Soil moisture dominates dryness stress on ecosystem production globally

Abstract Dryness stress can limit vegetation growth and is often characterized by low soil moisture (SM) and high atmospheric water demand (vapor pressure deficit, VPD). However, the relative role of SM and VPD in limiting ecosystem production remains debated and is difficult to disentangle, as SM and VPD are coupled through land-atmosphere interactions, hindering the ability to predict ecosystem responses to dryness. Here, we combine satellite observations of solar-induced fluorescence with estimates of SM and VPD and show that SM is the dominant driver of dryness stress on ecosystem production across more than 70% of vegetated land areas with valid data. Moreover, after accounting for SM-VPD coupling, VPD effects on ecosystem production are much smaller across large areas. We also find that SM stress is strongest in semi-arid ecosystems. Our results clarify a longstanding question and open new avenues for improving models to allow a better management of drought risk.

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Redefining droughts for the U.S. Corn Belt: The dominant role of atmospheric vapor pressure deficit over soil moisture in regulating stomatal behavior of Maize and Soybean

Agricultural and Forest Meteorology ◽

10.1016/j.agrformet.2020.107930 ◽

2020 ◽

Vol 287 ◽

pp. 107930 ◽

Cited By ~ 2

Author(s):

Hyungsuk Kimm ◽

Kaiyu Guan ◽

Pierre Gentine ◽

Jin Wu ◽

Carl J. Bernacchi ◽

...

Keyword(s):

Soil Moisture ◽

Vapor Pressure ◽

Vapor Pressure Deficit ◽

Dominant Role ◽

Corn Belt ◽

Stomatal Behavior ◽

The U.S

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Increased atmospheric vapor pressure deficit reduces global vegetation growth

Science Advances ◽

10.1126/sciadv.aax1396 ◽

2019 ◽

Vol 5 (8) ◽

pp. eaax1396 ◽

Cited By ~ 69

Author(s):

Wenping Yuan ◽

Yi Zheng ◽

Shilong Piao ◽

Philippe Ciais ◽

Danica Lombardozzi ◽

...

Keyword(s):

Vapor Pressure ◽

Vapor Pressure Deficit ◽

Vegetation Index ◽

Global Climate ◽

Gross Primary Production ◽

Ecosystem Responses ◽

Climate Conditions ◽

Vegetation Growth ◽

Current Century ◽

Global Vegetation

Atmospheric vapor pressure deficit (VPD) is a critical variable in determining plant photosynthesis. Synthesis of four global climate datasets reveals a sharp increase of VPD after the late 1990s. In response, the vegetation greening trend indicated by a satellite-derived vegetation index (GIMMS3g), which was evident before the late 1990s, was subsequently stalled or reversed. Terrestrial gross primary production derived from two satellite-based models (revised EC-LUE and MODIS) exhibits persistent and widespread decreases after the late 1990s due to increased VPD, which offset the positive CO2 fertilization effect. Six Earth system models have consistently projected continuous increases of VPD throughout the current century. Our results highlight that the impacts of VPD on vegetation growth should be adequately considered to assess ecosystem responses to future climate conditions.

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Conflicting drivers of land carbon uptake variability reconciled by land-atmosphere coupling

10.5194/egusphere-egu2020-11172 ◽

2020 ◽

Author(s):

Vincent Humphrey ◽

Alexis Berg ◽

Philippe Ciais ◽

Christian Frankenberg ◽

Pierre Gentine ◽

...

Keyword(s):

Soil Moisture ◽

Vapor Pressure ◽

Water Availability ◽

Vapor Pressure Deficit ◽

Sensitivity Analyses ◽

Carbon Uptake ◽

Annual Variability ◽

Vegetation Activity ◽

The Impact

Obtaining reliable estimates of the sensitivity of carbon fluxes to water availability, temperature and vapor pressure deficit is essential for constraining climate-carbon feedbacks in Earth system models. However, these variables often co-vary because of soil moisture &#8211; atmosphere feedbacks, especially in situations where they are most susceptible to strongly impact ecosystems (e.g. during droughts and heatwaves), leading to potentially conflicting results when sensitivities are assessed independently. In particular, there is conflicting evidence on the role of temperature versus water availability in explaining these variations at the global scale.Here, we show that accounting for the effect of soil moisture &#8211; atmosphere coupling resolves much of this controversy. Using idealized climate model experiments, we find that variability in soil moisture accounts for 90% of the inter-annual variability in land carbon uptake, mainly through its impact on photosynthesis. Without SM variability, the inter-annual variability (IAV) of land carbon uptake is almost eliminated. We show that the effects of soil moisture can be decomposed into 1) a direct ecosystem response to soil water stress and 2) a dominant indirect response to extreme temperature and vapor pressure deficit triggered by land-atmosphere coupling and controlled by anomalous soil moisture conditions. &#160;Importantly, these two mechanisms do not necessarily have the same spatial extent, and some regions can be more sensitive to indirect effects than to direct effects.These two pathways explain why results from coupled climate models suggest a dominant role of soil moisture, while uncoupled simulations diagnose a strong temperature effect. These findings have strong implications for offline model sensitivity analyses as well as field scale manipulation experiments (i.e. rainfall exclusion studies) where the impact of drought on carbon exchange and vegetation activity is often studied by intervening solely on soil moisture content with little consideration of the physical feedbacks on temperature and air humidity occurring in natural conditions.

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