Revisiting global vegetation controls using multi-layer soil moisture

Abstract

 Vertically resolved soil moisture improves the understanding of large-scale vegetation productivity. 11  Extended water-related control on vegetation productivity emerges when considering multi-layer soil moisture versus total soil moisture.  Sub-surface soil moisture is particularly important for vegetation productivity in semiarid regions.

Can Surface Soil Moisture Information Identify Evapotranspiration Regime Transitions?

“…There are, of course, exceptions where certain plants have adapted to frequent water stress by extending roots into far deeper soil layers and approaching underground aquifers (Fan et al., 2017; Gao et al., 2014). Nonetheless, for most ecosystems, more than 75% of vegetation roots are located in the top 50 cm of the soil column (Zeng, 2001), and soil moisture availability above 30 cm is most responsible for vegetation dynamics (Li et al., 2021). Therefore, θ v is expected to be representative of the general vegetation root water stress conditions.…”

“…However, plant roots can reach to soil water storages at several tens of centimeters and for certain limited species, to tens of meters (Fan et al., 2017; Gao et al., 2014; Yang et al., 2016). Therefore, evapotranspiration can draw water from soil layers that are considerably deeper than the θ s sampling depth (Buitink et al., 2020; Green et al., 2019; Humphrey et al., 2021; Li et al., 2021). Based on this line of reasoning, the expectation is that θ s is likely biased in its representation of evapotranspiration regime transitions or overall land surface energy partitioning (Hirschi et al., 2014; Mueller & Seneviratne, 2012).…”

Widespread increasing vegetation sensitivity to soil moisture

“…The latter is critical for mitigating climate change by absorbing human-emitted CO 2 4 . Vegetation requires sufficient energy and nutrients, and also soil moisture availability is essential, particularly in semi-arid regions 5 , 6 . As a result of ongoing climate change, soil moisture is declining in many regions as a consequence of decreased precipitation and higher evaporative water demand due to increased temperatures 7 .…”

“…In fact, leaf area index (LAI) products and other vegetation indices related to vegetation greenness and productivity can represent long-term global vegetation growth dynamics 8 , 16 – 19 . They are routinely employed to study land-atmosphere interactions as they are sensitive to soil moisture dynamics, and can diagnose temporal sensitivity to environmental drivers thanks to their relatively higher signal-to-noise ratio than photosynthesis-related indicators such as sun-induced fluorescence 5 . Furthermore, LAI is readily available as a key prognostic variable from land surface models (LSMs) 20 .…”

“…Furthermore, LAI is readily available as a key prognostic variable from land surface models (LSMs) 20 . From a modeling perspective, a more accurate representation of LAI response to soil water stress requires the differentiation between soil layers, as near-surface soil moisture primarily controls soil evaporation and precipitation infiltration and co-varies more with atmospheric conditions 21 , while sub-surface soil moisture is a more relevant plant water source 5 , 22 . State-of-the-art soil moisture reanalyses cover multiple layers and allow for comprehensive analyses of the vegetation-water interplay by benefiting from satellite-observed surface soil moisture and in-situ multi-depth soil moisture measurements 23 , 24 .…”