Timing and Magnitude of Drought Impacts on Carbon Uptake Across a Grassland Biome

Although drought is known to negatively impact grassland functioning, the timing and magnitude of these impacts within a growing season remains unresolved. Previous small-scale assessments indicate grasslands may only respond to drought during narrow periods within a year; however, large-scale assessments are now needed to uncover the general patterns and determinants of this timing. We combined remote sensing datasets of gross primary productivity and weather to assess the timing and magnitude of grassland responses to drought at 5 km2 temporal resolution across two expansive ecoregions of the western US Great Plains biome: the C4-dominated shortgrass steppe and the C3-dominated northern mixed prairies. Across over 700,000 pixel-year combinations covering more than 600,000 km2, we studied how the driest years between 2003-2020 altered the daily and bi-weekly dynamics of grassland carbon (C) uptake. Reductions to C uptake intensified into the early summer during drought and peaked in mid- and late June in both ecoregions. Stimulation of spring C uptake during drought was small and insufficient to compensate for losses during summer. Thus, total grassland C uptake was consistently reduced by drought across both ecoregions; however, reductions were twice as large across the more southern and warmer shortgrass steppe. Across the biome, increased summer vapor pressure deficit was strongly linked to peak reductions in vegetation greenness during drought. Rising vapor pressure deficit will likely exacerbate reductions in C uptake during drought across the western US Great Plains, with these reductions greatest during the warmest months and in the warmest locations. High spatiotemporal resolution analyses of grassland response to drought over large areas provide both generalizable insights and new opportunities for basic and applied ecosystem science in these water-limited ecoregions amid climate change

Diffuse Light and Wetting Differentially Affect Tropical Tree Leaf Photosynthesis

‐Most ecosystems experience frequent cloud cover resulting in light that is predominantly diffuse rather than direct. Moreover, these cloudy conditions are often accompanied by rain that results in wet leaf surfaces. Despite this, our understanding of photosynthesis is built upon measurements made on dry leaves experiencing direct light. ‐Using a modified gas exchange setup, we measured the effects of diffuse light and leaf wetting on photosynthesis in canopy species from a tropical montane cloud forest. ‐We demonstrate significant variation in species‐level response to light quality independent of light intensity. Some species demonstrated 100% higher rates of photosynthesis in diffuse light while others had 15% greater photosynthesis in direct light. Even at lower light intensities, diffuse light photosynthesis was equal to that under direct light conditions. Leaf wetting generally led to decreased photosynthesis, particularly when the leaf surface with stomata became wet, however, there was significant variation across species. ‐Ultimately, we demonstrate that ecosystem photosynthesis is significant altered in response to environmental conditions that are ubiquitous. Our results help explain the observation that net ecosystem exchange can increase in cloudy conditions and can improve the representation of these processes in earth systems models under projected scenarios of global climate change

Instructor Strategies to Alleviate Stress and Anxiety among College and University STEM Students

While student stress and anxiety are frequently cited as having negative effects on students’ academic performance, the role that instructors can play in mitigating these challenges is often underappreciated. We provide summaries of different evidence-based strategies, ranging from changes in instructional strategies to specific classroom interventions, that instructors may employ to address and ameliorate student stress and anxiety. While we focus on students in science, technology, engineering, and mathematics, the strategies we delineate may be more broadly applicable. We begin by highlighting ways in which instructors can learn about and prepare to act to alleviate stress and anxiety. We then discuss how to better connect with students and build an inclusive, equitable, and empowering classroom environment. When coupled with strategies to change student evaluation and assessment, these approaches may collectively reduce student stress and anxiety, as well as improve student performance. We then discuss the roles that instructors may play in empowering students with skills that improve their time management, studying, and approach toward learning, with an eye toward ensuring their success across all their academic endeavors. We conclude by noting areas in which further research is needed to determine best practices for alleviating student stress and anxiety

Evidence for Phylogenetic Signal and Correlated Evolution in Plant-Water Relations Traits

Evolutionary relationships are likely to play a significant role in shaping plant physiological and structural traits observed in contemporary taxa. We review research on phylogenetic signal and correlated evolution in plant-water relations traits, which play important roles in allowing plants to acquire, use and conserve water. We found more evidence for a phylogenetic signal in structural traits (e.g., stomatal length, stomatal density) than in physiological traits (e.g., stomatal conductance, water potential at turgor loss). Although water potential at turgor loss is the most-studied plant-water relations trait in an evolutionary context, it is the only trait consistently found to not have a phylogenetic signal. Correlated evolution was common among traits related to water movement efficiency and hydraulic safety in both leaves and stems. We conclude that evidence for phylogenetic signal varies depending on 1) the methodology used for its determination, i.e., model-based approaches to determine phylogenetic signal such as Blomberg\u27s K or Pagel\u27s λ vs. statistical approaches such as ANOVAs with taxonomic classification as a factor; 2) on the number of taxa studied (size of the phylogeny); and 3) the setting in which plants grow (field vs. common garden). More explicitly and consistently considering the role of evolutionary relationships in shaping plant ecophysiology could improve our understanding of how traits compare among species, how traits are coordinated with one another, and how traits vary with environment

Quantifying and Manipulating the Angles of Light in Experimental Measurements of Plant Gas Exchange

Diffuse light has been shown to alter plant leaf photosynthesis, transpiration and water-use efficiency. Despite this, the angular distribution of light for the artificial light sources used with common gas exchange systems is unknown. Here, we quantify the angular distribution of light from common gas exchange systems and demonstrate the use of an integrating sphere for manipulating those light distributions. Among three different systems, light from a 90° angle perpendicular to the leaf surface (±5.75°) was \u3c25% of the total light reaching the leaf surface. The integrating sphere resulted in a greater range of possible distributions from predominantly direct light (i.e., \u3e40% of light from a 90 ± 5.75° angle perpendicular to the leaf surface) to almost entirely diffuse (i.e., light from an even distribution drawn from a nearly 0° horizontal angle to a perpendicular 90° angle). The integrating sphere can thus create light environments that more closely mimic the variation in sunlight under both clear and cloudy conditions. In turn, different proportions of diffuse light increased, decreased or did not change photosynthetic rates depending on the plant species observed. This new tool should allow the scientific community to explore new and creative questions about plant function within the context of global climate change

Convergence in Water Use Efficiency Within Plant Functional Types across contrasting climates

Water use efficiency (WUE) provides a direct measure of the inextricable link between plant carbon uptake and water loss, and it can be used to study how ecosystem function varies with climate. We analysed WUE data from the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS), leveraging the high spatial resolution of ECOSTRESS to study the distribution of WUE values both within and among regions with different plant functional types. Our results indicate that despite wide local variability of WUE estimates, WUE tended to converge to common global optima (peaked distributions with variance \u3c0.5 g C per kg H2O, kurtosis \u3e3.0) for five of nine plant functional types (grassland, permanent wetland, savannah, deciduous broadleaf and deciduous needleleaf forest), and this convergence occurred in functional types that spanned distinct geographic regions and climates

Phylogenetic and Biogeographic Controls of Plant Nighttime Stomatal Conductance

The widely documented phenomenon of nighttime stomatal conductance (gsn) could lead to substantial water loss with no carbon gain, and thus it remains unclear whether nighttime stomatal conductance confers a functional advantage. Given that studies of gsn have focused on controlled environments or small numbers of species in natural environments, a broad phylogenetic and biogeographic context could provide insights into potential adaptive benefits of gsn. We measured gsn on a diverse suite of species (n = 73) across various functional groups and climates‐of‐origin in a common garden to study the phylogenetic and biogeographic/climatic controls on gsn and further assessed the degree to which gsn co‐varied with leaf functional traits and daytime gas exchange rates. Closely related species were more similar in gsn than expected by chance. Herbaceous species had higher gsn than woody species. Species that typically grow in climates with lower mean annual precipitation – where the fitness cost of water loss should be the highest – generally had higher gsn. Our results reveal the highest gsn rates in species from environments where neighboring plants compete most strongly for water, suggesting a possible role for the competitive advantage of gsn

Seeing Light from a Different Angle: The Effects of Diffuse Light Environments on the Function, Structure, and Growth of Tomato Plants

While considerable attention has been paid to how plants respond to changes in the spectral distribution, less attention has been paid to how plants respond to changes in the angular qualities of light. Evidence from both leaf- and ecosystem-scale measurements indicate that plants vary in their response to diffuse compared to direct light growing environments. Because of the significant implications for agricultural production, we quantified how changes in the angular quality of light affect the structure, function, and growth of Roma tomatoes in a greenhouse experiment with direct and diffuse light treatments. Diffuse light conditions (ca. 50-60% diffuse) were created with a glass coating that scattered incoming light. We measured leaf physiology and structure, as well as whole plant physiology, morphology, and growth. Light-saturated photosynthetic rates were set by the growing light environment and were unchanged by short-term exposure to the opposite light environment. After two months, plants in the diffuse light treatment demonstrated lower photosynthesis and had thinner leaves with higher chlorophyll concentration. However, relative growth rates did not differ between treatments and plants grown in diffuse light had significantly higher biomass at the conclusion of the experiment. While there was no difference in leaf or whole-plant water-use efficiency, plants in the diffuse light treatment demonstrated significantly lower leaf temperatures, highlighting the potential for diffuse light coatings and/or materials to reduce energy use for cooling. Our results highlight the need to advance our understanding of the effects of diffuse light conditions on agricultural crops growing on a changing planet

Seeing Light from a Different Angle: The Effects of Diffuse Light Environments on the Function, Structure, and Growth of Tomato Plants

While considerable attention has been paid to how plants respond to changes in the spectral distribution, less attention has been paid to how plants respond to changes in the angular qualities of light. Evidence from both leaf- and ecosystem-scale measurements indicate that plants vary in their response to diffuse compared to direct light growing environments. Because of the significant implications for agricultural production, we quantified how changes in the angular quality of light affect the structure, function, and growth of Roma tomatoes in a greenhouse experiment with direct and diffuse light treatments. Diffuse light conditions (ca. 50-60% diffuse) were created with a glass coating that scattered incoming light. We measured leaf physiology and structure, as well as whole plant physiology, morphology, and growth. Light-saturated photosynthetic rates were set by the growing light environment and were unchanged by short-term exposure to the opposite light environment. After two months, plants in the diffuse light treatment demonstrated lower photosynthesis and had thinner leaves with higher chlorophyll concentration. However, relative growth rates did not differ between treatments and plants grown in diffuse light had significantly higher biomass at the conclusion of the experiment. While there was no difference in leaf or whole-plant water-use efficiency, plants in the diffuse light treatment demonstrated significantly lower leaf temperatures, highlighting the potential for diffuse light coatings and/or materials to reduce energy use for cooling. Our results highlight the need to advance our understanding of the effects of diffuse light conditions on agricultural crops growing on a changing planet

Foliar Water Uptake: Processes, Pathways, and Integration into Plant Water Budgets

Nearly all plant families, represented across most major biomes, absorb water directly through their leaves. This phenomenon is commonly referred to as foliar water uptake. Recent studies have suggested that foliar water uptake provides a significant water subsidy that can influence both plant water and carbon balance across multiple spatial and temporal scales. Despite this, our mechanistic understanding of when, where, how, and to what end water is absorbed through leaf surfaces remains limited. We first review the evidence for the biophysical conditions necessary for foliar water uptake to occur, focusing on the plant and atmospheric water potentials necessary to create a gradient for water flow. We then consider the different pathways for uptake, as well as the potential fates of the water once inside the leaf. Given that one fate of water from foliar uptake is to increase leaf water potentials and contribute to the demands of transpiration, we also provide a quantitative synthesis of observed rates of change in leaf water potential and total fluxes of water into the leaf. Finally, we identify critical research themes that should be addressed to effectively incorporate foliar water uptake into traditional frameworks of plant water movement