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1. Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought‐induced tipping point

   https://doi.org/10.1111/gcb.16933  

   Restrepo‐Coupe, Natalia; O’Donnell Christoffersen, Bradley; Longo, Marcos; Alves, Luciana F.; Campos, Kleber Silva; da Araujo, Alessandro C.; de Oliveira, Jr, Raimundo C.; Prohaska, Neill; da Silva, Rodrigo; Tapajos, Raphael; et al (September 2023, Global Change Biology)

   Abstract Understanding the effects of intensification of Amazon basin hydrological cycling—manifest as increasingly frequent floods and droughts—on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change, including forest “tipping points”. Here, we investigated the impacts of hydrological extremes on forest function using 12+ years of observations (between 2001–2020) of water and energy fluxes from eddy covariance, along with associated ecological dynamics from biometry, at the Tapajós National Forest. Measurements encompass the strong 2015–2016 El Niño drought and La Niña 2008–2009 wet events. We found that the forest responded strongly to El Niño‐Southern Oscillation (ENSO): Drought reduced water availability for evapotranspiration (ET) leading to large increases in sensible heat fluxes (H). PartitioningETby an approach that assumes transpiration (T) is proportional to photosynthesis, we found that water stress‐induced reductions in canopy conductance (G_s) droveTdeclines partly compensated by higher evaporation (E). By contrast, the abnormally wet La Niña period gave higherTand lowerE, with little change in seasonalET. Both El Niño‐Southern Oscillation (ENSO) events resulted in changes in forest structure, manifested as lower wet‐season leaf area index. However, only during El Niño 2015–2016, we observed a breakdown in the strong meteorological control of transpiration fluxes (via energy availability and atmospheric demand) because of slowing vegetation functions (via shutdown ofG_sand significant leaf shedding). Drought‐reducedTandG_s, higherHandE, amplified by feedbacks with higher temperatures and vapor pressure deficits, signaled that forest function had crossed a threshold, from which it recovered slowly, with delay, post‐drought. Identifying such tipping point onsets (beyond which future irreversible processes may occur) at local scale is crucial for predicting basin‐scale threshold‐crossing changes in forest energy and water cycling, leading to slow‐down in forest function, potentially resulting in Amazon forests shifting into alternate degraded states. 

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2. Climate sensitivity of water use by riparian woodlands at landscape scales

   https://doi.org/10.1002/hyp.13942  

   Mayes, Marc; Caylor, Kelly K.; Singer, Michael Bliss; Stella, John C.; Roberts, Dar; Nagler, Pamela (November 2020, Hydrological Processes)

   Abstract Semi‐arid riparian woodlands face threats from increasing extractive water demand and climate change in dryland landscapes worldwide. Improved landscape‐scale understanding of riparian woodland water use (evapotranspiration, ET) and its sensitivity to climate variables is needed to strategically manage water resources, as well as to create successful ecosystem conservation and restoration plans for potential climate futures. In this work, we assess the spatial and temporal variability of Cottonwood (Populus fremontii)‐Willow (Salix gooddingii) riparian gallery woodland ET and its relationships to vegetation structure and climate variables for 80 km of the San Pedro River corridor in southeastern Arizona, USA, between 2014 and 2019. We use a novel combination of publicly available remote sensing, climate and hydrological datasets: cloud‐based Landsat thermal remote sensing data products for ET (Google Earth Engine EEFlux), Landsat multispectral imagery and field data‐based calibrations to vegetation structure (leaf‐area index, LAI), and open‐source climate and hydrological data. We show that at landscape scales, daily ET rates (6–10 mm day⁻¹) and growing season ET totals (400–1,400 mm) matched rates of published field data, and modelled reach‐scale average LAI (0.80–1.70) matched lower ranges of published field data. Over 6 years, the spatial variability of total growing season ET (CV = 0.18) exceeded that of temporal variability (CV = 0.10), indicating the importance of reach‐scale vegetation and hydrological conditions for controlling ET dynamics. Responses of ET to climate differed between perennial and intermittent‐flow stream reaches. At perennial‐flow reaches, ET correlated significantly with temperature, whilst at intermittent‐flow sites ET correlated significantly with rainfall and stream discharge. Amongst reaches studied in detail, we found positive but differing logarithmic relationships between LAI and ET. By documenting patterns of high spatial variability of ET at basin scales, these results underscore the importance of accurately accounting for differences in woodland vegetation structure and hydrological conditions for assessing water‐use requirements. Results also suggest that the climate sensitivity of ET may be used as a remote indicator of subsurface water resources relative to vegetation demand, and an indicator for informing conservation management priorities. 

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3. Increasing plant water stress and decreasing summer streamflow in response to a warmer and wetter climate in seasonally snow‐covered forests

   https://doi.org/10.1002/eco.2256  

   Christensen, Lindsey; Adams, Hallie R.; Tai, Xiaonan; Barnard, Holly R.; Brooks, Paul D. (October 2020, Ecohydrology)

   Abstract Warming temperatures and precipitation changes are expected to alter water availability and increase drought stress in western North America, yet uncertainties remain in how concurrent changes in the amount and seasonality of precipitation interact with warming to affect hydrologic partitioning. We combined over a century of streamflow (Q) and climate observations with two decades of tree growth data and remotely sensed vegetation activity to quantify how temperature and precipitation interact to control hydrologic partitioning in the Front Range of Colorado, Boulder Creek Watershed. Temperature and precipitation significantly increased over the last five decades, with precipitation increasing primarily in winter (11.2 mm decade⁻¹) and temperature increasing primarily during the growing season (0.12°C decade⁻¹). In response to wetter winters and warmer summers, streamflow decreased −9.8 mm decade⁻¹, with largest declines occurring during summer and autumn baseflow (−8.4 mm decade⁻¹). Spring warming was associated with increases in episodic, short spring melt events, earlier and slower snowmelt and an increase in fraction of precipitation available to plants (catchment wetting or W). Warming during the growing season resulted in an increase in the fraction of W lost as evapotranspiration (ET), earlier and lower peaks in remotely sensed normalized difference vegetation index (NDVI) and lower tree ring width index (RWI). These analyses highlight that vegetation is becoming increasingly water limited even as increases in precipitation and slower melt increase plant water availability. Further, catchment‐derived metrics like the Horton Index (ET/W) provide insight in to how simultaneous changes in temperature, precipitation and melt impact vegetation across complex watersheds. 

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4. Earth System Model Overestimation of Cropland Temperatures Scales With Agricultural Intensity

   https://doi.org/10.1029/2021GL097135  

   Coffel, Ethan D.; Lesk, Corey; Mankin, Justin S. (August 2022, Geophysical Research Letters)

   Abstract Intensive crop growth can modify regional climate by partitioning energy to latent heating through transpiration, cooling growing season temperatures. Recent work shows that cooling associated with agriculture can dampen anthropogenic warming over breadbasket regions. However, it is unknown whether climate models reproduce crop influences on regional climate, and thus the future risk of extreme climate events over global breadbasket regions. We show that models overestimate growing season temperatures and underestimate evapotranspiration (ET) over global croplands, and that these differences increase with cropped area. We trace this warm and dry difference through each model's representation of the surface energy budget, showing that model differences in transpiration, leaf area index, and the ratio of transpiration to total ET drive the overall effect. While the implications of these model deficiencies for future projections are uncertain, they point to the importance of improving representations of crop‐climate processes to better assess breadbasket vulnerability to climate change. 

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5. Assessing the influence of soil freeze–thaw cycles on catchment water storage–flux–age interactions using a tracer-aided ecohydrological model

   https://doi.org/10.5194/hess-23-3319-2019  

   Smith, Aaron; Tetzlaff, Doerthe; Laudon, Hjalmar; Maneta, Marco; Soulsby, Chris (January 2019, Hydrology and Earth System Sciences)

   Abstract. Ecohydrological models are powerful tools to quantify the effects that independent fluxes may have on catchment storage dynamics. Here, we adapted the tracer-aided ecohydrological model, EcH₂O-iso, for cold regions with the explicit conceptualization of dynamic soil freeze–thaw processes. We tested the model at the data-rich Krycklan site in northern Sweden with multi-criterion calibration using discharge, stream isotopes and soil moisture in three nested catchments. We utilized the model's incorporation of ecohydrological partitioning to evaluate the effect of soil frost on evaporation and transpiration water ages, and thereby the age of source waters. The simulation of stream discharge, isotopes, and soil moisture variability captured the seasonal dynamics at all three stream sites and both soil sites, with notable reductions in discharge and soil moisture during the winter months due to the development of the frost front. Stream isotope simulations reproduced the response to the isotopically depleted pulse of spring snowmelt. The soil frost dynamics adequately captured the spatial differences in the freezing front throughout the winter period, despite no direct calibration of soil frost to measured soil temperature. The simulated soil frost indicated a maximum freeze depth of 0.25 m below forest vegetation. Water ages of evaporation and transpiration reflect the influence of snowmelt inputs, with a high proclivity of old water (pre-winter storage) at the beginning of the growing season and a mix of snowmelt and precipitation (young water) toward the end of the summer. Soil frost had an early season influence of the transpiration water ages, with water pre-dating the snowpack mainly sustaining vegetation at the start of the growing season. Given the long-term expected change in the energy balance of northern climates, the approach presented provides a framework for quantifying the interactions of ecohydrological fluxes and waters stored in the soil and understanding how these may be impacted in future. 

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