Photosynthetic Response of Drooping Sheoak (Allocasuarina verticillata) to Root-Zone Soil Moisture

Author: Nasrin Sterling

Sterling, Nasrin, 2017 Photosynthetic Response of Drooping Sheoak (Allocasuarina verticillata) to Root-Zone Soil Moisture, Flinders University, School of the Environment

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Since European settlement in Australia, Allocasuarina verticillata (Lam.) L. Johnson (Drooping Sheoak) has undergone regional decline. In South Australia, Drooping Sheoak woodland is considered a threatened ecosystem, and in some areas is listed as vulnerable. Current land-use patterns (such as grazing, fire regimes, weeds and changes to the aquatic environment) have changed this natural woodland ecosystem and put pressure on biodiversity and the viability (or health) of woodland as well as the survival of other dependent flora and fauna. For a prospective conservation plan for Drooping Sheoak, this research focused on the leaf level photosynthetic response to root-zone soil moisture for two reasons: i) photosynthesis rate is the result of mechanical, physical and chemical processes, and thus can reveal the plant’s function under internal and external influences; ii) water is necessary for the plant’s structure and function; the failure of plant water uptake can result in mortality. In this research, four Drooping Sheoak trees (two male and two female) were selected throughout the period of the experiment, from mid-December 2016 utill the end of April 2017. The predawn stem water potential was measured to demonstrate the root-zone soil moisture at the point when the plant and soil reached equilibrium. In addition, the photosynthesis rate was measured using an LI-6400XT portable photosynthesis system. To eliminate any influential factor on photosynthesis other than soil moisture, the light saturation point was estimated and the assimilation rates after this point were taken as measurements of the response by the plant species to soil moisture. The predawn water potential was thus matched to each photosynthesis measurement, in order to illustrate the photosynthetic response to soil moisture. The results show stable photosynthesis within (0,-2) (MPa) and constant decline of photosynthesis after this point. The assimilation rate reached its lowest point within the range of soil moisture (-3,-4) (MPa), showing a 68% decline. These findings indicate that Drooping Sheoak’s function is suffering under soil moisture water stress, which can push the system into further degradation.

Keywords: Photosynthesis, Water potential, Water stress, Carbon assimilation, Threatened ecosystem, Land-use patterns, Vegetation, Woodlands, Viability, Light saturation point, Light response curve, Modification and degradation , Clearance and fragmentation, Disturbance, Time lag, Resilience, Vulnerable, Break point, Conservation, Recovery, Remnant, Mortality, Ecosystem services, Biodiversity, Cultural value, Flora and fauna, Exotic Species, Exotic Plants, Key ecological indicators, Grazing, Habitat loss, Habitat specialists, Glossy Black-cockatoo, Transition models, LI-6400XT, Stem Psychrometers,
Subject: Environmental Science thesis

Thesis type: Masters
Completed: 2017
School: School of the Environment
Supervisor: Huade Guan