Plant traits and community response to changing rainfall regimes in Australian pasture grassland

Abstract

Grasslands, including rangelands and pastures, cover ~40% of Earth's terrestrial surface area and more than 70% of the total agricultural area. They serve as a key repository of biodiversity and contribute significantly to global food security by providing a feed base for livestock production. Climate models project increasing mean atmospheric temperatures and an increase in associated drought intensity and frequency, along with more variable rainfall. Such changes in rainfall patterns impact many aspects of grassland systems, including productivity, species composition and associated functional traits. Plant traits describe various aspects of plant species' fitness, survival and reproductive success, including productivity responses to environmental variability and disturbance. Furthermore, flexibility in leaf traits (i.e. trait plasticity) help plants adapt to severe drought by minimizing water loss via transpiration (lower SLA or higher LDMC) and optimizing photosynthetic carbon assimilation associated with increasing leaf tissue nitrogen concentrations. It is crucial to understand relationships between plant traits and their productivity responses to changing rainfall patterns since this can inform predictions of forage availability for grazers and implications for the composition, biodiversity, ecological functioning in grassland systems under differing future rainfall scenarios.

Here, I investigated leaf trait response to predicted extreme rainfall scenarios (wet and dry, simulating La Nina and El Nino conditions, respectively) in six pasture species (grasses and legumes) using a well-replicated Pastures and Climate Extremes (PACE) field experiment, together with four grass species subjected to changes in the amount and frequency of rainfall inputs in the DRI-GRASS experimental field site at the Hawkesbury Campus of WSU, Australia.
We hypothesized that during future extreme drought conditions, grassland species reduce transpirational water loss by shifting their leaf traits towards stress avoidance (by reducing leaf area per unit leaf mass) and resource acquisition (higher carbon assimilation per unit leaf area associated with higher tissue nitrogen concentrations) strategies. We predicted that drought-induced leaf trait responses in plants will be lower during plant non-growing seasons relative to growing seasons. I specifically answered the following major research questions in my thesis: (1) How do pasture species' aboveground leaf functional traits vary across seasons? (2) Do leaf traits explain species’ productivity (in terms of biomass) responses to altered rainfall conditions? (3) How do variations in rainfall patterns affect the primary productivity and community-weighted traits of a local grassland ecosystem? These three major questions were addressed in three independent data chapters.

By examining a suite of plant characteristics relating to carbon and water use, this thesis contributes new knowledge on the variability of plant foliar traits across seasons and in response to altered rainfall patterns. It also provides a new understanding of trait-sensitivity relationships for important pasture species that can inform predictions of how different communities may respond to altered rainfall regimes in the future. Collectively, this information can help inform species selection in managed pastures in the context of current and future rainfall regimes, thereby guiding management decisions for sustainable pasture production in a changing and increasingly variable climate.

Date of Award	2024
Original language	English
Awarding Institution	Western Sydney University
Supervisor	Sally Power (Supervisor) & Manju Chandregowda (Supervisor)