Gas exchange and hydraulic strategies of pasture species under climate change

Abstract

Climate change-induced increases in global air temperature and concurrent modulations in the hydrological cycle have led to higher levels of drought stress globally. Grassland ecosystems such as pastures are particularly sensitive to climate change as they lack the deep roots and carbohydrate reserves that partially dampen the negative impact of abiotic stress in woody plant systems. Pastures have significant economic and ecological value, and it is important to assess the current and future impact of climate change on these systems. While many studies have investigated the response of pasture species to warming and drought separately, few studies have focused on their interactions. Furthermore, the mechanistic basis for the response of pasture species to abiotic stress, especially drought and heat stress, is not well understood. While the mechanisms underlying drought-induced impacts on hydraulic processes are well studied in woody species, there are currently few assessments in pasture grasses. In this thesis, I examined leaf gas exchange and hydraulic traits across a range of widely cultivated pasture species from different plant functional groups and investigated the response of these traits to warming and drought stress. My research sought to understand how these traits determine plant function under normal and abiotic stress conditions and how that will determine the response of pasture systems in a warmer, drier future. The primary objective was to identify more resilient pasture species and investigate the mechanistic basis for their resilience, aiding the development of species selection and management strategies that may mitigate the effects of climatic change. Findings show that increases in global air temperature may not have a positive impact on the productivity of pasture species in eastern Australia, even during the cool season and warming will have a more negative impact on C3 pasture species than C4 species during warmer periods. We found that the ability to resist xylem embolism and hydraulic dysfunction, rather than recovery, underpins pasture species resilience to drought, with early stomatal closure being crucial to species' survival under water limiting conditions. Collectively, climate change induced increases in air temperature and drought are likely to have negative impacts on growth and productivity of temperate C3 pasture species and physiological adjustment may not be sufficient to alleviate the impacts of rapid global change. Therefore, an overall decline in temperate C3 species performance and an increase in C4 dominance are expected in pastures in a warmer, drier future.

Date of Award	2021
Original language	English