Comment on gmd-2022-68

Abstract

Drought stress is an increasing threat for vegetation in tropical regions, especially in the context of human-induced increase of drought frequency and severity, as 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. Soil clay content was shown to be responsible for a significant fraction of the spatial variability in forest structure and productivity. Yet, large uncertainties remain for soil textural properties at the regional level and it is currently unclear how those uncertainties propagate into the outputs of Terrestrial Biosphere Models (TBMs) that are used to predict the response of vegetation ecosystems to future climate change scenarios. In this study, we evaluated the heterogeneity in soil textural properties from the global SoilGrids250m product, and we then assessed the sensitivity of the carbon cycle of tree TBMs (namely ORCHIDEEv2.2, ED2, and LPJ-GUESS) to the variability in soil textural properties at the regional level over the South-American tropics using model default pedotransfer functions. When the SoilGrids product was aggregated from its finest (250m) to a coarser spatial resolution typical of TBM simulations (0.5°), the intra-gridcell variability in soil texture rapidly increased to reach an order of magnitude similar to the inter-gridcell variability. All explored model outputs of each TBM, including gross primary productivity, aboveground biomass, soil carbon content and drought stress, were shown to be insensitive to soil texture changes, except for a limited region characterized by 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, which will be critical to properly account for future increasing drought stress conditions in tropical regions.