Date of Award

2000

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

School

School of Natural Sciences

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr Ray Froend

Abstract

Stable carbon isotope (δ13C) analysis is a tool used to understand the water use efficiency (WUE) of plants. The advantage of measuring the carbon isotope composition in plant tissue over other measures of WUE (such as the ratio of photosynthesis to transpiration) is that it provides a time-integrated, rather than an instantaneous measure of plant-water relations. The purpose of this study was to evaluate the applicability of using δ13C analysis in natural populations of the fringing tree Melaleuca preissiana based on short and long term sampling methods. There were three components to this study; a controlled, glasshouse experiment which established whether a relationship existed between δ13C and instantaneous WUE in M. preissiana, and two field studies which evaluated the use of time-integrated, short and long term sampling methods of δ13C in M. preissiana populations along a hydrological gradient. The glasshouse experiment compared measures of instantaneous WUE and time-integrated δ13C of M. preissiana seedlings. undergoing a drying regime (two five-week drying intervals) to those in a control (regular watering). The seedlings undergoing the drying regime became more water use efficient (using instantaneous measures) as soil moisture decreased. Time-integrated δ13C measurements of the same seedlings also increased significantly over the duration of the experiment. This established the positive relationship between δ13C and instantaneous WUE in M. preissiana, and supported the inverse relationship between water availability and δ13C, which has been documented by many researchers. Light intensity was also a likely factor influencing the δ13C discrimination of the seedlings. Future studies need to account for light variations when isolating water availability as the primary factor affecting δ13C. Short term sampling for δ13C of natural M. preissiana populations involved collecting recently-formed xylem tissue for isotopic analysis. Trees situated along a hydrological gradient (from areas of high to low relative water availability) at three wetlands were sampled for δ13C. δ13C of trees decreased significantly from areas of low to high water availability. Seasonal sampling also showed that δ13C of the trees decreased significantly from the dry to wet season. This suggested that short term δ13C measurements indicated a physiological response in WUE in M. preissiana, due to spatial and temporal variations in water availability. Long term δ13C measurements involved sampling annual growth ring tissue from the same M. preissiana populations sampled for short term δ13C. This provided a record of the trees WUE across annual periods of growth. Direct relationships between ring δ13C and historical water availability were not significant, however δ13C of annual rings did indicate a physiological response to past, episodic events such as fire and prolonged drought periods. From these results, it was concluded that short and long term δ13C measurements of M. preissiana populations were indicative of spatial and temporal variations in water availability. It was supported that the degree of isotopic discrimination in M. preissiana was inversely related to water availability due to the various physiological processes occurring during carbon assimilation. The applicability of the approaches used for measuring δ13C would increase by accounting for tree morphological factors such as age, size and health, and "external" environmental factors such as light intensity, all of which additionally influence δ13C discrimination.

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