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In an era where human influence pervades every corner of the natural world, improving our understanding the how ecosystems are structured and function has never been more critical. Dryland ecosystems, occupying over 40% of the plant’s land surface area, represent the largest terrestrial biome on earth. Drylands are particularly vulnerable to global change pressures such as rising temperatures, altered precipitation regimes, and the spread of invasive species. The Earth's climate is changing rapidly, exacerbating these pressures and threatening the productivity, biodiversity, and function of dryland ecosystems. The interactions between these shifting pressures and disturbances, both natural and anthropogenic, add layers of complexity that challenge our understanding of these ecosystems. In my dissertation, I used a combination of observational and experimental studies to investigate the impacts of disturbances on vegetation within dryland ecosystems, focusing particularly on the interactions between climate and biological factors known to influence the structure and function of plants. A rainfall manipulation and mechanical disturbance experiment repeated in three climatically distinct North America dryland ecosystems revealed complex, site-specific responses of dominant shrubs to environmental stressors. Findings indicated that individual traits, such as plant size, significantly influence sensitivity to climate changes, highlighting the need for localized management strategies. A study assessing the trophic impacts from a native twig girdling beetle on the above ground biomass of honey mesquite (Prosopis glandulosa) found significant year-to-year variability in beetle activity, with notable reductions in mesquite biomass due to girdling that exceeded estimates for annual net primary production for some years. A study assessing the influence of fire on biodiversity of soil seed bank across the Mojave found increased diversity in burned areas, but highlights the dominance of invasive species, ultimately leading to biodiversity loss and community homogenization. These findings underscore the significant impact of invasive species and the necessity of management practices to mitigate their spread. Collectively, this dissertation provides a nuanced understanding of how natural and novel disturbance regimes affect dryland ecosystems. The differential responses among species and ecosystems suggest that effective management strategies must consider local ecological contexts to preserve productivity and biodiversity amidst rapidly changing global pressures. 

Abstract Patches of plants in drylands can arise when larger plants facilitate recruitment via ‘nurse‐plant’ effects. Simultaneously, plant–soil feedback processes can increase soil nutrients beneath shrubs leading to spatial patterns known as ‘islands of fertility’ that may also promote plant performance. These phenomena shape dryland plant communities; however, their comparative relative influences on plant density and diversity remain poorly quantified.Using reciprocal soil transplants among pairs of unvegetated interspaces and neighbouring shrubs in combination with cover treatments—open interspace, shaded interspace, intact shrub canopy and trimmed shrub canopy—this study aimed to disentangle the influences of shading from soil properties in native plant recruitment under the foundation species of shrubsLarrea tridentata andNeltuma glandulosain the Chihuahuan Desert, NM, USA.Seedling density varied significantly across years in response to interannual climate variability and was consistently higher underL. tridentatathanN. glandulosa. Cover treatments, soil provenance, shrub species and spatial factors (study blocks) significantly affected seedling density, species richness and community structure. Shading from intact shrubs or shade clothes had a stronger effect on density and richness than soil provenance or shrub species. Seedling density and richness were also higher in soil from beneath shrubs regardless of shrub species, location (native or transplanted locations) or time (study year) since initial translocation.The density of seedlings emerging from shaded interspace soil was initially similar to that observed under intact shrubs indicating the importance of shading by nurse plants for recruitment. However, the effect of shade‐cloth canopy surrogates on seedling density in interspaces switched from positive to negative by the third year of the study, likely due to seed bank depletion.Synthesis. The significant influence of shading highlights the role of environmental factors in shaping plant communities of this dryland system, while the evidence of seed depletion combined with the influence of plot location suggests a concurrent influence of stochastic community assembly processes. Understanding how these processes interact to shape spatial and temporal patterns of vegetation community assembly and potentially reinforce alternate ecological states remains an important focus in dryland ecosystems. 

Variability of the terrestrial global carbon sink is largely determined by the response of dryland productivity to annual precipitation. Despite extensive disturbance in drylands, how disturbance alters productivity-precipitation relationships remains poorly understood. Using remote-sensing to pair more than 5600 km of natural gas pipeline corridors with neighboring undisturbed areas in North American drylands, we found that disturbance reduced average annual production 6 to 29% and caused up to a fivefold increase in the sensitivity of net primary productivity (NPP) to interannual variation in precipitation. Disturbance impacts were larger and longer-lasting at locations with higher precipitation (>450 mm mean annual precipitation). Disturbance effects on NPP dynamics were mostly explained by shifts from woody to herbaceous vegetation. Severe disturbance will amplify effects of increasing precipitation variability on NPP in drylands. 

Maintaining your research team’s productivity during the COVID-19 era can be a challenge. Developing new strategies to mentor your research trainees in remote work environments will not only support research productivity and progress toward degree, but also help to keep your mentees’ academic and research careers on track. We describe a three-step process grounded in reflective practice that research mentors and mentees can use together to reassess, realign, and reimagine their mentoring relationships to enhance their effectiveness, both in the current circumstances and for the future. Drawing on evidence-based approaches, a series of questions for mentees around documented mentoring competencies provide structure for remote mentoring plans. Special consideration is given to how these plans must address the psychosocial needs and diverse backgrounds of mentors and mentees in the unique conditions that require remote interactions. 

Fluorescent nucleobase surrogates capable of Watson–Crick hydrogen bonding are essential probes of nucleic acid structure and dynamics, but their limited brightness and short absorption and emission wavelengths have rendered them unsuitable for single-molecule detection. Aiming to improve on these properties, we designed a new tricyclic pyrimidine nucleoside analogue with a push–pull conjugated system and synthesized it in seven sequential steps. The resulting C -linked 8-(diethylamino)benzo[ b ][1,8]naphthyridin-2(1 H )-one nucleoside, which we name ABN, exhibits ε 442 = 20 000 M −1 cm −1 and Φ em,540 = 0.39 in water, increasing to Φ em = 0.50–0.53 when base paired with adenine in duplex DNA oligonucleotides. Single-molecule fluorescence measurements of ABN using both one-photon and two-photon excitation demonstrate its excellent photostability and indicate that the nucleoside is present to > 95% in a bright state with count rates of at least 15 kHz per molecule. This new fluorescent nucleobase analogue, which, in duplex DNA, is the brightest and most red-shifted known, is the first to offer robust and accessible single-molecule fluorescence detection capabilities. 

As droughts become longer and more intense, impacts on terrestrial primary productivity are expected to increase progressively. Yet, some ecosystems appear to acclimate to multiyear drought, with constant or diminishing reductions in productivity as drought duration increases. We quantified the combined effects of drought duration and intensity on aboveground productivity in 74 grasslands and shrublands distributed globally. Ecosystem acclimation with multiyear drought was observed overall, except when droughts were extreme (i.e., ≤1-in-100-year likelihood of occurrence). Productivity losses after four consecutive years of extreme drought increased by ~2.5-fold compared with those of the first year. These results portend a foundational shift in ecosystem behavior if drought duration and intensity increase, from maintenance of reduced functioning over time to progressive and profound losses of productivity when droughts are extreme. 

The T cell receptor (TCR) initiates the elimination of pathogens and tumors by T cells. To avoid damage to the host, the receptor must be capable of discriminating between wild-type and mutated self and nonself peptide ligands presented by host cells. Exactly how the TCR does this is unknown. In resting T cells, the TCR is largely unphosphorylated due to the dominance of phosphatases over the kinases expressed at the cell surface. However, when agonist peptides are presented to the TCR by major histocompatibility complex proteins expressed by antigen-presenting cells (APCs), very fast receptor triggering, i.e., TCR phosphorylation, occurs. Recent work suggests that this depends on the local exclusion of the phosphatases from regions of contact of the T cells with the APCs. Here, we developed and tested a quantitative treatment of receptor triggering reliant only on TCR dwell time in phosphatase-depleted cell contacts constrained in area by cell topography. Using the model and experimentally derived parameters, we found that ligand discrimination likely depends crucially on individual contacts being ∼200 nm in radius, matching the dimensions of the surface protrusions used by T cells to interrogate their targets. The model not only correctly predicted the relative signaling potencies of known agonists and nonagonists but also achieved this in the absence of kinetic proofreading. Our work provides a simple, quantitative, and predictive molecular framework for understanding why TCR triggering is so selective and fast and reveals that, for some receptors, cell topography likely influences signaling outcomes.