An ecophysiological approach to understanding recruitment in keystone Triodia species in arid zone restoration

[Truncated] Where topsoil is absent in arid zone restoration sites, seeding native species is the most cost-effective method for returning vegetation at scale. With Triodia R.Br. (spinifex) species forming the dominant vegetation cover in the arid zone of Australia, restoring vegetation to pre-existing densities is a key priority for restoration operations. However the recruitment of Triodia species after seeding has been severely limited by biotic and abiotic pressures affecting key processes in the life-stage transitions from seed and seedling. This thesis examined the impacts of seed dormancy, water stress and temperature on the regulation of germination, emergence and seedling ecophysiology (growth and function) in keystone Triodia species from the Pilbara region, located in the northwest of Western Australia.

For successful recruitment in the arid zone, seeds need to germinate, emerge and grow sufficient shoot and root biomass and structure to survive the oncoming drought in the dry season. Previous research into the seed biology and subsequent recruitment of Triodia is limited and often out-dated, and previous attempts to germinate species are hampered by innate seed dormancy mechanisms (i.e. seed coat-imposed dormancy, and physiological dormancy). The research in this thesis found that while germination can proceed over a wide temperature range between 20- 40 °C, seed coat-imposed dormancy limited germination to high water potentials (≥ -0.25 MPa). Wet/ dry cycling of intact florets improved initial germination performance by alleviating seed dormancy to a degree, but did not improve germination under increasing water-stress. Cleaning florets to seeds, and therefore removing restrictive tissues, enabled a wider germination range into water stress, with seeds of species such as T. epactia, T. pungens and T. wiseana able to germinate into water stress of as low as -0.9 MPa. In species associated with more arid environments, such as T. basedowii, T. lanigera and T. sp. Shovelanna Hill, seed germination improved after cleaning to seed, however never occurred below - 0.6 MPa. This result indicated species-level variation in germination potential due to a varied bet-hedging strategy for recruitment in Triodia species from the arid zone.