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1. Observed increasing water constraint on vegetation growth over the last three decades

   https://doi.org/10.1038/s41467-021-24016-9  

   Jiao, Wenzhe ; Wang, Lixin ; Smith, William K. ; Chang, Qing ; Wang, Honglang ; D’Odorico, Paolo ( June 2021 , Nature Communications)

   Despite the growing interest in predicting global and regional trends in vegetation productivity in response to a changing climate, changes in water constraint on vegetation productivity (i.e., water limitations on vegetation growth) remain poorly understood. Here we conduct a comprehensive evaluation of changes in water constraint on vegetation growth in the extratropical Northern Hemisphere between 1982 and 2015. We document a significant increase in vegetation water constraint over this period. Remarkably divergent trends were found with vegetation water deficit areas significantly expanding, and water surplus areas significantly shrinking. The increase in water constraints associated with water deficit was also consistent with a decreasing response time to water scarcity, suggesting a stronger susceptibility of vegetation to drought. We also observed shortened water surplus period for water surplus areas, suggesting a shortened exposure to water surplus associated with humid conditions. These observed changes were found to be attributable to trends in temperature, solar radiation, precipitation, and atmospheric CO₂. Our findings highlight the need for a more explicit consideration of the influence of water constraints on regional and global vegetation under a warming climate.

    

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2. Simulating Soil Moisture and Grass Growth in a Eurasian Steppe Grassland by SWAP

   Chereskin, A.D. ; Johnson, D.L. ; Zhang, A. ; Qin, Z. ; Dolan, D. ; Landis, Z.C. ; Potter, N.M. ; Gao, R. ; Luo, Y. ; Yu, R. ; et al ( January 2018 , AGU Fall Meeting 2018)

   The accelerating degradation of native grasslands is becoming a threat to the world’s biome supply and has raised serious environmental concerns such as desertification and dust storm. Given that the steppe grasslands, such as those located in the Inner Mongolia Plateau of north China, have a dry climatic condition, the grass growth closely relies on available soil water, which in turn depends on precipitation prior to the growing season (in particular from May to July). However, our understanding of steppe hydrology and water consumption by grasses is incomplete. In this study, the agro-hydrologic Soil Water Plant Atmosphere (SWAP) model was used to mimic the long-term variations in soil water and vegetation growth in a typical steppe grassland of north China to further understand how alterations of hydrologic processes are related to grassland degradation. A field experiment was conducted to collect the data needed to set up the model. The SWAP model was calibrated using continuous observations of soil moisture and soil temperature at various depths for a simulation period of 2014 to 2017. The results indicated that the SWAP model can be used to simulate the responses of soil moisture and vegetation growth to climates. Moreover, this study examines the water balance and chronological variations of precipitation, evapotranspiration, soil water, and runoff. This study will add new knowledge of steppe hydrologic processes into existing literature. 

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3. Spatial Variation in Catchment Response to Climate Change Depends on Lateral Moisture Transport and Nutrient Dynamics

   https://doi.org/10.1029/2021WR030577  

   Stephens, C. M. ; Marshall, L. A. ; Johnson, F. M. ; Ajami, H. ; Lin, L. ; Band, L. E. ( October 2022 , Water Resources Research)

   Future shifts in rainfall, temperature and carbon dioxide (CO₂) will impact hydrologic and ecosystem behavior. These changes are expected to vary in space because water and nutrient availability vary with terrain and soil properties, with feedbacks on vegetation and canopy adjustment. However, within‐basin patterns and spatial dependencies of ecohydrologic dynamics have often been ignored in future scenario modeling. We used a distributed process‐based ecohydrologic model, the Regional Hydro‐Ecological Simulation System, as a virtual catchment to examine spatial and temporal variability in climate change response. We found spatial heterogeneity in Leaf Area Index, transpiration and soil saturation trends, with some scenarios even showing opposite trends in different locations. For example, in a drying scenario, decreased vegetation productivity in water‐limited upslope areas enhanced downslope nutrient subsidies so that productivity increased in the nutrient‐limited riparian zone. In scenarios with both warming and rising CO₂, amplifying feedbacks between mineralization, vegetation water use efficiency and litter fall led to large increases in growth that were often strongest in the riparian area (depending on the coincident rainfall change). Modeled transpiration trends were determined by the competing effects of vegetation growth and changing water use efficiency. Overall, the riparian zone experienced substantially different (and even opposing) ecohydrologic trends compared to the rest of the catchment, which is important because productive riparian areas often contribute a disproportionate amount of vegetation growth, transpiration and nutrient consumption to catchment totals. Models that are spatially lumped, lack key ecosystem‐driving dynamics, or ignore lateral transport could misrepresent the complex ecohydrologic changes catchments could experience in the future.

    

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4. Impacts of abiotic and biotic factors on tundra productivity near Utqiaġvik, Alaska

   https://doi.org/10.1088/1748-9326/acf7d6  

   Zhang, Qingyuan ; Zhang, Xuesong ; Lara, Mark J. ; Li, Zhengpeng ; Xiao, Jingfeng ; Zhao, Kaiguang ; Hu, Tongxi ( September 2023 , Environmental Research Letters)

   Earlier snowmelt, warmer temperatures and herbivory are among the factors that influence high-latitude tundra productivity near the town of Utqiaġvik in northern Alaska. However, our understanding of the potential interactions between these factors is limited. MODIS observations provide cover fractions of vegetation, snow, standing water, and soil, and fractional absorption of photosynthetically active radiation by canopy chlorophyll (fAPAR_chl) per pixel. Here, we evaluated a recent time-period (2001–2014) that the tundra experienced large interannual variability in vegetation productivity metrics (i.e. fAPAR_chland APAR_chl), which was explainable by both abiotic and biotic factors. We found earlier snowmelt to increase soil and vegetation cover, and productivity in June, while warmer temperatures significantly increased monthly productivity. However, abiotic factors failed to explain stark decreases in productivity during August of 2008, which coincided with a severe lemming outbreak. MODIS observations found this tundra ecosystem to completely recover two years later, resulting in elevated productivity. This study highlights the potential roles of both climate and herbivory in modulating the interannual variability of remotely retrieved plant productivity metrics in Arctic coastal tundra ecosystems.

    

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5. 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)

   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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