Exceptional heat and atmospheric dryness amplified losses of primary production during the 2020 U.S. Southwest hot drought

Abstract

Earth's ecosystems are increasingly threatened by "hot drought," which occurs when hot air temperatures coincide with precipitation deficits, intensifying the hydrological, physiological, and ecological effects of drought by enhancing evaporative losses of soil moisture (SM) and increasing plant stress due to higher vapor pressure deficit (VPD). Drought-induced reductions in gross primary production (GPP) exert a major influence on the terrestrial carbon sink, but the extent to which hotter and atmospherically drier conditions will amplify the effects of precipitation deficits on Earth's carbon cycle remains largely unknown. During summer and autumn 2020, the U.S.Southwest experienced one of the most intense hot droughts on record, with recordlow precipitation and record-high air temperature and VPD across the region. Here, we use this natural experiment to evaluate the effects of hot drought on GPP and further decompose those negative GPP anomalies into their constituent meteorological and hydrological drivers. We found a 122 Tg C (>25%) reduction in GPP below the 2015-2019 mean, by far the lowest regional GPP over the Soil Moisture Active Passive satellite record. Roughly half of the estimated GPP loss was attributable to low SM (likely a combination of record-low precipitation and warming-enhancedThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Intensified Warming and Aridity Accelerate Terminal Lake Desiccation in the Great Basin of the Western United States

“…Outside of the GB, in parts of Montana and Wyoming, there are areas indicating an increasing number of days (#days) of snow cover (Figure S1 in Supporting Information ) especially in northern Montana, and roughly in the same location as the trends of decreasing LSTs seen in Figures S1 and S2 in Supporting Information . ET also tends to be reduced over much of the interior GB and California, where the extended drought has severely restricted water supplies available to support ET and vegetation growth (Dannenberg et al., 2022). Trends of decreasing ET from plants and soil in the GB and in California contrast with moderate increases in ET elsewhere as seen in Figure S3 in Supporting Information .…”

“…The focus herein is on the time period from Water Year (WY) 2001–2021 which is also mostly coincident with the timing of the current drought that began around 1999. The most severe drought area is largely centered over the US Southwest, including much of the GB (Dannenberg et al., 2022; Williams et al., 2022).…”

Upscaling dryland carbon and water fluxes with artificial neural networks of optical, thermal, and microwave satellite remote sensing

“…Despite its short calibration and validation period, DrylANNd's training data encompass much of the climate variability experienced by the western US, including both anomalously wet and dry years that may serve as analogues when running the model forward in time as new MODIS and SMAP data are released. However, it is possible that the historically atypical "megadrought" conditions (Williams et al, 2020(Williams et al, , 2022Dannenberg et al, 2022a) under which the model was trained may impose limitations on the model's predictive capability. Some of the model's limitations in capturing interannual variability could perhaps be ameliorated by incorporating additional remote sensing data that capture other aspects of dryland ecosystem functions.…”

“…Instead, light-use efficiency models often represent moisture stress using vapor pressure deficit (Running et al, 2004;Zhang et al, 2016). While vapor pressure deficit is well suited as a water stress indicator for mesic regions, it often does not fully capture water stress in drylands, where soil moisture plays a particularly important role in regulating surface conductance and carbon and water fluxes (Novick et al, 2016;Stocker et al, 2018;Dannenberg et al, 2022a). Soil moisture therefore needs to be incorporated into satellite-based carbon and water models to represent temporal variability in dryland water limitation (Stocker et al, 2018(Stocker et al, , 2019Smith et al, 2019).…”

How much does VPD drive tree water stress and forest disturbances?

“…While pioneer studies mainly focused on plant responses to reduced precipitation or soil moisture deficit ("soil drought", Beier et al, 2004;Bréda et al, 2006;Limousin et al, 2009;Pangle et al, 2015;Weltzin et al, 2003), the impact of recent massive heat-waves on plants has favored the emergence of the "globalchange-type drought" concept (Breshears et al 2005) and shed light on the key role of "atmospheric drought" (or vapor pressure deficit, VPD) on plant functioning (Park Williams et al 2012;Trenberth et al 2014;McDowell et al , 2022Yuan et al 2019;Grossiord et al 2020). Major efforts are currently underway to unravel the respective roles of VPD and soil drought on the different facets of plant functioning (e.g., gas exchanges, growth, mortality, vulnerability to wildfire) in studies ranging from controlled experiments (Grossiord et al 2017a, b;Schönbeck et al 2022) to analyses of climate impacts at regional (Trotsiuk et al 2021;Dannenberg et al 2022;Grünig et al 2022) and continental scales (Seager et al 2015;Humphrey et al 2021;Bauman et al 2022;Flo et al 2022;Fu et al 2022b, a). In these studies the predominant role of VPD is most of the time invoked as the main driver of plant responses (Grossiord et al 2017a, b;Flo et al 2021Flo et al , 2022Humphrey et al 2021;Bauman et al 2022;Fu et al 2022b, a;Grünig et al 2022;Schönbeck et al 2022).…”