Sean M. Gleason’s research while affiliated with Colorado State University and other places

We examine limited transpiration (LT) traits in crop species, which are claimed to conserve early season water for critical late season growth. Despite there being theoretical support for LT crops, we suggest that there is insufficient empirical evidence to support the general acceptance of this theory. Our criticism focuses on two main points: the undervaluation of early season carbon assimilation and investment over the lifetime of the plant; and the overestimation of soil water savings. We argue that forgoing early season water use, and therefore also future investment in deeper and denser roots (improved resource acquisition), will negatively impact plant performance in many soil and climate contexts. Furthermore, we challenge the assumption that conserved soil water remains available for later use without loss, noting significant losses resulting from evaporation and other sinks. We advocate for a re‐evaluation of LT traits, incorporating a balance of water and carbon dynamics throughout a plant's lifetime. We caution against the adoption of LT traits where they have not been empirically evaluated in the soils and climates of interest to individual research and breeding programs. We propose a more physiologically integrated approach to crop improvement, focusing on water extraction efficiency and strategic carbon investment.

Plant hydraulic dysfunction by embolism formation can impair photosynthesis, growth, and reproduction and, in severe cases, lead to death. Embolism reversal, or “refilling,” is a hypothesized adaptive process in which xylem functionality is rapidly and sustainably restored. This study investigated xylem embolism refilling during recovery from severe drought stress using entirely noninvasive measurements of the same plants. These results were considered in relation to functional traits to address long-standing gaps in understanding the consequences of severe drought stress. Leaf and stem xylem embolism as well as transpiration, photosynthesis, and stem water potential were characterized nondestructively on intact barnyard grass plants during an acute drought event. Plants were rewatered and returned to growth conditions for 10 d, during which time recovery of stem xylem embolism and transpiration were monitored. Leaf xylem embolism and declines in leaf gas exchange occurred mostly between −1.0 MPa and −2.0 MPa, whereas stem xylem embolism occurred mostly between −3.0 MPa and −4.0 MPa. In all measured plants, which included embolism levels up to 88%, stem xylem embolism reversed completely within 24 h after rewatering, and this refilling supported recovery of transpiration and growth after plants were returned to growth conditions. This study provides direct evidence of complete and functional stem xylem refilling. These results present a clear need to elucidate underlying mechanisms and the adaptive significance of this phenomenon as well as its prevalence in nature.

Plant water transport is essential to maintain turgor, photosynthesis and growth. Water is transported in a metastable state under large negative pressures, which can result in embolism, that is, the loss of function by the replacement of liquid xylem sap with gas, as a consequence of water stress. To avoid experimental artefacts, we used an optical vulnerability system to quantify embolism occurrence across six fully expanded maize leaves to characterize the sequence of physiological responses (photosynthesis, chlorophyll fluorescence, whole‐plant transpiration and leaf inter‐vein distance) in relation to declining water availability and leaf embolism during severe water stress. Additionally, we characterize the recovery of leaf function in the presence of sustained embolism during a 6‐day recovery period. Embolism formation occurred after other physiological processes were substantially depressed and were irreversible upon rewatering. Recovery of transpiration, net CO 2 assimilation and photosystem II efficiency were aligned with the severity of embolism, whereas these traits returned to near pre‐stress levels in the absence of embolism. A better understanding of the relationships between embolism occurrence and downstream physiological processes during stress and recovery is critical for the improvement of crop productivity and resilience.

Increasing wildfires across the western United States are affecting forest resilience, often leading to novel ecosystems after forest regeneration failures. Following fire, soil organic amendments are commonly applied to reduce erosion and accelerate recovery, but treatments effects on vegetation functional strategies and plant-microbe interactions are poorly understood. Here, we assess the impacts of soil amendments on plant and microbial community composition and plant functional traits 13 years following a severe wildfire in Colorado. Treatments included woody mulch, biochar, and mulch + biochar, which had unique effects on soil nutrients, chemistry, moisture, and microbiomes. We considered three plant functional traits: plant height, specific leaf area (SLA), and leaf dry matter content (LDMC), which had limited direct relationships with treatments. However, random forests and structural equation models revealed non-linear and indirect effects of treatments on soil physical and chemical properties and microbial composition, thereby affecting traits. There was a species-specific response of plant traits to soil N form; Vaccinium scoparium responded more to NH4 + and Oreochrysum parryi responded more to NO3-. Interactions among soil moisture, chemistry (pH and cations), and microbiome composition (especially ectomycorrhizal fungi and nitrifying bacteria and archaea) drove variation in plant traits. These findings show that soil amendment effects on plant traits or microbial composition may not be directly detected using simple statistical approaches, but emerge as indirect effects, revealing the multifaceted importance of soil biotic and abiotic characteristics. This work will benefit future research and restoration practices utilizing a trait-based approach to ecosystem recovery and improving forest resilience.

Microclimates are driven by fine-scale weather patterns, play a key role in shaping vegetation responses, and can serve as refuges against climate change. However, little is known about the meteorological factors that drive temporal variability across landscapes. In this study, we deployed a network of air temperature (Tair) and relative humidity (RH) sensors across a small grassland watershed in northern Colorado. We installed sensors at 2 m and 10 m heights to assess stability within the surface boundary layer along with anemometers and radiation sensors. We found significant within-field spatial variability with Tair fluctuating by >15°C and RH by >50% at 15 min intervals. The mean difference in Tair between the highest and lowest points in the watershed was 0.29 °C, corresponding to a near-surface lapse rate of 10 °C km-1, higher than the free-air lapse rate of 6.5 °C km-1. Within-field variability was driven primarily by atmospheric stability, being highest during periods of inversion, with secondary effects of wind speed and solar radiation. These variations resulted in annual variability in biophysical metrics within the field, such as growing degree days and potential evapotranspiration, that exceeded 7%. We found that as mean Tair increased, variability in within-field Tair decreased, indicating that warmer temperatures may reduce microclimate stability, with implications for microrefugia and species' range shifts under warming. This study highlights the temporal dynamics of microclimates which require more thorough representation of surface boundary layer conditions, especially the role of vertical temperature gradients, in shaping landscape-scale ecological, hydrological, and biophysical processes.

Members of the grass family Poaceae have adapted to a wide range of habitats and disturbance regimes globally. The cellular structure and arrangements of leaves can help explain how plants survive in different climates, but these traits are rarely measured in grasses. Most studies are focused on individual species or distantly related species within Poaceae. While this focus can reveal broad adaptations, it is also likely to overlook subtle adaptations within more closely-related groups (subfamilies, tribes). This study therefore investigated the scaling relationships between leaf size, Vein Length Area (VLA), and vessel size in five genera within the subfamily Pooideae. The scaling exponent of the relationship between leaf area and VLA was -0.46 (+/- 0.21), which is consistent with previous studies. In Poa and Elymus, however, minor vein number and leaf length were uncorrelated, whereas in Festuca these traits were positively correlated (slope = 0.82 +/- 0.8). These findings suggest there are broad-scale and fine-scale variation in leaf hydraulic traits among grasses. Future studies should consider both narrow and broad phylogenetic gradients.

In theory, there is a trade‐off between hydraulic efficiency and safety. However, the strength and direction of this trade‐off at the leaf level are not consistent across studies, and habitat climate may impact this trade‐off. We compiled a leaf hydraulic efficiency and safety dataset for 362 species from 81 sites world‐wide, with 280 paired observations of both traits, and tested whether climate was associated with departure from the proposed trade‐off. The leaf hydraulic efficiency–safety trade‐off was weak (R² = 0.144) at the global scale. Mean annual precipitation and solar radiation (SR) modified the trade‐off. Species from dry and high SR habitats (e.g. desert and tropical savanna) were generally located above the trade‐off line, indicating that these species tended to have higher leaf hydraulic safety and efficiency than species from wet habitats with low SR (e.g. subtropical monsoon forest and montane rainforest), which were located below the trade‐off line. Leaves with high vein density, dry leaf mass per area, and osmotic regulation enhanced safety without compromising hydraulic efficiency. Variation in the hydraulic efficiency–safety trade‐off at the leaf level likely facilitates plant survival in specific habitats and allows for a more nuanced view of leaf hydraulic adaption strategies at the global scale.

Limited transpiration (LT) traits aim to conserve early-season water to benefit late-season grain development. While theoretical and modeling efforts support LT efficacy, empirical tests directly measuring water loss from leaves and canopies are scarce. This study evaluates the performance of LT genotypes in achieving reduced early-season water use and improved late-season growth and yield in semi-arid Colorado. The research involved near-isogenic lines (NILs) derived from sorghum inbred lines, subjected to different irrigation treatments. Measurements included stomatal conductance, net CO2 assimilation, and photosystem II (PSII) efficiency. Results indicate that LT genotypes did not consistently exhibit lower early-season water use or higher late-season growth compared to non-LT genotypes. Early-season water use was positively correlated with above-ground biomass, challenging the assumption that early-season water conservation can be leveraged for late-season benefits. We question the efficacy of LT traits, highlighting the physiological link between water use and carbon gain, and the potential opportunity costs of reduced early-season growth. We suggests that breeding strategies should focus on enhancing deep soil water access and maximizing carbon gain rather than merely reducing transpiration or shifting water use in arid and semi-arid environments.

Leaf vein network cost (total vein surface area per leaf volume) for major veins and vascular bundles did not differ between monocot and dicot species in 21 species from the eastern Colorado steppe. Dicots possessed significantly larger minor vein networks than monocots. Across the tree of life, there is evidence that dendritic vascular transport networks are optimized, balancing maximum speed and integrity of resource delivery with minimal resource investment in transport and infrastructure. Monocot venation, however, is not dendritic, and remains parallel down to the smallest vein orders with no space-filling capillary networks. Given this departure from the “optimized” dendritic network, one would assume that monocots are operating at a significant energetic disadvantage. In this study, we investigate whether monocot venation networks bear significantly greater carbon/construction costs per leaf volume than co-occurring dicots in the same ecosystem, and if so, what physiological or ecological advantage the monocot life form possesses to compensate for this deficit. Given that venation networks could also be optimized for leaf mechanical support or provide herbivory defense, we measured the vascular system of both monocot and dicots at three scales to distinguish between leaf investment in mechanical support (macroscopic vein), total transport and capacitance (vascular bundle), or exclusively water transport (xylem) for both parallel and dendritic venation networks. We observed that vein network cost (total vein surface area per leaf volume) for major veins and vascular bundles was not significantly different between monocot species and dicot species. Dicots, however, possess significantly larger minor vein networks than monocots. The 19 species subjected to gas-exchange measurement in the field displayed a broad range of Amax and but demonstrated no significant relationships with any metric of vascular network size in major or minor vein classes. Given that monocots do not seem to display any leaf hydraulic disadvantage relative to dicots, it remains an important research question why parallel venation (truly parallel, down to the smallest vessels) has not arisen more than once in the history of plant evolution.