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Edith Cowan University (2003)

Variability in water use by phreatophytic Banksia woodland vegetation of the Swan Coastal Plain, Western Australia

Zencich, Sandra Jane

Titre : Variability in water use by phreatophytic Banksia woodland vegetation of the Swan Coastal Plain, Western Australia

Auteur : Zencich, Sandra Jane

Université de soutenance : Edith Cowan University

Grade : Doctor of Philosophy 2003

Résumé
Seasonal changes exist in source-water utilisation by Banksia woodland species overlying a shallow aquifer on the Swan Coastal Plain in Western Australia These were studied to identify the importance of groundwater as a water source to this vegetation. Intra-and interspecific variation in seasonal water use was investigated across the dominant overstorey and understorey species, with particular emphasis on species of different rooting morphologies. Seasonal water use was investigated across a topographical gradient ranging from a groundwater depth of 2.5 to 30 metres, a scenario representative of the coastal sand-dune system. An examination was also made of the response of woodland vegetation to an induced and rapid temporal separation from the groundwater able, beyond the normal range of groundwater fluctuation. To investigate a species’ seasonal water usage, measurements of the naturally occurring stable isotope of hydrogen in water (deuterium, δ2H) were used to distinguish among potential plant water sources. Isotopic ratios from vascular water of the study species were compared with those of potential sources of precipitation, soil moisture and groundwater. The primary assumptions of this method were validated for the study species in laboratory experiments, and field-sampling strategies were developed. Through this process, the water sources available to plants were identified. Mixing models were employed to calculate the proportion of groundwater used. The results of this study confirm that groundwater is an important water source to both overstorey and understorey components of the woodland. Phreatophytic species were characterised by a deep root system, in which the tap root played a significant role in groundwater uptake. These species derived some of their water from groundwater throughout the dry-wet cycle. With the onset of the hot, Mediterranean summer, they increased their use of deep moisture sources, including groundwater. Variability in groundwater usage was observed at the interspecific and intraspecific level. This was dependent on root-system distribution and maximum root depth. For those species capable of using groundwater, the degree to which it was utilised was dependent on the proximity of groundwater (i.e. the depth to the water table), and the availability of moisture in shallower horizons of the soil profile (i.e. the season). For individuals occurring at shallow depth to water (< 8 m), groundwater was an important water source : whereas, at greater depths (≥8 m) its importance decreased as it was used in lower proportions and/or for shorter durations. Comparisons of within-species variation revealed disparities in groundwater usage, and possibly dependency, between the dominant overstorey Banksia species. Banksia ilicifolio, a species confined to habitats where groundwater depth is ≤8 m, was dependent on groundwater during the dry summer period (up to 90% of water used was groundwater) and may be considered a summer obligate phreatophyte. B. attenuata is a summer facultative phreatophyte, occurring where groundwater depth is 2 to >30m with groundwater accounting for between 5 to 75% of water used depending on depth to the water table. These results suggest the possibility of an obligate-facultative phreatophyte distinction. The response of phreatophytic Banksia species to changes in the groundwater regime beyond the normal range of groundwater fluctuation confirmed the seasonal nature of groundwater dependence, and varied sensitivity of individuals according to their position within the sandplain landscape (i.e. proximity to groundwater).

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