climate change scenarios. In this study, we assessed the sensitivity of the carbon cycle of three state-of-the-art TBMs, i.e. ORganizing Carbon and Hydrology in Dynamic EcosystEms (ORCHIDEEv2.2), Ecosystem Demography model version 2 (ED2), and Lund-Potsdam-Jena General Ecosystem Simulator (LPJ-GUESS) to soil-textural properties at the regional level over the South American tropics using model default pedotransfer functions. For all three TBMs, the model outputs, including gross primary productivity (GPP), aboveground biomass (AGB), soil carbon content and drought stress, were shown to be mostly insensitive to soil-texture changes representative of the spatial variability in soil properties, except for a small region characterised by very low water availability in ORCHIDEEv2.2 and ED2. We argue that generic pedotransfer and simple drought stress functions, as currently implemented in TBMs, should be reconsidered to better capture the role of soil texture and its coupling to plant processes. Similarly, we suggest that better estimates of the soil-texture uncertainty resulting from soil-texture data aggregate should be considered in the future. Those steps forward are critical to properly account for future increasing drought stress conditions in tropical regions.