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  1. Adams, Henry (Ed.)

    The ubiquity of woody plant expansion across many rangelands globally has led to the hypothesis that the global rise in atmospheric carbon dioxide concentration ([CO2]) is a global driver facilitating C3 woody plant expansion. Increasing [CO2] also influences precipitation patterns seasonally and across the landscape, which often results in the prevalence of drought in rangelands. To test the potential for [CO2] to facilitate woody plant growth, we conducted a greenhouse study for 150 days to measure CO2 effects on juveniles from four woody species (Cornus drummondii C.A. Mey., Rhus glabra L., Gleditsia triacanthos L., Juniperus osteosperma Torr.) that are actively expanding into rangelands of North America. We assessed chronic water-stress (nested within CO2 treatments) and its interaction with elevated [CO2] (800 p.p.m.) on plant growth physiology for 84 days. We measured leaf-level gas exchange, tissue-specific starch concentrations and biomass. We found that elevated [CO2] increased photosynthetic rates, intrinsic water-use efficiencies and leaf starch concentrations in all woody species but at different rates and concentrations. Elevated [CO2] increased leaf starch levels for C. drummondii, G. triacanthos, J. osteosperma and R. glabra by 90, 39, 68 and 41%, respectively. We also observed that elevated [CO2] ameliorated the physiological effects of chronic water stress for all our juvenile woody species within this study. Elevated [CO2] diminished the impact of water stress on the juvenile plants, potentially alleviating an abiotic limitation to woody plant establishment in rangelands, thus facilitating the expansion of woody plants in the future.

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    Free, publicly-accessible full text available December 31, 2023
  2. Intermittent headwater streams are highly vulnerable to environmental disturbances, but effective management of these water resources requires first understanding the mechanisms that generate streamflow. This study examined mechanisms governing streamflow generation in merokarst terrains, a type of carbonate terrain that covers much of the central United States yet has received relatively little attention in hydrological studies. We used high-frequency sampling of precipitation, stream water, and groundwater during summer 2021 to quantify the contributions to streamflow from different water sources and characterize their short-term dynamics in a 1.2 km 2 merokarst catchment at the Konza Prairie Biological Station (Kansas, USA). Mixing calculations using stable water isotopes and dissolved ions indicate that streamflow is overwhelmingly contributed by groundwater discharge from thin (1–2 m) limestone aquifers, even during wet periods, when soil water and surface runoff are generally expected to be more important. Relationships between hydraulic heads in the aquifers and their contributions to streamflow differed early in the study period compared to later, after a major storm occurred, suggesting there is a critical threshold of groundwater storage that the bedrock needs to attain before fully connecting to the stream. Furthermore, contributions from each limestone unit varied during the study period in response to differences in their hydrogeological properties and/or their stratigraphic position, which in turn impacted both the length of streamflow and its composition. Taken together, we interpret that the subsurface storage threshold and variation in aquifer properties are major controllers of flow intermittency in merokarst headwater catchments. 
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  3. Cavaleri, Molly (Ed.)

    Leaf trait variation enables plants to utilize large gradients of light availability that exist across canopies of high leaf area index (LAI), allowing for greater net carbon gain while reducing light availability for understory competitors. While these canopy dynamics are well understood in forest ecosystems, studies of canopy structure of woody shrubs in grasslands are lacking. To evaluate the investment strategy used by these shrubs, we investigated the vertical distribution of leaf traits and physiology across canopies of Cornus drummondii, the predominant woody encroaching shrub in the Kansas tallgrass prairie. We also examined the impact of disturbance by browsing and grazing on these factors. Our results reveal that leaf mass per area (LMA) and leaf nitrogen per area (Na) varied approximately threefold across canopies of C. drummondii, resulting in major differences in the physiological functioning of leaves. High LMA leaves had high photosynthetic capacity, while low LMA leaves had a novel strategy for maintaining light compensation points below ambient light levels. The vertical allocation of leaf traits in C. drummondii canopies was also modified in response to browsing, which increased light availability at deeper canopy depths. As a result, LMA and Na increased at lower canopy depths, leading to a greater photosynthetic capacity deeper in browsed canopies compared to control canopies. This response, along with increased light availability, facilitated greater photosynthesis and resource-use efficiency deeper in browsed canopies compared to control canopies. Our results illustrate how C. drummondii facilitates high LAI canopies and a compensatory growth response to browsing—both of which are key factors contributing to the success of C. drummondii and other species responsible for grassland woody encroachment.

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  4. Summary

    Savannas cover a significant fraction of the Earth's land surface. In these ecosystems, C3trees and C4grasses coexist persistently, but the mechanisms explaining coexistence remain subject to debate. Different quantitative models have been proposed to explain coexistence, but these models make widely contrasting assumptions about which mechanisms are responsible for savanna persistence. Here, we show that no single existing model fully captures all key elements required to explain tree–grass coexistence across savanna rainfall gradients, but many models make important contributions. We show that recent empirical work allows us to combine many existing elements with new ideas to arrive at a synthesis that combines elements of two dominant frameworks: Walter's two‐layer model and demographic bottlenecks. We propose that functional rooting separation is necessary for coexistence and is the crux of the coexistence problem. It is both well‐supported empirically and necessary for tree persistence, given the comprehensive grass superiority for soil moisture acquisition. We argue that eventual tree dominance through shading is precluded by ecohydrological constraints in dry savannas and by fire and herbivores in wet savannas. Strong asymmetric grass–tree competition for soil moisture limits tree growth, exposing trees to persistent demographic bottlenecks.

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  5. The widespread extirpation of megafauna may have destabilized ecosystems and altered biodiversity globally. Most megafauna extinctions occurred before the modern record, leaving it unclear how their loss impacts current biodiversity. We report the long-term effects of reintroducing plains bison ( Bison bison ) in a tallgrass prairie versus two land uses that commonly occur in many North American grasslands: 1) no grazing and 2) intensive growing-season grazing by domesticated cattle ( Bos taurus ). Compared to ungrazed areas, reintroducing bison increased native plant species richness by 103% at local scales (10 m 2 ) and 86% at the catchment scale. Gains in richness continued for 29 y and were resilient to the most extreme drought in four decades. These gains are now among the largest recorded increases in species richness due to grazing in grasslands globally. Grazing by domestic cattle also increased native plant species richness, but by less than half as much as bison. This study indicates that some ecosystems maintain a latent potential for increased native plant species richness following the reintroduction of native herbivores, which was unmatched by domesticated grazers. Native-grazer gains in richness were resilient to an extreme drought, a pressure likely to become more common under future global environmental change. 
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  6. Abstract

    A frequently expressed viewpoint across the Earth science community is that global soil moisture estimates from satellite L‐band (1.4 GHz) measurements represent moisture only in a shallow surface layer (0–5 cm) and consequently are of limited value for studying global terrestrial ecosystems because plants use water from deeper rootzones. Using this argumentation, many observation‐based land surface studies avoid satellite‐observed soil moisture. Here, based on peer‐reviewed literature across several fields, we argue that such a viewpoint is overly limiting for two reasons. First, microwave soil emission depth considerations and statistical considerations of vertically correlated soil moisture information together indicate that L‐band measurements carry information about soil moisture extending below the commonly referenced 5 cm in many conditions. However, spatial variations of effective depths of representation remain uncertain. Second, in reviewing isotopic tracer field studies of plant water uptake, we find a prevalence of vegetation that primarily draws moisture from these upper soil layers. This is especially true for grasslands and croplands covering more than a third of global vegetated surfaces. Even some deeper‐rooted species (i.e., shrubs and trees) preferentially or seasonally draw water from the upper soil layers. Therefore, L‐band satellite soil moisture estimates are more relevant to global vegetation water uptake than commonly appreciated (i.e., relevant beyond only shallow soil processes like soil evaporation). Our commentary encourages the application of satellite soil moisture across a broader range of terrestrial hydrosphere and biosphere studies while urging more rigorous estimates of its effective depth of representation.

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    Free, publicly-accessible full text available February 1, 2024
  7. Abstract

    Grassland ecosystems are historically shaped by climate, fire, and grazing which are essential ecological drivers. These grassland drivers influence morphology and productivity of grasses via physiological processes, resulting in unique water and carbon-use strategies among species and populations. Leaf-level physiological responses in plants are constrained by the underlying anatomy, previously shown to reflect patterns of carbon assimilation and water-use in leaf tissues. However, the magnitude to which anatomy and physiology are impacted by grassland drivers remains unstudied. To address this knowledge gap, we sampled from three locations along a latitudinal gradient in the mesic grassland region of the central Great Plains, USA during the 2018 (drier) and 2019 (wetter) growing seasons. We measured annual biomass and forage quality at the plot level, while collecting physiological and anatomical traits at the leaf-level in cattle grazed and ungrazed locations at each site. Effects of ambient drought conditions superseded local grazing treatments and reduced carbon assimilation and total productivity inA. gerardii. Leaf-level anatomical traits, particularly those associated with water-use, varied within and across locations and between years. Specifically, xylem area increased when water was more available (2019), while xylem resistance to cavitation was observed to increase in the drier growing season (2018). Our results highlight the importance of multi-year studies in natural systems and how trait plasticity can serve as vital tool and offer insight to understanding future grassland responses from climate change as climate played a stronger role than grazing in shaping leaf physiology and anatomy.

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  8. Summary

    Models of tree–grass coexistence in savannas make different assumptions about the relative performance of trees and grasses under wet vs dry conditions. We quantified transpiration and drought tolerance traits in 26 tree and 19 grass species from the African savanna biome across a gradient of soil water potentials to test for a trade‐off between water use under wet conditions and drought tolerance.

    We measured whole‐plant hourly transpiration in a growth chamber and quantified drought tolerance using leaf osmotic potential (Ψosm). We also quantified whole‐plant water‐use efficiency (WUE) and relative growth rate (RGR) under well‐watered conditions.

    Grasses transpired twice as much as trees on a leaf‐mass basis across all soil water potentials. Grasses also had a lower Ψosmthan trees, indicating higher drought tolerance in the former. Higher grass transpiration and WUE combined to largely explain the threefold RGR advantage in grasses.

    Our results suggest that grasses outperform trees under a wide range of conditions, and that there is no evidence for a trade‐off in water‐use patterns in wet vs dry soils. This work will help inform mechanistic models of water use in savanna ecosystems, providing much‐needed whole‐plant parameter estimates for African species.

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