Anatomical constraints on grass physiological responses to changes in climate

Abstract

Grasslands occupy more of Earth’s terrestrial surface than any other biome and are characterized by the dominance of grasses (Poaceae), fire, grazing, and an inherently variable climate. Future climate projections forecast dramatic shifts in precipitation within the next 100 years for grassland systems, negatively impacting most native grassland species. Over the past 30 years, a substantial focus has been placed on understanding the ecological consequences of grassland responses to drought. During this period, most research has focused on identifying changes in productivity and species diversity and the larger scale consequences of drought on carbon, nutrient, and water cycling. While this ecosystem and community focus has been foundational for forecasting future scenarios, it typically lacks detailed mechanistic investigation of leaf physiological and anatomical responses to drought. For this reason, research focused on the physiological responses of dominant grassland species to changes in water availability provide a missing perspective for WHY and HOW drought impacts grassland function. Prior research on a dominant grass species, Andropogon gerardii, has illustrated that leaf-level physiological responses were constrained by anatomical traits that dictate carbon assimilation and water-use strategies (Bachle & Nippert, 2018 – Acta Oecologica). In this dissertation, I used both naturally occurring climate gradients and rainfall manipulation experiments to explore responses of a dominant grass species and provide a detailed perspective of differences in grass anatomy and physiology in response to water limitations across the climate gradients of the Great Plains. During a greenhouse dry down experiment I imposed an extreme drought by withholding water from 12 species of grasses from 6 tribes that are known to vary in drought tolerance (Bachle et al.- In Prep). I analyzed physiological responses at three stages: prior to drought (“Initial”), when samples reached stomatal closure (“Stressed”), and two days after re-watering (“Recovery”). I paired this physiological data with measurements of above- and belowground productivity, leaf and root economic traits, and leaf – level microanatomy. Results from this experiment revealed species response to drought were similar within tribe, while recovery from drought was highly variable across species. In addition, drought responses were framed by phylogenetic relatedness, physiology, morphology and microanatomical traits while drought recovery was influenced by one primary trait: the number of stomata. These data show the diverse physiological responses and variability of microanatomical traits across and within tribes. Furthermore, these results suggest that the biogeographic histories frame a species’ ability to respond to changes in low soil moisture. To investigate trait variability, I performed a literature search to assess intraspecific variability in commonly measured traits (such as specific leaf area - SLA) in A. gerardii from 13 climatically-distinct grasslands. Results indicated that SLA varied widely between locations, but with no discernable trends with climate parameters (Bachle et al., 2018 Frontiers in Ecology and Evolution). Ecologists commonly measure SLA because differences in this trait reflect growth strategies and leaf carbon investment, and because it’s a relatively easy trait to measure. However, the investment strategies inferred from SLA result from a suite of underlying anatomical tissues (bundle sheath, mesophyll, xylem) that are seldom measured. To understand how physiology/anatomy varies across latitudinal population gradients, I collected A. gerardii leaf tissues from locations varying in temperature and precipitation (KS, NE, SD, MN). I analyzed the relationship between SLA and internal leaf anatomical traits (Bachle & Nippert, 2020 – Annals of Botany). SLA was statistically similar in A. gerardii across locations, while anatomical traits related to carbon assimilation (mesophyll area, bundle sheath thickness) varied by mean annual temperature. Anatomical traits connected with water transport and storage (xylem area, cavitation resistance) exhibited 3 – 4 times the coefficient of variation than did carbon-related traits. Results from this chapter illustrate that variation in anatomical traits, influenced by climate, may underlie patterns of growth and productivity in this species at larger ecological scales. For my final chapter, I collected physiological and microanatomical data from A. gerardii in Kansas and Nebraska prairies managed with or without cattle in 2018 and 2019 (Bachle & Nippert – In Prep). The precipitation differences between successive years at each location had a larger impact on physiological and anatomical differences among sites compared to the grazing treatments. In addition, photosynthetic rates increased with leaf – level nitrogen content, while cavitation resistance increased with higher C:N ratios. Microanatomical traits, such as bundle sheath tissue area, were found to correlate with photosynthetic rates; however, the direction of this relationship was dependent on the year sampled. These results indicate that leaf-level nutrient content can influence microanatomical leaf structures and physiological responses to changes in climate. Overall, the results from my dissertation highlight the integral role of leaf anatomical traits in contextualizing our interpretation of physiological responses to drought, and across regional gradients in climate. My data clearly show that commonly used whole-leaf traits (e.g., SLA), unlike leaf microanatomy, typically don’t vary predictably with leaf physiology or climate gradients. Moving forward, the research framework I have organized can be applied to other species. The increased anatomical trait representation of other native species will help elucidate leaf form and function while also adding beneficial data that may be useful for forecasting future grassland responses in this era of global change.